Home Useful Tips Types of installations used in electrochemical research. Topic iii. electrochemical methods of analysis. "instruments and analytical technology"

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Types of installations used in electrochemical research. Topic iii. electrochemical methods of analysis. "instruments and analytical technology"

Work description

Modern branches of production and social life of people set their own specific tasks for physicochemical methods of analysis to control product quality. One of the main physicochemical methods of analysis are electrochemical methods of analysis.
These methods can quickly and accurately determine many indicators of product quality.
Electrochemical methods for analyzing the composition of a substance are widely used in various industries. They allow you to automate the receipt of results on product quality and correct violations without stopping production. In the food industry, these methods determine the acid-base balance of the product, the presence of harmful and toxic substances and other indicators that affect not only the quality, but also the safety of food.
Equipment for electrochemical analyzes is relatively cheap, affordable and easy to use. Therefore, these methods are widely used not only in specialized laboratories, but also in many industries.
In this regard, the purpose of this cu

INTRODUCTION 2
THEORETICAL PART 3

1.1 General characteristics of physical and chemical methods of analysis 3

1.2 Characterization of electrochemical methods 4

1.3 Classification of electrochemical methods of analysis 5

2 EXPERIMENTAL-PRACTICAL PART 15
CONCLUSION 21
REFERENCES 22

Electrochemical methods of analysis is a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the test medium or at the interface and associated with a change in the structure, chemical composition or concentration of the analyte.

Varieties of the method are electrogravimetric analysis (electroanalysis), internal electrolysis, contact metal exchange (cementation), polarographic analysis, coulometry, etc. In particular, electrogravimetric analysis is based on weighing a substance released from one of the electrodes. The method allows not only to carry out quantitative determinations of copper, nickel, lead, etc., but also to separate mixtures of substances.

In addition, electrochemical methods of analysis include methods based on measuring electrical conductivity (conductometry) or electrode potential (potentiometry). Several electrochemical methods are used to find the endpoint of a titration (amperometric titration, conductometric titration, potentiometric titration, coulometric titration).

Distinguish between direct and indirect electrochemical methods. Direct methods use the dependence of the current strength (potential, etc.) on the concentration of the analyte. In indirect methods, the current strength (potential, etc.) is measured in order to find the end point of the titration of the analyte with a suitable titrant, i.e. the dependence of the measured parameter on the titrant volume is used.

For any kind of electrochemical measurements, an electrochemical circuit or an electrochemical cell is required, of which the analyzed solution is an integral part.

Electrochemical methods are classified according to the type of phenomena measured during the analysis. There are two groups of electrochemical methods:

1. Methods without the imposition of extraneous potential, based on the measurement of the potential difference that occurs in an electrochemical cell, consisting of an electrode and a vessel with a test solution. This group of methods is called potentiometric. Potentiometric methods use the dependence of the equilibrium potential of the electrodes on the concentration of ions participating in the electrochemical reaction on the electrodes.

2. Methods with the imposition of an extraneous potential based on the measurement of: a) electrical conductivity of solutions - conductometry; b) the amount of electricity passed through the solution - coulometry; c) the dependence of the current value on the applied potential - volt-amperometry; d) the time required for the passage of the electrochemical reaction - chronoelectrochemical methods(chronovoltammetry, chronoconductometry). In the methods of this group, an extraneous potential is imposed on the electrodes of the electrochemical cell.

The main element of instruments for electrochemical analysis is the electrochemical cell. In methods without the imposition of extraneous potential, it represents galvanic cell, in which, due to the occurrence of chemical redox reactions, an electric current arises. In a cell of the type of a galvanic cell, in contact with the analyzed solution, there are two electrodes - an indicator electrode, the potential of which depends on the concentration of the substance, and an electrode with a constant potential - a reference electrode, relative to which the potential of the indicator electrode is measured. The potential difference is measured with special devices - potentiometers.

In methods with the imposition of extraneous potential, use electrochemical cell, named so because on the electrodes of the cell, under the action of the superimposed potential, electrolysis occurs - the oxidation or reduction of the substance. In conductometric analysis, a conductometric cell is used, in which the electrical conductivity of a solution is measured. According to the method of application, electrochemical methods can be classified into straight lines, in which the concentration of substances is measured according to the indication of the device, and electrochemical titration, where the indication of the equivalence point is recorded using electrochemical measurements. In accordance with this classification, potentiometry and potentiometric titration, conductometry and conductometric titration, etc. are distinguished.

Devices for electrochemical determinations, in addition to an electrochemical cell, stirrer, load resistance, include devices for measuring the potential difference, current, solution resistance, and the amount of electricity. These measurements can be carried out with dial gauges (voltmeter or microammeter), oscilloscopes, automatic recording potentiometers. If the electrical signal from the cell is very weak, then it is amplified using radio amplifiers. In the devices of methods with the imposition of an extraneous potential, an important part is the devices for supplying the corresponding potential of a stabilized DC or AC current to the cell (depending on the type of method). The power supply unit for electrochemical analysis instruments usually includes a rectifier and a voltage stabilizer, which ensures the constant operation of the instrument.

Potentiometry combines methods based on measuring the emf of reversible electrochemical circuits when the potential of the working electrode is close to the equilibrium value.

Voltammetry is based on the study of the dependence of the polarization current on the voltage applied to the electrochemical cell, when the potential of the working electrode differs significantly from the equilibrium value. It is widely used for the determination of substances in solutions and melts (for example, polarography, amperometry).

Coulometry combines analytical methods based on measuring the amount of a substance released on an electrode during an electrochemical reaction in accordance with Faraday's laws. With coulometry, the potential of the working electrode differs from the equilibrium value.

Conductometric analysis is based on changes in the concentration of a substance or the chemical composition of the medium in the interelectrode space; it is not related to the potential of the electrode, which is usually close to the equilibrium value.

Dielectrometry combines methods of analysis based on the measurement of the dielectric constant of a substance due to the orientation in the electric field of particles (molecules, ions) with a dipole moment. Dielectrometric titration is used to analyze solutions.

"Electrochemical methods of analysis and their modern hardware design: a review of the WEB-sites of firms-sellers of chemical-analytical equipment"

Introduction

Chapter 1. Classification of electrochemical methods

1.1 Voltammetry

1.2 Conductometry

1.3 Potentiometry

1.4 Amperometry

1.5 Coulometry

1.6 Other electrochemical phenomena and methods

1.7 Applied electrochemistry

Chapter 2. Electrochemical methods of analysis and their role in environmental protection

Chapter 3. Devices based on electrochemical methods of analysis

Chapter 4. Review of WEB - sites of firms - sellers of chemical analytical equipment

Literature

INTRODUCTION

Electrochemical methods of analysis (electroanalysis), which are based on electrochemical processes, occupy a worthy place among the methods of monitoring the state of the environment, since they are capable of determining a huge number of both inorganic and organic environmentally hazardous substances. They are characterized by high sensitivity and selectivity, quick response to changes in the composition of the analyzed object, ease of automation and the possibility of remote control. Finally, they do not require expensive analytical equipment and can be used in laboratory, production and field conditions. Three electroanalytical methods are directly related to the problem under consideration: voltammetry, coulometry, and potentiometry.

CHAPTER 1. CLASSIFICATION OF ELECTROCHEMICAL METHODS

Electrochemical methods of analysis (EMA) are based on the study of processes occurring on the electrode surface or in the near-electrode space. The analytical signal is an electrical parameter (potential, current strength, resistance, etc.), functionally related to the concentration of the analyte in the solution and amenable to correct measurement.

The classification of EMA, proposed by IUPAC, has undergone certain changes over the past decades; clarifications (explanations) and additions have been made to it.

Considerable attention is paid to electrochemical cells and analytical signal sensors (electrode systems, various electrochemical sensors); it is these primary electrochemical converters that determine the analytical capabilities of any method. Currently, the most perfect and fastest processing of the signal from the sensor, the calculation of the statistical characteristics of both the initial signal and the results of the entire analysis as a whole, is not a problem. That is why it is important to obtain a reliable raw signal in order to calibrate it in concentration units.

According to the general classification proposed

IUPAC, EMA are subdivided into methods in which the excited electrical signal is constant or equal to zero and into methods in which the excited signal changes over time. These methods are classified as follows:

voltammetry - voltammetry,I ≠ 0; E = f (t);

potentiometric – potentiometry, (I = 0);

amperometric – amperometry (I ≠ 0; E =const);

chronopotentiometric,E = f (t); I =const;

impedance, or conductometric- measurements using superposition of low amplitude AC voltage; other, combined(eg spectroelectrochemical).

1.1 VOLTAMPEROMETRY

VOLTAMPEROMETRY- a set of electrochemical research and analysis methods based on the study of the dependence of the current in the electrolytic cell on the potential of the indicator microelectrode immersed in the analyzed solution, on which the investigated electrochemically active (electroactive) substance reacts. In addition to the indicator, an auxiliary electrode with a significantly larger surface is placed in the cell so that its potential practically does not change when the current passes (non-polarizable electrode). The potential difference of the indicator and auxiliary electrodes E is described by the equation E = U - IR, where U is the polarizing voltage, R is the solution resistance. An indifferent electrolyte (background) is introduced into the analyzed solution in a high concentration in order, firstly, to reduce the value of R and, secondly, to exclude the migration current caused by the action of an electric field on electroactive substances (outdated - depolarizers). At low concentrations of these substances, the ohmic voltage drop IR in the solution is very small. To fully compensate for the ohmic voltage drop, potentiostation and three-electrode cells are used, which additionally contain a reference electrode. In these conditions

Stationary and rotating electrodes are used as indicator microelectrodes - made of metal (mercury, silver, gold, platinum), carbon materials (for example, graphite), as well as dripping electrodes (made of mercury, amalgam, gallium). The latter are capillaries from which liquid metal flows out dropwise. Voltammetry using dripping electrodes, the potential of which changes slowly and linearly, is called. polarography (the method was proposed by Ya. Geirovsky in 1922). The reference electrodes are usually of the second kind, for example. calomel or silver chloride (see. Reference electrodes). The dependence curves I = f (E) or I = f (U) (voltammograms) are recorded with special devices - polarographs of different designs.

Voltammograms obtained with a rotating or dripping electrode with a monotonic change (linear sweep) of the voltage have the form shown schematically in the figure. The section for increasing the current is called. wave. Waves m. B. anodic, if the electroactive substance is oxidized, or cathodic, if it is reduced. When oxidized (Ox) and reduced (Red) forms of a substance are present in the solution, which react quickly (reversibly) at the microelectrode, a continuous cathodic-anode wave is observed on the voltammogram, crossing the abscissa axis at a potential corresponding to the redox potential of the Ox / Red system in given environment. If the electrochemical reaction on the microelectrode is slow (irreversible), an anodic wave of oxidation of the reduced form of the substance and a cathodic wave of reduction of the oxidized form (at a more negative potential) are observed on the voltammogram. The formation of the limiting current area on the voltammogram is associated either with the limited rate of mass transfer of the electroactive substance to the electrode surface by convective diffusion (limiting diffusion current, I d), or with the limited rate of formation of the electroactive substance from the analyte in solution. Such a current is called the limiting kinetic current, and its strength is proportional to the concentration of this component.

The waveform for a reversible electrochemical reaction is described by the equation:

where R is the gas constant, T is the absolute temperature, E 1/2 is the half-wave potential, i.e. potential corresponding to half-height of the wave (I d / 2;). The E 1/2 value is characteristic for a given electroactive substance and is used to identify it. When the electrochemical reaction is preceded by the adsorption of the analyte on the electrode surface, the voltammograms show not waves, but peaks, which is associated with the extreme dependence of adsorption on the electrode potential. The voltammograms recorded with a linear change (sweep) of the potential with a stationary electrode or on one drop of a dropping electrode (obsolete - oscillographic polarogram) also exhibit peaks, the descending branch of which is determined by the depletion of the near-electrode layer of the solution with an electroactive substance. In this case, the height of the peak is proportional to the concentration of the electroactive substance. In polarography, the limiting diffusion current (in μA) averaged over the lifetime of a drop is described by the Ilkovich equation:

where n is the number of electrons participating in the electrochemical reaction, C is the concentration of the electroactive substance (mM), D is the diffusion coefficient (cm 2 / s), the lifetime of a mercury drop (s), m is the rate of mercury outflow (mg / s) ...

With a rotating disk electrode, the limiting diffusion current is calculated from the equation:

where S is the surface area of the electrode (cm 2), is the circular frequency of rotation of the electrode (rad / s), v is the kinematic viscosity of the solution (cm 2 / s), F is the Faraday number (C / mol).

Cyclic voltammetry (voltammetry with a relatively fast triangular potential sweep) makes it possible to study the kinetics and mechanism of electrode processes by observing voltammograms with anodic and cathodic potential sweeps on the screen of an oscilloscope tube with afterglow simultaneously, reflecting, in particular, the electrochemical reactions of electrolysis products.

The lower limit of the determined concentrations of C n in V. methods with a linear potential scan is 10 -5 -10 -6 M. To reduce it to 10-7 -10 -8 M, improved instrumental options are used - alternating-current and differential pulse voltammetry.

In the first of these options, a variable component of small amplitude of sinusoidal, rectangular (square-wave voltammetry), trapezoidal or triangular shape with a frequency usually in the range of 20-225 Hz is imposed on the constant component of the polarization voltage. In the second variant, voltage pulses of the same magnitude (2-100 mV) with a duration of 4-80 ms with a frequency equal to the dripping frequency of a dropping mercury electrode are imposed on the constant component of the polarization voltage, or with a frequency of 0.3-1.0 Hz when using stationary electrodes. In both variants, the dependence on U or E of the alternating current component with phase or time selection is recorded. Voltammograms in this case have the form of the first derivative of a conventional voltammetric wave. The height of the peak on them is proportional to the concentration of the electroactive substance, and the peak potential serves to identify this substance according to reference data.

The peaks of various electroactive substances, as a rule, are better resolved than the corresponding voltammetric waves, and the peak height in the case of an irreversible electrochemical reaction is 5-20 times less than the peak height in the case of a reversible reaction, which also determines the increased resolution of these voltammetric options. For example, irreversibly reducing oxygen practically does not interfere with the determination of electroactive substances by the method of alternating current voltammetry. Peaks on alternating-current voltammograms reflect not only the electrochemical reactions of electroactive substances, but also the processes of adsorption - desorption of non-electroactive substances on the electrode surface (peaks of non-Paradean admittance, obsolete - tensammetric peaks).

For all variants of voltammetry, a method for reducing C n is used, based on preliminary electrochemical, adsorption or chemical accumulation of the determined component of the solution on the surface or in the volume of a stationary microelectrode, followed by registration of a voltammogram reflecting the electrochemical reaction of the accumulation product. This type of voltammetry is called stripping (the outdated name of stripping voltammetry with accumulation on a stationary mercury microelectrode is amalgam polarography with accumulation). In stripping voltammetry with preliminary accumulation of С n reaches 10 -9 -10 -11 M. The minimum values of С n are obtained using thin-film mercury indicator electrodes, incl. mercury-graphite, consisting of the smallest droplets of mercury electrolytically isolated on a specially processed graphite substrate.

For phase and elemental analysis of solids, stripping voltammetry with electroactive carbon electrodes (the so-called mineral-carbon paste electrodes) is used. They are prepared from a mixture of coal powder, an investigated powdery substance and an inert binder, for example. petroleum jelly. A version of this method has been developed, which makes it possible to analyze and determine the thickness of metal coatings. In this case, a special device (pressure cell) is used, which makes it possible to register a voltammogram using a drop of background electrolyte applied to the surface under study.

Application

Voltammetry is used: for the quantitative analysis of inorganic and organic substances in a very wide range of contents - from 10 -10% to tens of%; to study the kinetics and mechanism of electrode processes, including the stage of electron transfer, previous and subsequent chemical reactions, adsorption of initial products and products of electrochemical reactions, etc .; to study the structure of the electric double layer with, the equilibrium of complexation in solution, the formation and dissociation of intermetallic compounds in mercury and on the surface of solid electrodes; to select the conditions for amperometric titration, etc.

1.2 Conductometry

Conductometry - based on the measurement of the electrical conductivity of a solution and is used to determine the concentration of salts, acids, bases, etc. In conductometric determinations, electrodes of the same materials are usually used, and the conditions for their conduct are selected in such a way as to minimize the contribution of potential surges at both electrode / electrolyte interfaces (for example, high-frequency alternating current is used). In this case, the main contribution to the measured cell potential is made by the ohmic voltage drop IR, where R is the solution resistance. The electrical conductivity of a one-component solution can be related to its concentration, and the measurement of the electrical conductivity of electrolytes of complex composition makes it possible to estimate the total content of ions in the solution and is used, for example, to control the quality of distilled or deionized water. In another type of conductometry - conductometric titration - a known reagent is added in portions to the analyzed solution and the change in electrical conductivity is monitored. The equivalence point, at which a sharp change in electrical conductivity is noted, is determined from the graph of the dependence of this value on the volume of the added reagent.

1.3 Potentiometry

Potentiometry - used to determine various physicochemical parameters based on data on the potential of a galvanic cell. The electrode potential in the absence of current in the electrochemical circuit, measured relative to the reference electrode, is related to the concentration of the solution by the Nernst equation. In potentiometric measurements, ion-selective electrodes are widely used, which are mainly sensitive to one ion in a solution: a glass electrode for measuring pH and electrodes for selective determination of sodium, ammonium, fluorine, calcium, magnesium ions, etc. enzymes, and the result is a system that is sensitive to the corresponding substrate. Note that the potential of an ion-selective electrode is determined not by the transfer of electrons, as in the case of substances with electronic conductivity, but mainly by the transfer or exchange of ions. However, the Nernst equation, which relates the electrode potential to the logarithm of the concentration (or activity) of a substance in a solution, is also applicable to such an electrode. In potentiometric titration, the reagent is added to the analyzed solution in portions and the change in potential is monitored. The S-curves, typical of this type of titration, allow you to determine the equivalence point and find such thermodynamic parameters as equilibrium constant and standard potential.

1.4 Amperometry

The method is based on measuring the limiting diffusion current passing through the solution at a fixed voltage between the indicator electrode and the reference electrode. In amperometric titration, the equivalence point is determined by the bend in the current versus volume of the added working solution. Chronoamperometric methods are based on measuring the dependence of current on time and are mainly used to determine diffusion coefficients and rate constants. According to the principle of amperometry (like voltammetry), miniature electrochemical cells operate, serving as sensors at the outlet of liquid chromatograph columns. Galvanostatic methods are similar to amperometric ones, but they measure the potential when a current of a certain magnitude passes through the cell. So, in chronopotentiometry, the change in potential over time is controlled. These methods are mainly used to study the kinetics of electrode reactions.

1.5 Coulometry.

In coulometry at a controlled potential, a complete electrolysis of the solution is carried out by intensively stirring it in an electrolyzer with a relatively large working electrode (bottom mercury or platinum mesh). The total amount of electricity (Q, C) required for electrolysis is related to the amount of the forming substance (A, g) by Faraday's law:

where M is a pier. mass (g / mol), F  Faraday number. Coulometric titration means that at a constant current, a reagent is electrolytically generated that interacts with the substance to be determined. The titration progress is controlled potentiometrically or amperometrically. Coulometric methods are convenient in that they are absolute in nature (i.e., they allow one to calculate the amount of the analyte without resorting to calibration curves) and are insensitive to changes in electrolysis conditions and cell parameters (electrode surface area or stirring intensity). In coulomb gravimetry, the amount of electrolyzed substance is determined by weighing the electrode before and after electrolysis.

There are also other electroanalytical methods. In alternating-current polarography, a sinusoidal voltage of small amplitude in a wide frequency range is applied to a linearly varying potential and either the amplitude and phase shift of the resulting alternating current or impedance is determined. From these data, information is obtained on the nature of substances in solution and on the mechanism and kinetics of electrode reactions. Thin-layer methods use electrochemical cells with an electrolyte layer 10–100 µm thick. In such cells, electrolysis is faster than in conventional electrolysers. To study electrode processes, spectrochemical methods with spectrophotometric registration are used. To analyze the substances formed on the surface of the electrode, measure their absorption of light in the visible, UV and IR regions. Changes in the properties of the electrode surface and the medium are monitored using electroreflection and ellipsometry methods, which are based on measuring the reflection of radiation from the electrode surface. These include methods of specular reflection and Raman scattering of light (Raman spectroscopy), second harmonic spectroscopy (Fourier spectroscopy).

1.6 Other electrochemical phenomena and methods

With the relative movement of the electrolyte and charged particles or surfaces, electrokinetic effects arise. An important example of this kind is electrophoresis, which separates charged particles (for example, protein molecules or colloidal particles) moving in an electric field. Electrophoretic methods are widely used to separate proteins or deoxyribonucleic acids (DNA) in a gel. Electrical phenomena play an important role in the functioning of living organisms: they are responsible for the generation and propagation of nerve impulses, the emergence of transmembrane potentials, etc. Various electrochemical methods are used to study biological systems and their components. The study of the effect of light on electrochemical processes is also of interest. So, the subject of photoelectrochemical research is the generation of electrical energy and the initiation of chemical reactions under the influence of light, which is very important for increasing the efficiency of converting solar energy into electrical energy. It usually uses semiconductor electrodes made of titanium dioxide, cadmium sulfide, gallium arsenide and silicon. Another interesting phenomenon is electrochemiluminescence, i.e. generation of light in an electrochemical cell. It is observed when high-energy products are formed on the electrodes. The process is often carried out in a cyclic manner to obtain both oxidized and reduced forms of a given compound. Their interaction with each other leads to the formation of excited molecules, which pass into the ground state with the emission of light.

1.7 Applied electrochemistry

Electrochemistry has many practical applications. With the help of primary galvanic cells (disposable cells) connected to batteries, they convert chemical energy into electrical energy. Secondary sources of current - batteries - store electrical energy. Fuel cells are primary power sources that generate electricity through a continuous supply of reactants (such as hydrogen and oxygen). These principles are at the heart of portable power supplies and batteries used in space stations, electric vehicles and electronic devices.

Large-scale production of many substances is based on electrochemical synthesis. During the electrolysis of brine in the chlor-alkali process, chlorine and alkali are formed, which are then used to obtain organic compounds and polymers, as well as in the pulp and paper industry. Electrolysis products are compounds such as sodium chlorate, persulfate, sodium permanganate; Industrial important metals are obtained by electro-extraction: aluminum, magnesium, lithium, sodium and titanium. It is better to use molten salts as electrolytes, since in this case, in contrast to aqueous solutions, the reduction of metals is not complicated by the evolution of hydrogen. Fluorine is obtained by electrolysis in molten salt. Electrochemical processes serve as the basis for the synthesis of some organic compounds; for example, the hydrodimerization of acrylonitrile produces adiponitrile (an intermediate in the synthesis of nylon).

It is widely practiced to apply electroplating coatings to various items of silver, gold, chrome, brass, bronze and other metals and alloys in order to protect steel items from corrosion, for decorative purposes, for the manufacture of electrical connectors and printed circuit boards in the electronics industry. Electrochemical methods are used for high-precision dimensional processing of workpieces made of metals and alloys, especially those that cannot be processed by conventional mechanical methods, as well as for the manufacture of parts with a complex profile. When the surface of metals such as aluminum and titanium is anodized, protective oxide films are formed. Such films are created on the surface of billets of aluminum, tantalum and niobium in the manufacture of electrolytic capacitors, and sometimes for decorative purposes.

In addition, studies of corrosion processes and the selection of materials that slow down these processes are often based on electrochemical methods. Corrosion of metal structures can be prevented by means of cathodic protection, for which an external source is connected to the protected structure and the anode and the potential of the structure is maintained so that its oxidation is excluded. Possibilities of practical application of other electrochemical processes are being investigated. So, electrolysis can be used to purify water. A very promising direction is the conversion of solar energy using photochemical methods. Electrochemical monitors are being developed, the principle of which is based on electrochemiluminescence.

Brief historical background... The beginning of the development of electroanalysis is associated with the emergence of the classical electrogravimetric method (about 1864, W. Gibbs). The discovery of the laws of electrolysis by M. Faraday in 1834 formed the basis of the coulometry method, but the application of this method began in the 30s of the twentieth century. A real turning point in the development of electroanalysis took place after the discovery in 1922 of the method of polarography by Ya. Geyrovsky. Polarography can be defined as electrolysis with a dripping mercury electrode. This method remains one of the main methods of analytical chemistry. In the late 50s - early 60s, the problem of environmental protection stimulated the rapid development of analytical chemistry, and in particular electroanalytical chemistry, including polarography. As a result, improved polarographic methods were developed: alternating current (Barker, B. Breuer) and pulsed polarography (Barksr, A. Gardnsr), which significantly exceeded in their characteristics the classical version of polarography proposed by Ya. Geirovsky. When solid electrodes made of various materials were used instead of mercury ones (used in polarography), the corresponding methods came to be called voltammetric ones. In the late 50s, the work of V. Kemuli and Z. Kublik laid the foundation for the method of stripping voltammetry. Along with the methods of coulometry and voltammetry, methods are developing based on the measurement of electrode potentials and electromotive forces of galvanic cells - methods of potentiometry and ionometry (see).

Voltammetry... This is a group of methods based on the study of the dependence of the current in the electrolytic cell on the value of the potential applied to the indicator microelectrode immersed in the analyzed solution. These methods are based on the principles of electrolysis; the analytes present in the solution are oxidized or reduced at the indicator electrode. In addition to the indicator, a reference electrode with a much larger surface is placed in the cell so that its potential practically does not change when the current passes. Stationary and rotating electrodes made of platinum or graphite are most often used as indicator microelectrodes, as well as a dropping mercury electrode, which is a long narrow capillary, at the end of which small mercury drops with a diameter of 1–2 mm are periodically formed and detached (Fig. 1). The qualitative and quantitative composition of the solution can be established from voltammograms.

Rice. 4. Electrochemical cell with a dripping mercury electrode: 1 - analyzed solution, 2 - dripping mercury electrode, 3 - reservoir with mercury, 4 - reference electrode

Voltammetric methods, especially sensitive options such as differential pulsed polarography and stripping voltammetry, are consistently used in all areas of chemical analysis and are most useful in solving environmental problems. These methods are applicable for the determination of both organic and inorganic substances, for example, for the determination of most chemical elements. The method of stripping voltammetry is most often used to solve the problem of determining traces of heavy metals in waters and biological materials. So, for example, voltammetric methods for the simultaneous determination of Cu, Cd and Pb, as well as Zn and Pb or TI in drinking water are included in the standard Germany. An important advantage of voltammetry is the ability to identify the forms of metal ions in waters. This makes it possible to assess the quality of water, since different chemical forms of the existence of metals have different degrees of toxicity. Organic substances can be used to determine compounds with groups capable of reduction (aldehydes, ketones, nitro-, nitroso compounds, unsaturated compounds, halogen-containing compounds, azo compounds) or oxidation (aromatic hydrocarbons, amines, phenols, aliphatic acids, alcohols, sulfur-containing compounds). The possibilities of determining organic matter by stripping voltammetry are significantly expanded when using chemically modified electrodes. By modifying the electrode surface with polymer and inorganic films that include reagents with specific functional groups, including biomolecules, it is possible to create conditions for the component being determined such that the analytical signal is practically specific. The use of modified electrodes provides for the selective determination of compounds with similar redox properties (for example, pesticides and their metabolites) or electrochemically inactive on conventional electrodes. Voltammetry is used for the analysis of solutions, but it can also be used for the analysis of gases. Many simple voltammetric analyzers have been designed for use in the field.

Coulometry... An analysis method based on measuring the amount of electricity (Q) that has passed through an electrolyzer during the electrochemical oxidation or reduction of a substance on a working electrode. According to Faraday's law, the mass of an electrochemically converted substance (P) is related to Q by the ratio:

P = QM/ Fn,

where M is the molecular or atomic mass of a substance, n is the number of electrons involved in the electrochemical transformation of one molecule (atom) of a substance, p is Faraday's constant.

Distinguish between direct coulometry and coulometric titration. In the first case, an electrochemically active substance is determined, which is deposited (or converted to a new oxidation state) on the electrode at a given electrolysis potential, while the amount of electricity consumed is proportional to the amount of the reacted substance. In the second case, an electrochemically active auxiliary reagent is introduced into the analyzed solution, from which a titrant (coulometric titrant) is electrolytically generated, and it quantitatively chemically interacts with the substance to be determined. The content of the analyte is estimated by the amount of electricity passed through the solution when generating the titrant until the end of the chemical reaction, which is established, for example, using colored indicators. It is important that when conducting coulometric analysis in the test solution there are no foreign substances that can enter into electrochemical or chemical reactions under the same conditions, that is, no side electrochemical and chemical processes occur.

Coulometry is used to determine both trace (at the level of 109-10 R mol / l) and very large amounts of substances with high accuracy. Many inorganic (almost all metals, including heavy metals, halogens, S, NO 3, NO 2) and organic substances (aromatic amines, nitro- and nitroso compounds, phenols, azo dyes) can be determined coulometrically. Automatic coulometric analyzers for the determination of very low contents (up to 104%) of gaseous pollutants (SO2 "Oz, H 2 S, NO, NO 2) in the atmosphere have successfully proven themselves in the field.

Potentiometry. An analysis method based on the dependence of the equilibrium electrode potential E on the activity a of the components of the electrochemical reaction: aA + bB + ne = mM + pP.

In potentiometric measurements, a galvanic cell is made up of an indicator electrode, the potential of which depends on the activity of one of the components of the solution, and a reference electrode, and the electromotive force of this element is measured.

A distinction is made between direct potentiometry and potentiometric titration. Direct potentiometry is used to directly determine the activity of ions by the value of the potential (E) of the corresponding indicator electrode. In the method of potentiometric titration, the change in E is recorded during the reaction of the analyte with a suitable titrant.

When solving problems of environmental protection, the most important method of direct potentiometry using membrane ion-selective electrodes (ISE) - ionometry. Unlike many other methods of analysis, which make it possible to assess only the total concentration of substances, ionometry makes it possible to assess the activity of free ions and therefore plays an important role in studying the distribution of ions between their various chemical forms. Methods of automated monitoring are especially important for monitoring environmental objects, and the use of ISE is very convenient for this purpose.

One of the main indicators in characterizing the state of the environment is the pH value of the environment, which is usually determined using glass electrodes. Glass electrodes covered with a semi-permeable membrane with a film of the corresponding electrolyte are used in the analysis of water and atmosphere to control pollution (NH 3, SO 2 NO, NO 2, CO 2, H 2 S). ISE is usually used to control the content of anions, for which there are traditionally significantly fewer determination methods than for cations. To date, ISEs have been developed and are widely used for the determination of F, CI, Br, I, C1O 4, CN, S 2, NO] and NO 2, which make it possible to determine the listed ions in the concentration range from 10 -6 to 10 -1 mol / l ...

One of the important areas of application of ionometry is hydrochemical studies and the determination of the concentration of anions and cations in different types of water (surface, sea, rainwater). Another area of application of ISE is food analysis. An example is the determination of NO - 3 and NO 2 - in vegetables, meat and dairy products, baby food. A miniature ISE in the shape of a needle has been created for the determination of NO - 3 directly in the pulp of fruits and vegetables.

Ionometry is also widely used to determine various biologically active compounds and drugs. At present, we can already say that there are carriers that are selective for almost any type of organic compounds, which means that it is possible to create an unlimited number of corresponding ISEs. A promising direction is the use of enzymatic electrodes, the membrane of which includes immobilized enzymes. These electrodes are highly specific for enzymatic reactions. With their help, for example, it will be possible to determine inhibiting cholinesterase, insecticides (organophosphorus compounds, carbamates) at concentrations of -1 ng / ml. The future of the method is associated with the creation of compact specific sensors, which are modern electronic devices in combination with ion-selective membranes, which will make it possible to dispense with the separation of sample components and significantly accelerate the analysis in the field.

Wastewater analysis

Electroanalytical methods, which are usually used in water analysis for the determination of inorganic components, are often inferior in sensitivity to methods of gas and liquid chromatography, atomic adsorption spectrometry. However, cheaper equipment is used here, sometimes even in the field. The main electroanalytical methods used in water analysis are voltammetry, potentiometry and conductometry. The most effective voltammetric methods are differential pulsed polarography (DIP) and inversion electrochemical analysis (IEA). The combination of these two methods makes it possible to carry out the determination with a very high sensitivity - approximately 10 -9 mol / L, the instrumentation is simple, which makes it possible to carry out analyzes in the field. Fully automated monitoring stations operate on the principle of using the IEA method or a combination of IEA and RIP. The DIP and IEA methods in the direct version, as well as in combination with each other, are used to analyze water pollution with heavy metal ions and various organic substances. Moreover, sample preparation methods are often much simpler than in spectrometry or gas chromatography. The advantage of the IEA method is (in contrast to other methods, for example, atomic adsorption spectrometry) also the ability to “distinguish” free ions from their bound chemical forms, which is important both for assessing the physicochemical properties of the analyzed substances, and from the point of view of biological control ( for example, when assessing the toxicity of waters). The analysis time is sometimes reduced to a few seconds by increasing the scan rate of the polarizing voltage.

Potentiometry with the use of various ion-selective electrodes is used in water analysis to determine a large number of inorganic cations and anions. The concentrations that can be determined in this way are 10 0 -10 -7 mol / l. The control with the help of ion-selective electrodes is characterized by simplicity, rapidity and the ability to carry out continuous measurements. At present, ion-selective electrodes have been created that are sensitive to certain organic substances (for example, alkaloids), surfactants and detergents (detergents). In the analysis of water, compact analyzers such as probes are used with the use of modern ion-selective electrodes. At the same time, a circuit that processes the response and a display are mounted in the probe handle.

Conductometry it is used in the work of analyzers of detergents in wastewater, in determining the concentration of synthetic fertilizers in irrigation systems, in assessing the quality of drinking water. In addition to direct conductometry, indirect methods can be used to determine certain types of pollutants, in which the analytes interact with specially selected reagents before measurement and the recorded change in electrical conductivity is caused only by the presence of the corresponding reaction products. In addition to the classical versions of conductometry, its high-frequency version (oscillometry) is also used, in which the indicator electrode system is implemented in conductometric continuous analyzers.

Chapter 3. Devices based on electrochemical methods of analysis

The voltammetric method of analysis is today considered one of the most promising among electrochemical methods, due to its wide capabilities and good operational characteristics.

Modern stripping voltammetry, which has replaced classical polarography, is a highly sensitive and rapid method for the determination of a wide range of inorganic and organic substances with redox properties.

This is one of the most versatile methods for the determination of trace amounts of substances, which is successfully used for the analysis of natural geo- and biological, as well as medical, pharmaceutical and other objects.

Voltammetric analyzers make it possible to simultaneously determine several components (up to 4 - 5) in one sample with a fairly high sensitivity of 10 -8 - 10 -2 M (and stripping voltammetry - up to 10-10 - 10 -9 M).

The most promising in analytical chemistry today is considered to be adsorption stripping voltammetry, based on the preliminary adsorption concentration of the determined element on the electrode surface and the subsequent registration of the voltammogram of the resulting product. Thus, it is possible to concentrate many organic substances, as well as metal ions in the form of complexes with organic ligands (especially nitrogen - and sulfur-containing ones). With a sequential accumulation time of 60 s and using a differential pulse mode for recording voltammograms, it is possible to achieve the detection limits at the level of 10 -10 - 10 -11 mol / L (10 -8 - 10 -9 g / L or 0.01 - 0.001 μg / dm 3 ).

Voltammetric complex for the analysis of metals "IVA - 400MK" (NPKF "Aquilon", Moscow) designed for the analysis of 30 elements (Cu, Zn, Pb, Cd, As, Co, Ni, Cr, and other metals), sensitivity 0.1 - 10 -3 μg / dm 3.

Voltammetric analyzer with UV irradiation of samples - TA-1M (Tomsk), which, in addition to metal ions, allows the determination of a number of organic compounds. The device has the following features:

Simultaneous analysis in three electrochemical cells,

Small amount of sample (0.1 - 1.0 g),

· Low cost of sample preparation and analysis.

In St. Pereburg NFT "Volta" produces a voltammetric complex "AVS-1" with a rotating disk glassy carbon electrode, which allows the analysis of toxic elements in waters, food products and various materials. The detection limit without sample concentration is: 0.1 mg / L for Pb, 0.5 mg / L for Cd, 1.0 μg / L for Cu. The sample volume is 20 ml, the time to obtain the volt-ampere curve is no more than 3 minutes.

"AZHE - 12" (Vladikavkaz) is intended for express analysis of the ionic composition of waste and circulating waters. The analyzer uses a traditional mercury electrode. Controlled components - Cu, Zn, Pb, Cd, In, Bi, Tl, Sb, As, Co, Ni, Cr, CN -, Cl -, S 2-. The analyzer allows measurements without sample preparation.

Ecotest-VA (Econix, Moscow) - portable voltammetric analyzer. It is made on a modern microprocessor element base and is equipped with a whole complex of electrodes - graphite, glassy carbon, microelectrodes made of noble metals and a mercury dropping electrode.

Devices of this series are intended for the determination of metals Cu, Zn, Pb, Cd, As, Bi, Mn, Co, Ni, Cr, as well as acetaldehyde, furfural, caprolactam and other substances in samples of drinking, natural, waste water, soil, and after appropriate sample preparation - in food and feed.

The capabilities of many analytical methods for water analysis can be significantly expanded when using in the process of sample preparation flow-injection concentrating attachments operating in an automatic mode, for example, of the BPI-M and BPI-N type.

BPI-M - designed for automated sample preparation, it includes microcolumns with highly efficient sorbents. The unit's productivity is 30-60 analyzes per day with full automation of the process. The use of the block allows you to increase the sensitivity 20 times per minute of concentration. The unit works best in combination with atomic absorption detection, as well as with X-ray fluorescence, atomic absorption and electrochemical methods.

BPI-N- is intended for concentration of metal ions on selective sorbents simultaneously in four microcolumns with DEETATA - sorbent or on 4 thin-layer sorption DEETATA - filters. It can be used with X-ray fluorescence, atomic absorption, atomic emission, electrochemical methods.

Voltammetric Analyzers

Devices based on the principle of inverse voltammetry have recently been in great demand. They combine selectivity and high sensitivity with ease of analysis.

With regard to the determination of the elemental composition (for example, for heavy metals), these devices successfully compete with atomic absorption spectrophotometers, since they are not inferior to them in sensitivity, but are much more compact and cheap (about 5-10 times). They do not require additional consumables, and also enable the simultaneous express determination of several elements.

Polarograph ABC - 1.1 (NTF "Volta" St. Petersburg).

The detection limits for metals without sample concentration are (mg / l): Cd, Pb, Bi - 0.0001, Hg - 0.00015, Cu - 0.0005, Zn, Ni - 0.01. The cost is $ 1700.

Analyzers based on conductometric principle are intended for the quantitative determination of the total content of salts in water. EKA-2M (St. Petersburg) measures salinity in a wide range of values from 0.05 to 1000 μS / cm ($ 900). ANION, MARK, KSL (from 330 to 900 $), COD - analyzers (750 $).

Gas analyzers of harmful substances

An automatic gas analyzer is a device in which air sampling, determination of the amount of a controlled component, issuance and recording of analysis results are carried out automatically according to a given program without the participation of an operator. Gas analyzers are used to monitor the air environment, the operation of which is based on various principles.

Thermal conductometric gas analyzers.

The principle of operation is based on the dependence of the thermal conductivity of the gas mixture on its composition. Thin platinum filaments are the sensitive element of this type of analyzers. Depending on the composition of the gas, the temperature of the sensitive element changes, a current arises, the strength of which is proportional to the concentration of the controlled component.

Coulometric Gas Analyzers.

The principle of operation is based on measuring the limiting electric current that occurs during the electrolysis of a solution that contains the substance to be determined, which is an electrochemical depolarizer. The mixture to be analyzed, containing, for example, sulfur dioxide, is fed into an electrochemical cell. It reacts with iodine to form hydrogen sulfide, which is then electrooxidized at the measuring electrode. Electric current is a measure of the concentration of an analyte.

CHAPTER 4. OVERVIEWWEB- WEBSITES OF FIRMS - SELLERS OF CHEMICAL - ANALYTICAL EQUIPMENT

"AGILENT.RU"

Modern test, measuring and monitoring equipment for the development, manufacture and implementation of new electronic devices and technologies ...

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"ACADEMLINE", JSC, Moscow

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"AKTAKOM"

The registered trademark AKTAKOM unites a wide range of world-class instrumentation. All the best from foreign and domestic manufacturers ...

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"ANALITPRIBOR"

Offers gas analyzers

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"WATSON", JSC, Mytishchi, Moscow region

Instruments and measuring instruments;

http://www.watson.ru/

"DIPOL", NPF, St. Petersburg

http://www.dipaul.ru/

"EuroLab SPb", Ltd., St. Petersburg

Spectral analysis instruments, chromatographs.

http://www.eurolab.ru

"IZME.RU"

http://www.izme.ru/

"INSOVT", JSC

Development and production of gas analyzers

http://www.insovt.ru

"Institute of Information Technologies", Minsk, Belarus

Specializing in the design and manufacture of fiber optics measuring instruments ...

"KIPARIS", Ltd., St. Petersburg

http://www.kiparis.spb.ru/

"CONTINENT", Gomel

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"Control and measuring devices and equipment", Volgograd

http://www.oscilloscop.ru

"Kontur", ITC, OOO, Novosibirsk

http://www.kip.ru/

"KraySibStroy", Ltd., Krasnoyarsk

http://www.kipkr.ru/

"Krismas +" JSC, St. Petersburg

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"KURS", Ltd., St. Petersburg

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"LUMEX", St. Petersburg

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"METTEK"

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"METTLER TOLEDO"

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"MONITORING", STC, St. Petersburg

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"Scientific Instruments" JSC, St. Petersburg

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"NevaLab" JSC, St. Petersburg

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"OWEN", PO, Moscow

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"OCTAVA +", Moscow

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"OPTEK", JSC, St. Petersburg

Develops and manufactures gas analyzers and analytical systems for various purposes for use in ecology, industry and scientific research ...

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"POLYTECHFORM", Moscow

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"Praktik-NTs" JSC, Moscow, Zelenograd

http://www.pnc.ru/

"INSTRUMENTS AND ANALYTICAL TECHNOLOGY"

Devices for chemical analysis.

http://www.zhdanov.ru/

"Sartogosm" JSC, St. Petersburg

http://www.sartogosm.ru

"Special", JSC, Moscow

http://www.special.ru

"TKA"

http://www.tka.spb.ru/

"TST", JSC, St. Petersburg

http://www.tst-spb.ru

"EKOPRIBOR", NPO, Moscow

Offers gas analyzers and gas analysis systems ...

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"ECOTECH", SME, Ukraine

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"EKOTEKHINVEST", NPF, Moscow

http://ecotechinvest.webzone.ru

"Exis" JSC, Moscow, Zelenograd

http://www.eksis.ru/

"ELIX"

http://www.eliks.ru/

"EMI", LLC, St. Petersburg

Production of optical gas analyzers, oil product analyzers.

http://www.igm.spb.ru

"ENERGOTEST" JSC, Moscow

http://www.energotest.ru, http://www.eneffect.ru

HIMMED

Analytical Instruments and Chromatography

e-mail:[email protected]

LITERATURE

1. Geyrovsky Y., Kuta Y., Fundamentals of polarography, trans. from Czech., M., 1965;

2. Galius 3., Theoretical foundations of electrochemical analysis, trans. from Polish., M., 1974;

3. Kaplan B. Ya., Pulse polarography, M., 1978;

4. Brainina X. 3., Neiman E. Ya., Solid-phase reactions in electroanalytical chemistry, M., 1982;

5. Kaplan B. Ya., Pats R. G., Salikhdzhanova R. M.-F., Voltammetry of alternating current, M., 1985.

6. Plambek J. Electrochemical methods of analysis. / Per. from English Moscow: Mir, 1985.496 p.

7. Brief chemical encyclopedia. Moscow: Soviet Encyclopedia, 1964. Volume 1. A – E. 758 s.

8. Classification and nomenclature of electrochemical methods // Zhurn. analyte. chemistry. 1978.Vol. 33, no. 8, pp. 1647-1665.

9. Recommended Terms, Symbols and Definitions for Electroanalytical Chemistry // Pure & Appl. Chem. 1979. Vol. 51. P. 1159-1174.

10. On the use of the concept of "chemical equivalent" and related quantities: Zhurn. analyte. chemistry. 1989. T. 44, no. 4. P. 762–764; Journal. analyte. chemistry. 1982.Vol. 37, no. 5, p. 946; Journal. analyte. chemistry. 1982.Vol. 37, no. 5, p. 947.

11. Neiman E.Ya. Terminology of modern analytical chemistry and its formation // Zh. analyte. chemistry. 1991.Vol. 46, no. 2.P. 393–405.

12. Presentation of the results of chemical analysis (Recommendations IUPAC 1994) // Zhurn. analyte. chemistry. 1998. T. 53. No. 9. P. 999–1008.

13. Compendium of Analytical Nomenclature (Definitive Rules 1997). 3rd ed., IUPAC, Blackwell Science, 1998. 8.1-8.51 (Electrochemical Analysis).

Electrochemical methods of analysis are based on the measurement of potentials, currents and other characteristics during the interaction of an analyte with an electric current.

Electrochemical methods are divided into three groups:

¨ methods based on electrode reactions proceeding in the absence of current (potentiometry);

¨ methods based on electrode reactions proceeding under the influence of current (voltammetry, coulometry, electrogravimetry);

¨ methods based on measurements without an electrode reaction (conductometry - low-frequency titration and oscillometry - high-frequency titration).

According to the methods of application, electrochemical methods are classified into straight based on the direct dependence of the analytical signal on the concentration of the substance, and indirect(setting the equivalence point during titration).

To register the analytical signal, two electrodes are required - indicator and comparison. The electrode, the potential of which depends on the activity of the detected ions, is called indicator... It should quickly and reversibly respond to changes in the concentration of the ions being determined in the solution. An electrode, the potential of which does not depend on the activity of the ions being determined and remains constant, is called reference electrode.

POTENTIOMETRY

Potentiometric method based on the measurement of the electromotive forces of reversible galvanic cells and is used to determine the concentration of ions in a solution.

The method was developed at the end of the last century, after, in 1889, Walter Nernst derived an equation that relates the potential of an electrode to the activity (concentration of substances):

where is the standard electrode potential, V; 0.059 is a constant including the universal gas constant (), absolute temperature and Faraday's constant (); - the number of electrons taking part in the electrode reaction; and - the activity of the oxidized and reduced forms of the substance, respectively.

When a metal plate is immersed in a solution, equilibrium is established at the metal-solution interface

Me 0 ↔ Me n + + nē

and an electrode potential arises. You cannot measure this potential, but you can measure the electromotive force of a galvanic cell.

The investigated galvanic cell consists of two electrodes, which can be immersed in the same solution (cell without transfer) or in two solutions of different composition, having liquid contact with each other (transfer circuit).

The electrode, the potential of which depends on the activity of the detected ions, is called indicator: E = f (c). An electrode whose potential does not depend on the concentration of the ions being determined and remains constant is called reference electrode... It is used to measure the potential of an indicator electrode.

Introduction

The use of electrochemical methods in quantitative analysis is based on the use of dependences of the values of the measured parameters of electrochemical processes (the difference in electrical potentials, current, amount of electricity) on the content of the analyte in the analyzed solution participating in the given electrochemical process. Electrochemical processes are processes that are accompanied by the simultaneous occurrence of chemical reactions and changes in the electrical properties of the system, which in such cases can be called electrochemical system. In analytical practice, an electrochemical system usually contains electrochemical cell, including a vessel with an electrically conductive solution to be analyzed, in which the electrodes are immersed.

Classification of electrochemical methods of analysis

Electrochemical analysis methods are classified in different ways. ... Classification based on the nature of the source of electrical energy in the system. There are two groups of methods. - Methods without imposing external (extraneous) potential. The source of electrical energy is the electrochemical system itself, which is a galvanic cell (galvanic circuit). These methods include potentiometric methods; the electromotive force (EMF) and electrode potentials in such a system depend on the content of the analyte in the solution. - Methods with the imposition of external (extraneous) potential. These methods include:

O conductometric analysis- based on measuring the electrical conductivity of solutions as a function of their concentration;

O voltammetric analysis- based on current measurement as a function of the applied known potential difference and solution concentration;

O coulometric analysis- based on measuring the amount of electricity passing through the solution as a function of its concentration;

O electrogravimetric analysis- based on measuring the mass of the product of an electrochemical reaction.

Classification according to the method of application of electrochemical methods. Distinguish between direct and indirect methods.

- Direct methods. The electrochemical parameter is measured as a known function of the concentration of the solution, and the content of the analyte in the solution is found according to the indication of the corresponding measuring device.

- Indirect methods. Titration methods in which the end of the titration is recorded based on the measurement of the electrical parameters of the system.

In accordance with this classification, there are, for example, direct conductometry and conductometric titration, direct potentiometry and potentiometric titration etc.

This manual contains laboratory work only for the following electrochemical methods:

Direct potentiometry;

Potentiometric titration;

Coulometric titration.

All these methods are pharmacopoeial and are used to control the quality of medicines.

General characteristics of potentiometric analysis

Method principle

Potentiometric analysis (potentiometry) is based on measuring the EMF and electrode potentials as a function of the concentration of the analyzed solution.

If in an electrochemical system - in a galvanic cell - a reaction occurs on the electrodes:

with carry n electrons, then the Nernst equation for the EMF E this reaction has the form:

where is the standard EMF of the reaction (the difference between the standard electrode potentials); R- universal gas constant; T- the absolute temperature at which the reaction proceeds; F- Faraday number; -

activity of reagents - participants in the reaction.

Equation (1) is valid for the EMF of a reversibly working galvanic cell.

For room temperature, equation (1) can be represented in the form:

(2)

Under conditions when the activity of the reagents is approximately equal to their concentration, equation (1) turns into equation (3):

(3)

where are the concentrations of reagents.

For room temperature, this equation can be represented as:

(4)

For potentiometric measurements in an electrochemical cell, two electrodes are used:

. indicator electrode, whose potential depends on the concentration of the determined (potential-determining) substance in the analyzed solution;

. reference electrode, the potential of which under the conditions of the analysis remains constant.

That is why the value of the EMF determined by equations (14) can be calculated as the difference in the real potentials of these two electrodes.

In potentiometry, electrodes of the following types are used: electrodes of the first, second kind, redox, membrane.

First-class electrodes. These are electrodes that are cation-reversible in common with the electrode material. There are three types of electrodes of the first kind:

a) Metal M immersed in a salt solution of the same metal. A reversible reaction occurs on the surface of such electrodes:

The real potential of such an electrode of the first kind depends on the activity metal cations and is described by equations (5-8). In general, for any temperature:

(5)

For room temperature:

(6)

At low concentrations when activity cations

metal is approximately equal to their concentration,

(7)

For room temperature:

(8)

b) Gas electrodes such as a hydrogen electrode, including a standard hydrogen electrode. The potential of a reversibly operating gas hydrogen electrode is determined by the activity of hydrogen ions, i.e. the pH value of the solution, and at room temperature is equal to:

since for a hydrogen electrode the standard potential is taken to be zero , and in accordance with the electrode reaction

the number of electrons participating in this reaction is equal to one: n= 1;

v) Amalgam electrodes, which are an amalgam of a metal immersed in a solution containing cations of the same metal. Potential

The number of such electrodes of the first kind depends on the activity ka-

metal ions in solution and activity a (M) metal in amalgam:

Amalgam electrodes are highly reversible. Type II electrodes are anion reversible. There are the following types of electrodes of the second kind:

A. A metal, the surface of which is covered with a slightly soluble salt of the same metal, immersed in a solution containing the anions that make up this poorly soluble salt. An example is the silver chloride electrode , or calomel electrode ,

A silver chloride electrode consists of a silver wire coated with a salt that is poorly soluble in water and immersed in an aqueous solution of potassium chloride. A reversible reaction occurs on the silver chloride electrode:

The calomel electrode consists of metallic mercury coated with a paste of poorly soluble mercury (I) chloride - calomels, contact

ting with an aqueous solution of potassium chloride. A reversible reaction occurs on the calomel electrode:

The real potential of electrodes of the second kind depends on the activity of anions and for a reversibly working electrode on which the reaction takes place

is described by the Nernst equations (9-12).

Generally at any acceptable temperature T:

. (9)

For room temperature:

For conditions in which the activity of anions is approximately equal to their concentration :

. (11)

For room temperature:

(12)

For example, the real potentials, respectively, of silver chloride and calomel electrodes at room temperature can be represented as:

In the latter case, 2 electrons are involved in the electrode reaction (n= 2) and 2 chloride ions are also formed, therefore the factor at the logarithm is also 0.059.

Electrodes of the second kind of the considered type have high reversibility and are stable in operation, therefore they are often used as reference electrodes capable of stably maintaining a constant potential value;

b) gas electrodes of the second kind, for example, chlorine electrode,Gas electrodes of the second kind in quantitative potential

cyometric analysis is rarely used.

Redox electrodes. They consist of an inert material (platinum, gold, tungsten, titanium, graphite, etc.) immersed in a solution containing oxidized Ox and reduced Red forms of this substance. There are two types of redox electrodes:

1) electrodes, the potential of which does not depend on the activity of hydrogen ions, for example, etc .;

2) electrodes, the potential of which depends on the activity of hydrogen ions, for example, a quinhydrone electrode.

On a redox electrode, the potential of which does not depend on the activity of hydrogen ions, a reversible reaction occurs:

The real potential of such a redox electrode depends on the activity of the oxidized and reduced form of a given substance and for a reversibly working electrode is described, depending on the conditions (by analogy with the above potentials), by the Nernst equations (13-16):

(13) (14) (15) (16)

where all designations are traditional.

If hydrogen ions participate in the electrode reaction, then their activity (concentration) is taken into account in the corresponding Nernst equations for each specific case.

Membrane, or ion-selective, electrodes- electrodes reversible for certain ions (cations or anions) sorbed by a solid or liquid membrane. The real potential of such electrodes depends on the activity of those ions in the solution that are sorbed by the membrane.

Membrane electrodes with a solid membrane contain a very thin membrane, on both sides of which there are different solutions containing the same detectable ions, but with different concentrations: a solution (standard) with a precisely known concentration of the ions to be determined and an analyzed solution with an unknown concentration of the ions to be determined. Due to the different concentration of ions in both solutions, ions on different sides of the membrane are sorbed in different amounts, and the electric charge arising from the sorption of ions on different sides of the membrane is also different. As a result, a membrane potential difference arises.

The determination of ions using membrane ion-selective electrodes is called ionometry.

As mentioned above, during potentiometric measurements, the electrochemical cell includes two electrodes - an indicator

and a reference electrode. The magnitude of the EMF generated in the cell is equal to the potential difference between these two electrodes. Since the potential of the reference electrode remains constant under the conditions of potentiometric determination, the EMF depends only on the potential of the indicator electrode, i.e. from the activity (concentration) of certain ions in the solution. This is the basis for the potentiometric determination of the concentration of a given substance in the analyzed solution.

For potentiometric determination of the concentration of a substance in a solution, both direct potentiometry and potentiometric titration are used, although the second method is used much more often than the first.

Direct potentiometry

Determination of the concentration of a substance in direct potentiometry. It is usually carried out by the method of a calibration graph or by the method of adding a standard.

. Calibration graph method. Prepare a series of 5-7 standard solutions with a known content of the analyte. The concentration of the analyte and the ionic strength in standard solutions should not differ greatly from the concentration and ionic strength of the analyzed solution: under these conditions, the determination errors are reduced. The ionic strength of all solutions is maintained by the constant introduction of an indifferent electrolyte. The reference solutions are sequentially introduced into the electrochemical (potentiometric) cell. Typically, this cell is a glass beaker in which the indicator and reference electrodes are placed.

Measure the EMF of the standard solutions by thoroughly rinsing the electrodes and the glass with distilled water before filling the cell with each standard solution. Based on the data obtained, a calibration graph is plotted in coordinates where with- concentration determined

th substance in the reference solution. Typically, such a graph is a straight line.

Then, the analyzed solution is introduced into the electrochemical cell (after washing the cell with distilled water) and the EMF of the cell is measured. According to the calibration schedule, find , where is the concentration of the analyte in the analyzed solution.

. Standard addition method. A known volume V (X) of the analyzed solution with concentration is introduced into the electrochemical cell and the EMF of the cell is measured. Then, in the same solution, add precisely measured small the volume of a standard solution with a known, up to

with a sufficiently high concentration of the analyte and again determine the EMF of the cell.

Calculate the concentration of the analyte in the analyzed solution according to the formula (17):

(17)

where - difference between two measured values of EMF; - the number of electrons involved in the electrode reaction.

Application of direct potentiometry. The method is used to determine the concentration of hydrogen ions (pH of solutions), anions, metal ions (ionometry).

When using direct potentiometry, the selection of a suitable indicator electrode and accurate measurement of the equilibrium potential play an important role.

When determining the pH of solutions, electrodes are used as indicator, the potential of which depends on the concentration of hydrogen ions: glass, hydrogen, quinhydrone and some others. Most often, a membrane glass electrode, reversible for hydrogen ions, is used. The potential of such a glass electrode is determined by the concentration of hydrogen ions; therefore, the EMF of a circuit including a glass electrode as an indicator is described at room temperature by the equation:

where is the constant K depends on the material of the membrane, the nature of the reference electrode.

The glass electrode allows you to determine pH in the range of pH 0-10 (more often in the range of pH 2-10) and has high reversibility and stability in operation.

Quinhydron electrode, often used earlier - a redox electrode, the potential of which depends on the concentration of hydrogen ions. It is a platinum wire immersed in an acid solution (usually HC1) saturated with quinhydrone, an equimolecular compound of quinone with hydroquinone (dark green powder, slightly soluble in water). Schematic designation of a quinhydrone electrode:

A redox reaction takes place on the quinhydrone electrode:

The potential of a quinhydrone electrode at room temperature is described by the formula:

The quinhydron electrode makes it possible to measure the pH of solutions in the range of pH 0-8.5. At pH< 0 хингидрон гидролитически расщепляется; при рН >8.5 hydroquinone, which is a weak acid, enters into a neutralization reaction.

The quinhydrone electrode must not be used in the presence of strong oxidizing and reducing agents.

Membrane ion-selective electrodes are used in ionometry as indicator for the determination of various cations

Etc.) and anions ,

and etc.).

The advantages of direct potentiometry include the simplicity and speed of measurements. Measurements require small volumes of solutions.

Potentiometric titration

Potentiometric titration is a method for determining the volume of titrant spent on titrating an analyte in an analyzed solution by measuring the EMF (during titration) using a galvanic circuit composed of an indicator electrode and a reference electrode. In potentiometric titration, the analyzed solution in the electrochemical cell is titrated with a suitable titrant, fixing the end of the titration by a sharp change in the EMF of the measured circuit - the potential of the indicator electrode, which depends on the concentration of the corresponding ions and changes sharply at the equivalence point.

Measure the change in the potential of the indicator electrode during the titration, depending on the volume of the added titrant. Based on the data obtained, a potentiometric titration curve is plotted and the volume of consumed titrant in TE is determined from this curve.

Potentiometric titration does not require the use of indicators that change color near the TE.

The electrode pair (reference electrode and indicator electrode) is made up so that the potential of the indicator electrode depends on the concentration of ions involved or formed in the reaction proceeding during the titration. The potential of the reference electrode must remain constant during titration. Both electrodes are installed directly in the electrochemical cell or placed in separate vessels with conductive solutions (the indicator electrode is in the analyzed solution), which are connected by an electrolytic bridge filled with an indifferent electrolyte.

The titrant is added in equal portions, each time measuring the potential difference. At the end of the titration (near the TE), the titrant is added dropwise, also measuring the potential difference after the addition of the next portion of the titrant.

The potential difference between the electrodes is measured using high resistance potentiometers.

Potentiometric titration curves

Potentiometric titration curve is a graphical representation of the change in the EMF of an electrochemical cell depending on the volume of added titrant.

Potentiometric titration curves are plotted in different coordinates:

Titration curves in coordinates , sometimes such curves are called integral titration curves;

Differential titration curves - in coordinates

Titration curves according to the Gran method - in coordinates

where is the EMF of the potentiometric cell, - the volume of the added

th titrant, is the change in potential corresponding to the addition of the titrant.

In fig. 3-8 show schematically different types of potentiometric titration curves.

Based on the constructed titration curves, the titrant volume is determined

in the TE, as shown in Fig. 3-8. Titrant volume added to TE, you can determine

not only graphically, but also by calculation using the formula (18):

where is the volume of added titrant corresponding to the last measurement before TE; is the volume of added titrant corresponding to the first measurement after TE;

Rice. 3-8. Types of potentiometric titration curves (E - measured EMF, - volume of added titrant, - the volume of the titrant, at-

added at the equivalence point): a - titration curve in coordinates ; b, c - differential titration curves; d - titration curve according to the Gran's method

Table 3-9 shows the results of determinations and calculations in potentiometric titration as an example (pharmacopoeial).

Let us calculate by the formula (18) the value V(TE) using the data table. 3-9. Obviously, the maximum value = 1000. Therefore, = 5.20 and = 5.30; = 720,. = -450. Hence:

Table 3-9. An example of processing the results of potentiometric titration

The use of potentiometric titration. The method is universal, it can be used to indicate the end of the titration in all types of titrations: acid-base, redox, compleximetric, precipitation, when titrating in non-aqueous media. Glass, mercury, ion-selective, platinum, silver electrodes are used as indicator electrodes, and calomel, silver chloride, and glass electrodes are used as reference electrodes.

The method is highly accurate and highly sensitive; allows titration in turbid, colored, non-aqueous media, separate determination of mixture components in one analyzed solution, for example, separate determination of chloride and iodide ions during argentometric titration.

Many medicinal substances are analyzed by potentiometric titration methods, for example, ascorbic acid, sulfa drugs, barbiturates, alkaloids, etc.

Assignment for self-preparation for laboratory studies on the topic "Potentiometric Analysis"

The purpose of studying the topic

Based on knowledge of the theory of potentiometric analysis and the development of practical skills, learn to reasonably choose and practically apply the methods of direct potentiometry and potentiometric titration for the quantitative determination of a substance; be able to conduct a statistical assessment of the results of potentiometric analysis.

Target tasks

1. Learn to quantitatively determine the content of fluoride ion in solution by direct potentiometry using a fluoride-selective electrode.

2. To learn how to quantitatively determine the mass fraction of novocaine in the preparation by potentiometric titration.

Two laboratory sessions are allocated for the study of the topic. In one lesson, students perform the first laboratory work and solve typical computational problems in the main sections of potentiometric analysis; in another lesson, students perform a second laboratory work. The sequence of the lessons does not really matter.

Bibliography

1. Tutorial. - Book 2, chapter 10. - pp. 447-457; 493-507; 510-511.

2.Kharitonov Yu.Ya. Grigorieva V.Yu. Examples and tasks in analytical chemistry. - M .: GEOTAR-Media, 2007. - P. 214-225; 245-259; 264-271.

3. Lectures on the topic: "Potentiometric analysis".

4.Efremenko O.A. Potentiometric analysis. - M .: MMA im. THEM. Sechenov, 1998.

You need to know for the lesson

1. The principle of methods of potentiometric analysis. Nernst equation.

2. Varieties of methods of potentiometric analysis.

3. Installation diagram for direct potentiometry.

4. Indicator electrodes and reference electrodes used in direct potentiometry.

5. The essence of determining the concentration of a substance by the method of direct potentiometry using a calibration graph.

6. The essence of determining the content of fluoride ion in solution by direct potentiometry using a fluoride-selective electrode.

For the lesson you need to be able to

1. Calculate the weight of the sample for the preparation of a standard solution of the substance.

2. Prepare standard solutions by dilution method.

3. Build calibration curves and use them for quantitative determination of a substance.

Self-test questions

1. What is the principle behind the direct potentiometry method?

3. What electrochemical parameter is measured when determining a substance by the method of direct potentiometry?

4. Give a diagram of the setup for determining a substance by the direct potentiometry method.

5. What electrodes are called indicator? What are the most commonly used indicator ion-selective electrodes?

6. What electrodes are called reference electrodes? Which reference electrode is accepted as the international standard? How does it work? What are the most commonly used reference electrodes? How do they work:

a) saturated calomel electrode;

b) saturated silver chloride electrode?

7. What is the essence of potentiometric determination of a substance by the method of a calibration graph?

8. Name the range of the determined concentrations and the percentage (relative) error in the determination of the substance by the direct potentiometry method.

9. What is the principle behind the determination of fluoride ion by direct potentiometry? List the main stages of the analysis.

Laboratory work "Determination of the content of fluoride ion in solution using a fluoride-selective electrode"

purpose of work

Learn to apply the method of direct potentiometry using an ion-selective electrode for the quantitative determination of a substance using a calibration graph.

Target tasks

1. Preparation of a standard solution of sodium fluoride, the concentration of which is exactly equal to the specified one.

2. Preparation by dilution method of a series of standard solutions of sodium fluoride, in composition and ionic strength close to the analyzed solution.

3. Measurement of the electromotive force (EMF) of a galvanic cell composed of an indicator fluoride selective electrode and a silver chloride reference electrode as a function of the concentration of fluoride ion.

4. Construction of a calibration graph in coordinates: "EMF - an indicator of the concentration of fluoride ion".

5. Determination of the content of fluoride ion in the analyzed solution using a calibration graph.

Material security

Reagents

1. Sodium fluoride, chemically pure.

2. Acetate buffer solution, pH ~ 6.

3. Distilled water. Glassware

1. Volumetric flask, 100 ml - 1 pc.

2. Volumetric flask, 50 ml - 6 pcs.

3. Measuring pipette, 5 ml - 1 pc.

4. Beaker chemical for 200-250 ml - 1 pc.

5. Beaker chemical 50 ml - 2 pcs.

6. Byux - 1 pc.

7. Funnel - 1 pc.

8. Glass stick - 1 pc.

9. Wash bottle for 250 or 500 ml - 1 pc.

Devices

2. Indicator electrode, fluoride-selective. Before operation, the fluoride electrode is kept in 0.01 mol / L sodium fluoride solution for 1-2 hours.

3. Reference electrode, auxiliary laboratory silver chloride EVL-IMZ or similar. Before use, the silver chloride electrode is filled through the side hole with a concentrated, but unsaturated, approximately 3 mol / l solution of potassium chloride. When a saturated solution of potassium chloride is used, salt crystallization is possible in the immediate vicinity of the contact zone of the electrode with the measured solution, which prevents the passage of current and leads to irreproducible readings of the measuring device. After filling the electrode with 3 mol / L potassium chloride solution, the side hole is closed with a rubber stopper, the electrode is immersed in a potassium chloride solution of the same concentration and kept in this solution for ~ 48 h. During operation, the plug from the side hole of the electrode must be removed. The flow rate of the potassium chloride solution through the electrolytic switch of the electrode at a temperature of 20 ± 5 ° C is 0.3-3.5 ml / day.

4. A stand for fixing two electrodes.

5. Magnetic stirrer.

Other materials

1. Strips of filter paper 3 5 cm.

2. Millimeter paper 912 cm.

3. Ruler.

The essence of the work

The determination of the fluoride ion by the direct potentiometry method is based on measuring the electromotive force of a galvanic cell, in which a fluoride-selective electrode serves as an indicator electrode, and a silver chloride or calomel electrode as a reference electrode, as a function of the concentration of fluoride ions in solution.

The sensitive part of the fluoride electrode (Fig. 3-9) is a membrane made of a single crystal of lanthanum (III) fluoride, activated with europium (II).

Rice. 3-9. Diagram of the device of a fluoride-selective electrode: 1 - single crystal membrane 2 - internal half-cell (usually silver chloride -

ny); 3 - internal solution with constant ion activity (0.01 mol / l and imol / l); 4 - electrode body; 5 - wire for connecting the electrode to the measuring device

The equilibrium potential of the fluoride electrode in accordance with the Nernst equation for anion-selective electrodes depends on the activity (concentration) of the fluoride ion in the solution:

(19) or at 25 ° C:

(20)

where is the standard potential of the fluoride electrode, V; -

respectively, activity, activity coefficient, molar concentration of fluoride ion in solution.

The first term on the right-hand side of equation (20) is a constant value. For solutions with approximately the same ionic strength, the activity coefficient of the fluoride ion, and hence the second term on the right-hand side of Eq. (20), is also a constant. Then the Nernst equation can be represented as:

E= const - 0.0591gc (F -) = const + 0.059pF, (21)

where pF = -1gc (F -) is an indicator of the concentration of fluoride ion in the solution.

Thus, for constant ionic strength solutions, the equilibrium potential of the fluoride electrode is linearly dependent on the concentration of the fluoride ion. The existence of such a dependence makes it possible to determine the concentration of fluoride ion using a calibration graph, which is plotted in coordinates for a series of standard solutions of sodium fluoride, in composition and ionic strength close to the analyzed solution.

The fluoride electrode is used in the pH range of 5-9, since at pH< 5 наблюдается неполная ионизация или образование and at pH> 9 - the interaction of the electrode material with hydroxydion:

To maintain a constant pH value and create a constant ionic strength in standard and analyzed solutions, a buffer solution (for example, acetate or citrate) is usually used. When analyzing solutions with a complex salt composition, the buffer solution also serves to eliminate the interfering influence of extraneous cations by binding them into stable acetate, citrate, or other complex compounds. For the same purpose, additional complexing reagents (for example, EDTA) are introduced into the buffer solution.

The selectivity of determination with a fluoride electrode is very high; only hydroxide ions interfere and those few cations that form more stable complex compounds with the fluoride ion than with the components of the buffer solution

The range of determined concentrations of fluoride ion is very wide: from 10 -6 to 1 mol / l; in this case, the percentage determination error is ± 2%.

The fluoride selective electrode is widely used in the analysis of various objects: drinking water, pharmaceuticals, biological materials, environmental pollution control, etc.

Since in this work, sodium fluoride solutions that do not contain foreign ions are analyzed, the buffer solution can be omitted. In this case, a slight deviation of the calibration curve from the linear dependence should be expected, since in standard solutions with an increase in the concentration of the fluoride ion, the ionic strength increases, and the activity coefficient of the fluoride ion does not remain constant.

Work order

1. (see Appendix 1).

2. Acquaintance with the purpose, principle of operation and the "Operating Instructions for the EV-74 universal ion meter" (or a similar device) (see Appendices 2, 3).

ATTENTION! This work provides for the use of an EV-74 type ionomer. When using devices of a different type, it is necessary to give an additional description of them.

3.1. A galvanic cell is assembled from an indicator fluoride selective electrode and a silver chloride reference electrode.

ATTENTION! When working with ion-selective electrodes, care must be taken not to damage the working surface of the electrode - the membrane, which should be smooth, free from scratches and deposits.

Before installation, shake the fluoride electrode vigorously like a medical thermometer, holding it upright with the membrane facing down. This is done in order to remove air bubbles that are invisible from the outside, which can form between the surface of the membrane and the internal solution of the electrode (see Fig. 3-9) and lead to instability of the meter readings.

The fluoride electrode is fixed in a stand next to the reference electrode.

ATTENTION! Holders for attaching electrodes to the tripod are usually pre-installed properly; it is not recommended to change their position. In order to fix the fluoride electrode or change the solution in the cell, you must first carefully remove the magnetic stirrer from under the cell.

When fixing, the fluoride electrode is brought into the tripod leg from below so that its lower end is level with the lower end of the reference electrode. The electrode is connected to the ionomer through the "Measure" socket located on the rear panel of the device (Appendix 3, p. 1.1). The reference electrode must be connected to the ionomer through the "Aux." Socket.

The electrodes are repeatedly washed with distilled water from a washing bottle over a glass with a capacity of 200-250 ml, after which a 50 ml glass with distilled water is placed under the electrodes, which is installed in the center of the table of a magnetic stirrer. Correctly attached electrodes should not touch the walls and bottom

glasses, as well as a magnetic rod, which is used later to stir the solution.

3.2. The ionomer is connected to the network under the supervision of a teacher, guided by the instruction manual for the device (Appendix 3, items 1.2-1.7). Allow the appliance to warm up for 30 minutes.

4. Preparation of a standard 0.1000 mol / L sodium fluoride solution. Calculate, with an accuracy of 0.0001 g, the mass of a sample of sodium fluoride required for the preparation of 100 ml of a 0.1000 mol / l solution according to the formula:

where c, - respectively, molar concentration (mol / l) and volume (l) of a standard solution of sodium fluoride; - molar mass of sodium fluoride, g / mol.

First, a clean and dry weighing bottle is weighed on an analytical balance with an accuracy of ± 0.0002 g, and then a sample of reagent grade is weighed in this weighing bottle. sodium fluoride, the mass of which must be accurately calculated.

The weighed portion is quantitatively transferred into a volumetric flask with a capacity of 100 ml through a dry funnel, rinsing salt particles from the walls of the bottle and funnel with an acetate buffer solution (pH ~ 6). The solution from the bottle is poured into a flask on a glass rod, leaning it against the edge of the bottle. Complete dissolution of the salt is achieved, after which the volume of the solution is brought up to the mark of the flask with the buffer solution. The contents of the flask are mixed.

5. Preparation of a series of constant ionic strength sodium fluoride standard solutions. A series of standard solutions with a fluoride ion concentration equal to 10 -2, 10 -3, 10 -4, 10 -5, and 10 -6 mol / l are prepared in 50 ml volumetric flasks from a standard 0.1000 mol / l sodium solution fluoride by successive dilution with buffer solution.

So, to prepare a 10 -2 mol / L solution, 5 ml of 0.1000 mol / L sodium fluoride solution is pipetted into a 50 ml volumetric flask with a pipette, after rinsing the pipette with a small amount of this solution 2-3 times, the volume of the solution is adjusted to the mark with the buffer solution. , the contents of the flask are stirred. In the same way, a 10 -3 mol / l solution is prepared from a 10 -2 mol / l solution, etc. up to 10 -6 mol / l sodium fluoride solution.

6. Measurement of the electromotive force of a galvanic cell as a function of the concentration of fluoride ion. In a beaker with a capacity of 50 ml, the prepared standard solutions are sequentially placed on

sodium fluoride, starting with the most diluted one, after rinsing the glass with the measured solution 2-3 times. Carefully dry the surface of the fluoride and silver chloride electrodes with filter paper, after which the electrodes are immersed in the solution to be measured, the magnetic rod is lowered and the cell is installed in the center of the magnetic stirrer table. If instructed by the teacher, open the side hole of the silver chloride electrode by removing the rubber stopper from it. Turn on the magnetic stirrer and measure the EMF of the element (positive potential of the fluoride electrode) using an EV-74 ion meter in a narrow measurement range - 14 as indicated in Appendix 3, p. 2.1-2.5. The measurement results are entered in table. 3-10.

Table 3-10. Results of measuring the electromotive force of a galvanic cell as a function of the concentration of fluoride ion

7. Construction of a calibration graph. According to the table. 3-10, a calibration graph is plotted on graph paper, plotting the concentration of fluoride ion along the abscissa, and the EMF of the element in millivolts on the ordinate (E, mV). If dependence (21) is satisfied, then a straight line is obtained, the slope of which to the abscissa axis is 59 ± 2 mV (at 25 ° C). The graph is glued to the laboratory journal.

8. Determination of the content of fluoride ion in the analyzed solution using a calibration graph. The analyzed solution containing fluoride ion is obtained from the teacher in a 50 ml volumetric flask. The volume of the solution is made up to the mark with an acetate buffer solution. The contents of the flask are stirred and the EMF of an element composed of fluoride and silver chloride electrodes is measured in the resulting solution.

At the end of the measurements, close the hole of the silver chloride electrode with a rubber stopper and turn off the device, as indicated in Appendix 3, clause 2.6.

According to the calibration graph, an indicator of the concentration of the fluoride ion is found, corresponding to the EMF of the element in the analyzed solution, then the molar concentration is determined and the content of the fluoride ion in the solution is calculated by the formula:

where - titer of fluoride ion in the analyzed solution, g / ml; - molar

ny concentration of fluoride ion, found using the calibration graph, mol / l; - molar mass of fluoride ion, g / mol.

The titer is calculated with an accuracy of three significant figures.

9. Determination of the content of fluoride ion in the analyzed solution according to the equation of the calibration graph. The pF value for the analyzed solution can be found from the equation of the calibration graph, which seems to be more accurate than using the calibration graph. This equation is:

where chains with test solution ;chains at = 0 -

line cut by the ordinate ;- tangent of an angle

the slope of the straight line to the abscissa axis:

where n- the number of standard solutions. Thus:

Having determined according to the schedule and calculated count on

according to the formula:

Then the molar concentration is determined and the content of the fluoride ion in the solution is calculated according to the formula indicated above.

Control questions

1. Name the components of a galvanic cell used to determine the concentration (activity) of a fluoride ion in a solution by direct potentiometry.

2. What is the mathematical relationship underlying the determination of the concentration (activity) of the fluoride ion in the solution by the direct potentiometry method?

3. Describe the design of the fluoride-selective electrode. What factors determine its potential?

4. Why is it necessary to create the same ionic strength in the analyzed and standard solutions when determining the concentration of fluoride ion by the method of direct potentiometry?

5. What is the optimal pH range for fluoride ion determination using a fluoride selective electrode?

6. How is the optimal pH value and constant ionic strength maintained during the determination of fluoride ion in solutions with a complex salt composition?

7. What ions interfere with the determination of fluoride ion in solution using a fluoride-selective electrode? How is their interfering influence eliminated?

8. List the main stages of determining the concentration of fluoride ion in a solution by the potentiometric method using a calibration curve.

9. In what coordinates is the calibration graph plotted when determining the concentration of the fluoride ion by the direct potentiometry method?

10. What should be equal to the slope (tangent of the slope) of the calibration graph plotted in coordinates , for standard solutions of sodium fluoride with the same ionic strength at 25 ° C?

11. How to calculate the concentration of fluoride-ion in solution using the data of the calibration graph, built in coordinates , if the EMF of the element in the analyzed solution is known?

12. How to prepare a standard solution from a crystalline substance of sodium fluoride with a concentration exactly equal to the specified one, for example, 0.1000 mol / l?

13. How to prepare a standard sodium fluoride solution from a more concentrated solution?

14. Name the range of the determined concentrations and the percentage error in determining the fluoride ion using a fluoride-selective electrode by the method of the calibration curve.

15. Name the areas of application of the fluoride-selective electrode.

Lesson 2. Potentiometric titration

You need to know for the lesson

1. The principle of methods of potentiometric analysis. Nernst equation. Varieties of methods of potentiometric analysis.

2. Schematic diagram of a potentiometric titration setup.

3. Indicator electrodes used in potentiometric titration, depending on the type of titration reaction; reference electrodes.

4. Methods for indicating the equivalence point in potentiometric titration.

5. Advantages of potentiometric titration over titrimetric analysis with visual indication of the equivalence point.

6. The essence of the determination of novocaine by potentiometric titration.

For the lesson you need to be able to

1. Prepare the solution to be analyzed by dissolving a weighed portion of the test sample with a precisely known mass.

2. Calculate the mass fraction of the substance in the analyzed sample based on the titration results.

3. Write the equation of the reaction taking place during titration.

Self-test questions

1. What is the principle behind the potentiometric titration method?

2. What equation expresses the dependence of the electrode potential on the concentration (activity) of the potential-determining components in the solution?

3. What electrochemical parameter is measured when determining a substance by potentiometric titration?

4. Give a definition to the terms "indicator electrode", "reference electrode".

5. What is the reason for the sharp change in the electromotive force of the galvanic cell (potential of the indicator electrode) in the titrated solution near the equivalence point?

6. Name the known methods for determining the equivalence point based on potentiometric titration data.

7. For what types of chemical reactions can the potentiometric titration method be used? What electrodes are used for this?

8. What is the advantage of potentiometric titration over titrimetric analysis with visual indication of the equivalence point?

9. Name the range of the determined concentrations and the percentage (relative) error in the determination of the substance by potentiometric titration.

10. What is the chemical reaction underlying the determination of a substance containing a primary aromatic amino group by nitrite titration? What are the conditions for holding it? Applied indicators?

11. What is the principle underlying the determination of novocaine by potentiometric titration? List the main stages of the analysis.

Laboratory work "Determination of the mass fraction of novocaine in the preparation"

purpose of work

Learn to use the potentiometric titration method for the quantitative determination of a substance.

Target tasks

1. Approximate potentiometric titration of novocaine with sodium nitrite solution.

2. Precise potentiometric titration of novocaine with sodium nitrite solution.

3. Finding the end point of potentiometric titration.

4. Calculation of the mass fraction of novocaine in the preparation.

Material security

Reagents

1. Sodium nitrite, standard ~ 0.1 mol / l solution.

2. Novocaine powder.

3. Potassium bromide, powder.

4. Concentrated hydrochloric acid (= 1.17 g / ml).

5. Distilled water. Glassware

1. Volumetric flask, 100 ml.

2. Volumetric flask, 20 ml.

3. 25 ml burette.

4. Measuring cylinder for 20 ml.

5. Measuring cylinder for 100 ml.

6. Beaker for titration, 150 ml.

7. Byux.

8. Funnel.

9. 250 ml or 500 ml wash bottle.

Devices

1. Universal ionomer EV-74 or similar.

2. Platinum indicator electrode ETPL-01 M or similar.

3. Reference electrode, auxiliary laboratory silver chloride EVL-1MZ or similar.

Preparation of the silver chloride electrode for use - see above, previous laboratory work.

4. A stand for fixing two electrodes and a burette.

5. Magnetic stirrer.

6. Analytical balance with weights.

7. Weights technochemical with weights.

Other materials: see "Material Support" in previous work.

The essence of the work

Potentiometric titration is based on the indication of the equivalence point by a sharp change (jump) in the potential of the indicator electrode during the titration.

To determine novocaine, a substance containing a primary aromatic amino group, a nitritometric titration method is used, according to which novocaine is titrated with a standard 0.1 mol / L sodium nitrite solution in a hydrochloric acid medium in the presence of potassium bromide (accelerates the reaction) at a temperature not higher than 18-20 ° C. Under such conditions, the titration reaction proceeds quantitatively and rather quickly:

The progress of the diazotization reaction is monitored using a platinum indicator electrode, which, together with a suitable reference electrode (silver chloride or calomel), is immersed in the solution to be titrated, and the electromotive force is measured element depending on

the difference from the volume of the added titrant

The potential of the indicator electrode according to the Nernst equation depends on the concentration (activity) of the substances participating in the titration reaction. Near the point of equivalence (TE), the concentration of potential-determining substances changes sharply, which is accompanied by a sharp change (jump) in the potential of the indicator electrode. The EMF of an element is determined by the potential difference between the indicator electrode and the reference electrode. Since the potential of the reference electrode remains constant, a jump in the potential of the indicator electrode causes a sharp change in the EMF of the element, which indicates the achievement of TE. For greater accuracy of the TE determination, the titrant is added dropwise at the end of the titration.

Graphical methods, usually used to find TE, in this case is hardly advisable to use, since the titration curve plotted in coordinates , is asymmetric with respect to TE; it is rather difficult to establish TE with a sufficiently high accuracy.

The percentage error in the determination of novocaine in the preparation by potentiometric titration does not exceed 0.5%.

Similar to the determination of novocaine by potentiometric titration, many other organic compounds and drugs containing a primary aromatic amino group can be determined, for example, sulfacil, norsulfazole, derivatives of n-aminobenzoic acid, etc.

Note. The diazotization reaction is slow. Various factors affect the rate of its flow. An increase in acidity leads to a decrease in the reaction rate; therefore, during titration, try to avoid a large excess of hydrochloric acid. To accelerate the reaction, potassium bromide is introduced into the reaction mixture. Temperature has the usual effect

on the reaction rate: an increase in temperature by 10 ° C leads to an increase in the rate of about 2 times. However, titration, as a rule, is carried out at a temperature not higher than 18-20 ° C, and in many cases even lower, when the reaction mixture is cooled to 0-10 ° C, since the diazo compounds formed as a result of the reaction are unstable and decompose at a higher temperature.

Titration using the diazotization reaction is carried out slowly: first at a rate of 1–2 ml / min, and at the end of titration — 0.05 ml / min.

Work order

ATTENTION! This work provides for the use of a universal ionomer EV-74. When using devices of a different type, it is necessary to additionally give their description in the laboratory methodological instructions.

1. Acquaintance with the "Safety instructions for working with electrical appliances"(see Appendix 1).

2. Acquaintance with the purpose, principle of operation and "Operating instructions for the universal ion meter EV-74"(see Appendices 2, 3) or a similar device.

3. Preparation of the ionomer for measurements.

3.1. A galvanic cell is assembled from a platinum indicator electrode and a silver chloride reference electrode.

The platinum electrode is fixed in a stand next to the reference electrode.

ATTENTION! Holders designed for attaching electrodes and burettes to the tripod are usually pre-installed properly. It is not recommended to change their position. In order to fix the platinum electrode or replace the solution in the cell, first carefully remove the magnetic stirrer from under the cell.

For fixing, the platinum electrode is brought into the tripod leg from below so that its lower end is slightly higher (by about 0.5 cm) than the lower end of the reference electrode. The indicator electrode is connected to the ionomer through the "Measure" socket located on the rear panel of the device (see Appendix 3, p. 1.1). The reference electrode must be connected to the ionomer through the "Aux." Socket.

The electrodes are repeatedly washed with distilled water from a washing bottle over a 200-250 ml beaker, after which a 150 ml beaker with distilled water is placed under the electrodes, which is installed in the center of the magnetic stirrer table. Correctly fixed electrodes should not touch the walls and bottom of the glass, as well as the magnetic rod used later to stir the solution.

3.2. The ionomer is connected to the network under the supervision of a teacher, guided by the instruction manual for the device (Appendix 3, p. 1.2-1.7). Allow the appliance to warm up for 30 minutes.

4. Preparation of the analyzed solution of novocaine. Prepare about 0.05 mol / l novocaine solution in 2 mol / l hydrochloric acid solution. To do this, about 0.9 g of the drug (the sample is weighed in a weighing bottle on an analytical balance with an accuracy of ± 0.0002 g) is placed in a 100 ml volumetric flask, 20-30 ml of distilled water, 16.6 ml of concentrated hydrochloric acid solution ( = 1.17 g / ml). The mixture is stirred until the drug is completely dissolved, the volume of the solution is brought to the mark with distilled water, the contents of the flask are stirred.

5. Indicative titration. In a glass with a capacity of 150 ml with a pipette, place 20 ml of the analyzed solution of novocaine, add 60 ml of distilled water using a cylinder and about 2 g of potassium bromide. Electrodes - indicator platinum and auxiliary silver chloride - are immersed in the titrated solution, the magnetic rod is lowered and the cell is installed in the center of the magnetic stirrer table. If instructed by the teacher, open the side hole of the silver chloride electrode by removing the rubber stopper from it. A 25 ml burette is filled with a standard 0.1 mol / L sodium nitrite solution and fixed in a stand so that the lower end of the burette is lowered into the beaker 1–2 cm below its edge. Turn on the magnetic stirrer. Stirring is not stopped during the entire titration process.

The device is switched on in the millivoltmeter mode to measure positive potentials (+ mV). In an approximate titration, the EMF of the system is measured over a wide range (-119) as indicated in Appendix 3, p. 2.1-2.5, the titrant solution is added in portions of 1 ml, each time measuring the EMF of the system after the reading of the device assumes a steady value.

A sharp change in the EMF (titration jump) is observed, and then another 5-7 ml of the titrant is added in portions of 1 ml and the measured value changes insignificantly. At the end of the titration, turn off the magnetic stirrer. The measurement results are entered in table. 3-11.

Based on the results of the approximate titration, the titrant volume is established, after the addition of which a titration jump is observed. This volume is considered to be close to the endpoint titration (CTT) volume.

In the table. 3-11 in the example, the volume of titrant consumed for approximate titration is 11 ml.

Table 3-11. Indicative titration (example)

Based on the results of the approximate titration, a titration curve is plotted in coordinates. The asymmetric nature of the curve is noted, which makes it difficult to determine the CTT graphically with appropriate accuracy.

6. Accurate titration. A new portion of the analyzed solution of novocaine, distilled water, potassium bromide in the same quantities as in the approximate titration are placed in a clean 150 ml beaker. The electrodes, previously washed with distilled water, are immersed in the solution, the magnetic bar is lowered and the magnetic stirrer is turned on. For accurate titration, the EMF measurement is carried out over a narrow range (49) as indicated in Appendix 3, clause 2.5.

First, to the titrated solution at a rate of 1 ml / min, add such a volume of titrant, which should be 1 ml less than the volume spent on approximate titration, after which the EMF of the element is measured. In the example shown, the volume of titrant added is: 11 - 1 = 10 ml.

Then the titrant is added in portions of 2 drops, each time measuring the EMF after the reading of the device assumes a steady-state value. A sharp change in the EMF (titration jump) is observed, and then titration is continued in portions of 2 drops and a decrease and a small change are confirmed. At the end of the titration, the total volume of the added titrant is noted with an accuracy of hundredths of a milliliter.

Turn off the magnetic stirrer. The titration results are entered in table. 3-12.

Accurate titration is carried out at least three times. At the end of the measurements, close the hole of the silver chloride electrode with a rubber stopper and turn off the device, as indicated in Appendix 3, clause 2.6.

7. Calculation of the analysis result. Based on the exact titration data, first the volume of one drop is calculated and then the volume of the titrant corresponding to by the formulas:

where is the volume of the titrant, after the addition of which the titration is continued dropwise, ml; is the volume of the titrant at the end of the titration, ml; n- the total number of titrant drops added; - the number of titrant drops added before the titration jump appeared; - the number of drops that make up the portion of the titrant solution that caused the titration jump.

Table 3-12. Accurate titration (example)

Example. Calculation according to the table. 3-12.

The volume of titrant consumed for titration is determined for each i-th titration.

Mass fraction (percentage) of novocaine in the preparation calculate

melt with an accuracy of hundredths of a percent according to the formula:

where with- molar concentration of the titrant: standard sodium nitrite solution, mol / l; - the volume of the titrant spent on the i-th exact titration, ml;

The volume of an aliquot of novocaine solution, ml; - the total volume of the analyzed solution of novocaine, ml; M- molar mass of novocaine, equal to 272.78 g / mol; m- weight of a sample of a preparation containing novocaine, g.

The obtained values of the mass fraction of novocaine in the preparation are processed by the method of mathematical statistics, presenting the analysis result in the form of a confidence interval for a confidence level of 0.95.

Control questions

1. What is the principle of determining novocaine by potentiometric titration?

2. What is the chemical reaction underlying the determination of novocaine by potentiometric titration?

3.What electrodes can be used to monitor the progress of the diazotization reaction during titration of novocaine with sodium nitrite solution?

4. What caused the jump in EMF (jump in the potential of the indicator electrode) in the region of the equivalence point during titration of novocaine with sodium nitrite solution?

5. Under what conditions does the diazotization reaction (with the participation of novocaine) proceed quantitatively and quickly enough?

6. At what rate is the potentiometric titration of novocaine carried out with sodium nitrite solution?

7. What is the form of the titration curve of novocaine with sodium nitrite solution, plotted in the coordinates "EMF - titrant volume"?

8. Is it advisable to use graphic methods for determining the equivalence point in potentiometric titration of novocaine?

10. What is the percentage (relative) error in the determination of novocaine in the preparation by potentiometric titration?

11. What are the advantages of the potentiometric method of indicating the equivalence point in comparison with the visual one in the determination of novocaine by nitrite titration?

12. What substances can be determined by potentiometric titration by analogy with the determination of novocaine?

Annex 1

Safety instructions for working with electrical appliances

Work with ungrounded appliances;

Leave the turned on device unattended;

Relocating the included device;

Work near open current-carrying parts of the device;

Switch the device on and off with wet hands.

2. In the event of a power outage, turn off the appliance immediately.

3. In case of fire of wires or an electrical appliance, it is necessary to immediately de-energize them and extinguish the fire with a dry fire extinguisher, blankets of asbestos, sand, but not with water.

Appendix 2

Purpose and principle of operation of the universal ionomer EV-74

1. Purpose of the device

Universal ionometer EV-74 is designed to determine, complete with ion-selective electrodes, the activity (activity indicator - pX) of singly and doubly charged ions (for example, , and others), as well as for measuring redox potentials (electromotive force) -corresponding electrode systems in aqueous solutions of electrolytes.

The ionomer can also be used as a high resistance millivoltmeter.

2. The principle of operation of the device

The work of the ionomer is based on converting the electromotive force of the electrode system into a direct current proportional to the measured value. The conversion is carried out using a high-resistance autocompensating type converter.

The electromotive force of the electrode system is compared to the opposite voltage drop across the precision resistance R, through which the amplifier current flows The voltage is applied to the amplifier input:

With a sufficiently large gain, the voltage differs little from the electromotive force, and due to this, the current flowing through the electrodes during the measurement is very small, and the current flowing through the resistance R, proportional to the electromotive force of the electrode system:

By measuring the current with a microammeter A, it is possible to determine as well in the test solution.

Appendix 3

Operating instructions for the universal ion meter EV-74 for measuring redox potentials (EMF) of electrode systems

Measurements can be carried out both in millivolts and in pX units on the instrument scale. When measuring the EMF, no correction for the temperature of the test solution is introduced.

1. Preparation of the EV-74 ionomer for measurements.

1.1. Select the required electrodes and fix them in the tripod. The indicator electrode is connected to the "Meas." directly or using an adapter plug, and the reference electrode - to the "Aux." on the back of the instrument. The electrodes are washed and immersed in a glass of distilled water.

1.2. Check the presence of grounding of the device case.

1.3. Set the mechanical zero of the indicating device, for which, by turning the zero corrector with a screwdriver, set the arrow to the zero (initial) mark of the scale.

1.4. Press the lower button "t °" to select the type of work and the upper button "-119" to select the measurement range.

1.5. Connect the device to a 220 V network using a cord.

1.6. Turn on the device using the "Network" toggle switch. When voltage is applied, the switch-on indicator light comes on.

1.7. The device warms up for 30 minutes.

2. Measurement of oxidation-reduction potentials (EMF) of electrode systems.

2.1. The electrodes are immersed in a glass with the test solution, after removing the excess of distilled water from the surface of the electrodes with filter paper.

2.2. Turn on the magnetic stirrer.

2.3. Press the button and a button for the selected measuring range.

2.4. The button "anion | cation; + | - ", if positive potentials are measured, and pressed when negative potentials are measured.

2.5. The readings of the device are allowed to stabilize and the potential value in millivolts is read on the appropriate scale of the indicating device, multiplying the reading of the device by 100:

When measuring on a wide range of "-119", the reading is carried out on the lower scale with digitization from -1 to 19;

When measuring on a narrow range of "-14", the reading is carried out on the upper scale with digitization from -1 to 4;

When measuring at one of the narrow ranges "49", "914", "1419", the reading is carried out on the upper scale with digitization from 0 to 5, and the reading of the device is added to the value of the lower limit of the selected range.

Example. The range switch is set to position "49", and the arrow of the device is set at a value of 3.25. In this case, the measured value is: (4 + 3.25). 100 = 725 mV.

2.6. At the end of the measurements, press the "t °" and "-119" buttons, turn off the device using the "Network" toggle switch and disconnect the device and the magnetic stirrer from the network. The electrodes and the rod of the magnetic stirrer are washed with distilled water and handed over to the laboratory assistant.

Lesson 3. Coulometric analysisMethod principle

Coulometric analysis (coulometry) based on the use of the relationship between mass m the substance reacted during electrolysis in the electrochemical cell, and the amount of electricity Q passed through the electrochemical cell during the electrolysis of only this substance. In accordance with the unified law of electrolysis M. Faraday, the mass m(in grams) is related to the amount of electricity Q(in pendants) with the ratio:

(1)

where M- molar mass of the substance reacted during electrolysis, g / mol; n- the number of electrons participating in the electrode reaction; F= 96 487 C / mol - Faraday number.

The amount of electricity (in pendants) passed through the electrochemical cell during electrolysis is equal to the product of the electric current (in amperes) by the electrolysis time (in seconds):

(2)

If the amount of electricity is measured, then according to (1), you can calculate the mass m. This is true when the entire amount of electricity passed during electrolysis through an electrochemical cell is consumed only for the electrolysis of a given substance; side-

certain processes should be excluded. In other words, the current output (efficiency) must be 100%.

Since, in accordance with the unified law of electrolysis by M. Faraday (1), to determine the mass m (g) of a substance reacted during electrolysis, it is necessary to measure the amount of electricity Q, spent on the electrochemical conversion of the analyte, in pendants, then the method is called coulometry. The main task of coulometric measurements is to determine the amount of electricity as accurately as possible. Q.

Coulometric analysis is carried out either in amperostatic (galvanostatic) mode, i.e. with constant electric current i= const, or at a controlled constant potential of the working electrode (potentiostatic coulometry), when the electric current changes (decreases) during electrolysis.

In the first case, to determine the amount of electricity Q it is enough to measure as accurately as possible the electrolysis time, direct current and calculate the value Q by formula (2). In the second case, the quantity Q determined either by calculation or using chemical coulometers.

Distinguish between direct and indirect coulometry (coulometric titration).

Direct coulometry

The essence of the method

Direct coulometry at constant current is rarely used. Coulometry with a controlled constant potential of the working electrode or direct potentiostatic coulometry is used more often.

In direct potentiostatic coulometry, a directly determined substance is subjected to electrolysis. The amount of electricity consumed for the electrolysis of this substance is measured, and the mass is calculated using equation (1) m analyte.

During electrolysis, the potential of the working electrode is kept constant, what devices are usually used for - potentiostats.

Constant potential value E are selected in advance on the basis of consideration of the volt-ampere (polarization) curve plotted in the coordinates "current i- potential E ", obtained under the same conditions in which electrolysis will be carried out. Usually choose

potential value E, corresponding to the region of the limiting current for the substance to be determined and slightly exceeding its half-wave potential (by ~ 0.05-0.2 V). At this potential value, the background electrolyte should not undergo electrolysis.

A platinum electrode is most often used as a working electrode, on which electrochemical reduction or oxidation of the analyte occurs. In addition to the working electrode, the electrochemical cell includes 1 or 2 other electrodes - a reference electrode, for example, silver chloride, and an auxiliary electrode, for example, made of steel.

As the electrolysis process proceeds at a constant potential, the electric current in the cell decreases, since the concentration of the electroactive substance participating in the electrode reaction decreases. In this case, the electric current decreases with time according to the exponential law from the initial value at the moment of time to the value at the moment of time

(3)

where the coefficient depends on the nature of the reaction, the geometry of the electrochemical cell, the area of the working electrode, the diffusion coefficient of the analyte, the stirring rate of the solution and its volume.

The graph of function (3) is schematically shown in Fig. 3-10.

Rice. 3-10. Variation of tox with time in direct potentiostatic coulometry

The current output will be quantitative when the current decreases to zero, i.e. with an infinite time. In practice, electrolysis

The analyte is considered quantitative when the current reaches a very small value, not exceeding ~ 0.1% of the value. In this case, the determination error is about ~ 0.1%.

Since the amount of electricity is defined as the product of current and time of electrolysis, it is obvious that the total amount of electricity Q, spent on electrolysis of the analyte is equal to:

(4)

those. is determined by the area bounded by the coordinate axes and the exponent in Fig. 3-10.

To find the mass m of the reacted substance is required according to (1) to measure or calculate the amount of electricity Q.

Methods for determining the amount of electricity passed through a solution in direct potentiostatic coulometry

The value Q can be determined by calculation methods or using a chemical coulometer.

. Calculation of the value of Q from the area under the curve of the dependence of i on Measure the area bounded by the coordinate axes and the exponent (3) (see Fig. 3-10). If the current i expressed in amperes, and time in seconds, then the measured area is equal to the amount of electricity Q in pendants.

For determining Q without noticeable error, the method requires almost complete completion of the electrolysis process, i.e. long time. In practice, the area is measured at a value m corresponding to i= 0.001 (0.1% of.

. Calculation of the Q value based on the relationship from In accordance with (3) and (4) we have:

insofar as:

Thus, and to determine the value Q necessary

find values

According to (3) ... After taking the logarithm of this equation,

we get a linear dependence on

(5)

If you measure several values at different points in time (for example, using a curve of the type shown in Fig. 3-10 or directly experimentally), you can build a graph of function (5), schematically shown in Fig. 3-11 and representing a straight line.

The segment cut off by a straight line on the ordinate is equal to the tangent of the angle of inclination of the straight line to the abscissa is:

Knowing the values and therefore, it is possible to calculate the value

Well and then the mass m according to the formula (1).

Rice. 3-11. Time dependence of electrolysis in direct potentiostatic coulometry

... Determination of the Q value using a chemical coulometer. With this method, a chemical coulometer is connected in series with an electrochemical cell in the electric circuit of the coulometric installation, in which the analyte is electrolyzed. Electricity quantity Q, passing through a coulometer and an electrochemical cell connected in series is the same. The design of the coulometer makes it possible to experimentally determine the value Q.

The most commonly used are silver, copper and gas coulometers, less often some others. The use of silver and copper coulometers is based on the electrogravimetric determination of the mass of silver or copper deposited on a platinum cathode during electrolysis.

Knowing the mass of the metal precipitated at the cathode in the coulometer, we can calculate the amount of electricity Q using equation (1).

Coulometers, especially silver and copper, allow you to determine the amount of electricity Q with high accuracy, however, working with them is quite laborious and time-consuming.

In coulometry, electronic integrators are also used, which allow registering the amount of electricity. Q, spent on electrolysis, according to the indications of the corresponding device.

Application of direct coulometry

The method possesses high selectivity, sensitivity (up to 10 -8 -10 -9 g or up to ~ 10 -5 mol / l), reproducibility (up to ~ 1-2%), allows to determine the content of trace impurities. The disadvantages of the method include the high complexity and duration of the analysis, the need for expensive equipment.

Direct coulometry can be used to determine metal ions, organic nitro and halogen derivatives, chloride, bromide, iodide, thiocyanate anions, metal ions in lower oxidation states when they are converted to higher oxidation states, for example:

Etc.

In pharmaceutical analysis, direct coulometry is used to determine ascorbic and picric acids, novocaine, oxyquinoline, and in some other cases.

Direct coulometry is rather laborious and time consuming. In addition, in a number of cases, side processes begin to occur noticeably even before the completion of the main electrochemical reaction, which reduces the current efficiency and can lead to significant analysis errors. That is why indirect coulometry is often used - coulometric titration.

Coulometric titration

The essence of the method

In coulometric titration, the analyte X, which is in solution in the electrochemical cell, reacts with the titrant T, a substance that is continuously formed (generated) on the generator electrode during the electrolysis of an auxiliary substance also present in the solution. The end of the titration is the moment when all the analyte X completely reacts with the generated titrant T;

house, introducing into the solution an appropriate indicator that changes color near the TE, or using instrumental methods - potentiometric, amperometric, photometric.

Thus, during coulometric titration, the titrant is not added from the burette to the solution to be titrated. The role of the titrant is played by the substance T, which is continuously generated during the electrode reaction on the generator electrode. Obviously, there is an analogy between the usual titration, when a titrant is introduced from the outside into the titrated solution and, as it is added, reacts with the substance to be determined, and the generation of substance T, which, as it is formed, also reacts with the substance to be determined, therefore, the method under consideration is called coulometric titration ".

Coulometric titration is carried out in amperostatic (galvanostatic) or potentiostatic mode. More often coulometric titration is carried out in amperostatic mode, maintaining the electric current constant during the entire electrolysis time.

Instead of the volume of titrant added in coulometric titration, the time t and the current are measured i electrolysis. The process of formation of a substance T in a coulometric cell during electrolysis is called generation of titrant.

Coulometric constant current titration

In coulometric titration in amperostatic mode (at constant current), the time during which the electrolysis was carried out and the amount of electricity are measured Q, consumed during electrolysis is calculated by the formula (2), after which the mass of the analyte X is found according to the ratio (1).

So, for example, the standardization of a solution of hydrochloric acid by the coulometric titration method is carried out by titrating hydrogen ions for a standardized solution containing HCl, hydroxide ions OH generated at a platinum cathode - during electrolysis of water:

The resulting titrant - hydroxide ions - reacts with ions in solution:

Titration is carried out in the presence of phenolphthalein indicator and is stopped when a light pink color of the solution appears.

Knowing the magnitude of the direct current in amperes) and the time (in seconds) spent on titration, calculate the amount of electricity by the formula (2) Q(in pendants) and according to the formula (1) - the mass (in grams) of the reacted HCl contained in an aliquot of the standardized HCl solution introduced into the coulometric cell (into the generator vessel).

In fig. 3-12 schematically shows one of the variants of an electrochemical cell for coulometric titration with a visual (by changing the color of the indicator) indication of the end of titration, with a generator cathode and an auxiliary anode.

The generator platinum electrode 1 (in this case, the anode) and the auxiliary platinum electrode 2 (in this case, the cathode) are placed, respectively, in the generation (generator) vessel 3 and the auxiliary vessel 4. The generation vessel 3 is filled with a test solution containing the analyte X, background electrolyte with an auxiliary electroactive substance and indicator. The excipient itself can play the role of a supporting electrolyte; in such cases, there is no need to add another supporting electrolyte to the solution.

The generation and auxiliary vessels are connected by an electrolytic (salt) bridge 5 filled with a strong indifferent electrolyte to ensure electrical contact between the electrodes. The ends of the electrolytic bridge tube are closed with filter paper plugs. The generation vessel has a magnetic bar 6 for stirring the solution by means of a magnetic stirrer.

The electrochemical cell is included in the electrical circuit of the coulometric titration setup capable of maintaining a constant current of the required value (for example, a universal power supply such as a UIP-1 laboratory instrument and similar equipment is used).

Before coulometric titration, the electrodes are thoroughly washed with distilled water, a solution with an auxiliary electroactive (under these conditions) substance is introduced into the generation vessel, and, if necessary, a background electrolyte and an indicator.

Since the background solution prepared in this way may contain electroreductive or electrooxidizing impurities, then first carry out preelectrolysis background solution for the purpose of electroreduction or electrooxidation of impurities. To do this, close the electrical circuit of the installation and conduct electrolysis for

some (usually short) time until the color of the indicator changes, after which the circuit is opened.

Rice. 3-12. Diagram of an electrochemical cell for coulometric titration with visual indicator fixation of the end of titration: 1 - working generator platinum electrode; 2 - auxiliary platinum electrode; 3 - a generation vessel with a test solution; 4 - an auxiliary vessel with a solution of a strong indifferent electrolyte; 5 - electrolytic bridge; 6 - magnetic stirrer rod

After the completion of pre-electrolysis, a precisely measured volume of the analyzed solution is introduced into the generation vessel, the magnetic stirrer is turned on, the electrical circuit of the installation is closed, simultaneously turning on the stopwatch, and electrolysis is carried out at constant current until the color of the indicator (solution) abruptly changes, when the stopwatch is immediately stopped and the electrical installation chain.

If the analyzed solution introduced into the coulometric cell for titration contains impurities of electroreductive or electrooxidizing substances, the transformation of which requires a certain amount of electricity during electrolysis, then after pre-electrolysis (before adding the analyzed solution to the cell), carry out blank titration, by introducing into the coulometric cell instead of the analyzed solution exactly the same volume of solution, which contains all the same substances and in the same quantities as the added analyzed solution, with the exception of the analyte X. In the simplest case, distilled water is added to the background solution in a volume equal to the volume of an aliquot of the analyzed solution with the analyte.

The time spent on blank titration is subsequently subtracted from the time spent on titrating the test solution with the analyte.

Conditions for carrying out coulometric titration. Must provide 100% current efficiency. To do this, at least the following requirements must be met.

1. The auxiliary reagent, from which the titrant is generated on the working electrode, must be present in the solution in a large excess with respect to the analyte (~ 1000-fold excess). Under these conditions, side electrochemical reactions are usually eliminated, the main of which is the oxidation or reduction of the background electrolyte, for example, hydrogen ions:

2. The magnitude of the constant current i= const during electrolysis should be less than the value of the diffusion current of the auxiliary reagent in order to avoid the occurrence of a reaction with the participation of ions of the background electrolyte.

3. It is necessary to determine as accurately as possible the amount of electricity consumed during electrolysis, for which it is required to accurately record the beginning and end of the time count and the magnitude of the electrolysis current.

End of titration indication. In coulometric titration, TE is determined either by visual indicator or instrumental (spectrophotometric, electrochemical) methods.

For example, when titrating a sodium thiosulfate solution with electrogenerated iodine, an indicator — a starch solution — is added to the coulometric cell. After reaching the TE, when all the thiosulfate ions have been titrated in the solution, the very first portion of the electrogenerated iodine stains the solution blue. The electrolysis is interrupted.

In the case of electrochemical indication of TE, a pair of electrodes, included in an additional indicator electric circuit, are placed in the test solution (in the generation vessel). The end of the titration can be recorded using an additional indicator electric circuit potentiometrically (pH-metric) or biamperometrically.

With biamperometric indication of TE, titration curves are plotted in coordinates by measuring the current i in additional indi-

electric circuit as a function of electrolysis time in a coulometric cell.

Coulometric constant potential titration

Potentiostatic mode is used less frequently in coulometric titration.

Coulometric titration in potentiostatic mode is carried out at a constant potential value corresponding to the discharge potential of the substance on the working electrode, for example, during the cathodic reduction of metal cations M n + on the platinum working electrode. As the reaction proceeds, the potential remains constant until all metal cations have reacted, after which it sharply decreases, since there are no potential-determining metal cations in the solution.

Application of coulometric titration. In coulometric titration, you can use all types of reactions of titrimetric analysis: acid-base, redox, precipitation, complexation reactions.

Small amounts of acids (up to ~ 10 -4 -10 -5 mol / l) can be determined by coulometric acid-base titration with electrogenerated ions formed during the electrolysis of water at the cathode:

It is possible to titrate bases with hydrogen ions generated at the anode during water electrolysis:

In redox bromometric coulometric titration, it is possible to determine compounds of arsenic (III), antimony (III), iodides, hydrazine, phenols and other organic substances. Bromine electrically generated at the anode acts as a titrant:

By precipitation coulometric titration, halide ions and organic sulfur-containing compounds can be determined by electrogenerated silver cations, zinc cations, by electrogenerated ferrocyanide ions, etc.

Complexometric coulometric titration of metal cations can be carried out with EDTA anions electrogenerated at a mercury (II) complexonate cathode.

Coulometric titration has high accuracy, a wide range of applications in quantitative analysis, allows the determination of small amounts of substances, low-stability compounds (since they enter into reactions immediately after their formation), for example, copper (I), silver (II), tin (II) , titanium (III), manganese (III), chlorine, bromine, etc.

The advantages of the method also include the fact that the preparation, standardization and storage of the titrant is not required, since it is continuously formed during electrolysis and is immediately consumed in the reaction with the analyte.

Objectives of studying the topic

Based on knowledge of the theoretical foundations of the coulometric titration method and the development of practical skills, learn how to reasonably choose and practically apply this method of analysis for the quantitative determination of a substance; be able to carry out a statistical assessment of the results of coulometric titration.

Target tasks

1. Learn to quantitatively determine the mass of sodium thiosulfate in solution by coulometric titration.

2. Learn to standardize a hydrochloric acid solution by coulometric titration.

3. Solution of typical computational problems.

One of the two laboratory sessions described in this manual is devoted to the study of the topic. It is recommended to carry out the laboratory work "Determination of the mass of sodium thiosulfate in solution by coulometric titration".

Self-study assignment

You need to know for the lesson

1. The principle of coulometry methods.

2. The essence of the coulometric titration method for determining:

a) sodium thiosulfate;

b) hydrochloric acid.

You must be able to

1. Write the equations of electrochemical reactions occurring on electrodes during coulometric titration:

a) sodium thiosulfate;

b) hydrochloric acid.

2. Write the equations of electrochemical reactions occurring in solution during coulometric titration:

a) sodium thiosulfate;

b) hydrochloric acid.

3. Calculate the amount of electricity and the mass (concentration) of the substance based on the results of coulometric titration.

4. To process the results of parallel determinations of a substance by the method of mathematical statistics.

Bibliography

1. Tutorial. - Book 2, chapter 10. - S. 481-492; 507-509; 512-513.

2.Kharitonov Yu.Ya., Grigorieva V.Yu. Examples and tasks in analytical chemistry.- M .: GEOTAR-Media, 2009.- pp. 240-244; 261-264; 277-281.

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