Electrochemical methods– the most dynamically developing in terms of their application in environmental monitoring. Most often, MOS systems use voltammetry (including polarography), potentiometry (including ionometry), coulometry, and conductometry.
Electrochemical methods of analysis use the dependence of various electrical properties of the medium on the quantitative content and qualitative composition of the substances analyzed in it:
· change capacity electrode depending on the physicochemical processes occurring in the substance ( potentiometric method), incl. selective reactions of ion-selective electrodes, individually sensitive to a large number of cations and anions ( ionometric method);
· change conductivity (current) and permittivity of the substance, depending on the nature of the medium and the concentration of its components ( conductometric and amperometric methods);
changes amount of electricity when the analyte enters the electrochemical cell ( coulometric method);
recovery of the analyzed compound on a mercury dropping or rotating electrode, as a rule, in the analysis of trace amounts of substances in different aggregate states ( polarographic or voltammetric method).
Polarographs of all devices in this group have the highest sensitivity, equal to 0.005–1 µg/ml of sample.
Voltammetry includes a group of electrochemical methods of analysis based on the study of polarization curves. These methods are polarography and amperometric titration - have many varieties and modifications. The most common direct current polarography.
A polarographic setup consists of a direct current source, a voltage divider, a drip (usually mercury) or rotating electrode, and an auxiliary (usually also mercury or other) electrode. To measure the current strength, a microammeter is connected to the system. The electrodes are placed together with the test solution in the electrolyzer (cell).
The voltage applied to the electrolytic cell causes polarization of the anode and cathode E= f a– f k +iR, where i– current strength; TO - solution resistance; f a and f k are the anode and cathode potentials.
If we reduce the resistance of the solution by adding a strong electrolyte (background), then the value iR(potential drop in solution) can be neglected.
The anode potential remains practically constant during the operation of the cell, since the current density is low and the relatively large anode surface is not polarized. Then the potential of a dropping polarizing cathode with a small surface will be equal to: E= -f k. Often in polarographic measurements, instead of a layer of mercury at the bottom of the vessel, a non-polarizable saturated calomel electrode is used, the potential of which is assumed to be zero.
Polarographic data is obtained by measuring the current passing through the electrolytic cell as a function of the potential applied to the electrodes. The graphical dependence of the current strength on the potential is called the polarographic wave ( rice. 2).
At the beginning of electrolysis, with small values of the superimposed EMF, the current strength will be almost constant and increase only very slowly. This is the so-called residual current, which is maintained throughout the electrolysis.
Rice. 2. Polarogram of 10–3 M zinc chloride solution and 1 M potassium chloride solution (curve 1) and 1 M potassium chloride solution (curve 2)
As soon as the ion reduction potential is reached (for example, for the zinc ions being determined, it is equal to -1.0 V), their discharge on a drop of mercury begins:
Zn 2+ + 2 + Hg ® Zn (Hg).
A dilute zinc amalgam Zn (Hg) is formed on the cathode, which decomposes into its constituents as soon as the falling drop comes into contact with the anode:
Zn (Hg) - 2 ® Zn 2+ + Hg.
At the reduction potential of zinc ions, the current increases sharply ( rice. 2), but after reaching a certain value, despite the increase in the applied emf, it remains almost constant. This current is called the limiting or diffusion current, its value is usually proportional to the concentration of the analyte.
When taking polarograms, an indifferent electrolyte with cations that are much more difficult to recover than the analyzed cation is added to the electrolyte under study, for example, KCl, KNO 3, NH 4 Cl; at a concentration 100-1000 times higher than the concentration of the analyte. Such an electrolyte is called "background". It is created in the test solution to increase the electrical conductivity and to shield the electric field of the indicator electrode (cathode). Therefore, the cations of the analyte are not attracted by the electric field of the cathode, but move towards it due to diffusion.
The most important characteristic of a polarogram is the half-wave potential E 1/2 and polarographic wave height h(limiting diffusion current). The half-wave potential is used in quality polarographic analysis. The half-wave potentials of various substances, arranged in ascending order of their negative value, constitute the so-called "polarographic spectrum". Since the half-wave potential significantly depends on the composition of the solution (analyzed medium), the background is always indicated in the polarographic tables.
AT quantitative In polarographic analysis, the methods of calibration curve, additives, comparison and calculation method are used to measure the concentration.
Among the various options for polarography, the method differential pulse polarography (DIP ) is most effective for solving environmental monitoring problems, mainly due to its high sensitivity. The DIP method makes it possible to estimate the content of all substances determined by the classical polarography method. Among other polarographic methods, especially suitable for trace analysis square wave polarography, which provides a detection limit close to the DIP detection limit, but only in the case of reversible electrode processes, and therefore this method is often used to determine traces of heavy metals. The DIP method can also be used to determine surfactants that change the capacitance of the electrical double layer of the electrode.
Methods can be used to determine the microcontents of heavy metal ions. inverse electrochemical analysis (IEA) or in another way, stripping voltammetric analysis (IVA ), in which the metals to be determined are preliminarily deposited on the electrode and then dissolved under polarographic control. This option, in combination with DIP, is one of the most sensitive methods of electrochemical analysis. The hardware design of the IEA (IVA) is relatively simple, which makes it possible to carry out analyzes in the field, and automated continuous control (monitoring) stations can also work on this principle.
IEA (IVA) methods provide the determination of Cu, Pb, Bi, Sb, As, Sn In, Ga, Ag, Tl, Cd, Zn, Hg, Au, Ge, Te, Ni, Co and many anions. An important advantage of IEA (IVA) methods is (unlike other methods, such as atomic absorption spectrometry, for example) the ability to distinguish free ions from their bound chemical forms, which is also important for assessing the physicochemical properties of the analyzed substances from the point of view of ecoanalytical control (for example, when assessing water quality). Many organic substances can also be determined by IEA (IVA) after their adsorption accumulation on the electrode surface.
Polarographic methods can also be used to determine aerosols of various metals in the atmosphere and air of industrial premises after they are captured on appropriate filters, followed by transferring the concentrates into solution. Organic compounds found in the form of gases and vapors in the atmosphere can be determined polarographically after they have been absorbed by specially selected solutions. Metals and various compounds in biological materials are usually determined polarographically after their extraction. All polarographic measurements, including IEA (IVA), can be fully automated, which is essential when performing serial analyses.
One of the most important applications of polarography is the determination of oxygen in water. For this, amperometric detectors are used that generate a current proportional to the oxygen concentration in the solution.
By applying the enzyme to the surface of the detector membrane, it is possible to obtain various enzyme amperometric sensors that are convenient for biochemical and clinical analyzes. Such sensors are also used in environmental monitoring systems.
Electrodes operating on the electrocatalytic principle are suitable for monitoring various gases (SO 2 , H 2 S, CO, NO x) in the air of industrial premises. The electrochemical reactions of these gases (playing the role of a catalyst) occurring on the electrode surface generate a current in the electrode system that is functionally related to the concentration of gases in the air.
The use of polarography is not limited to the analysis of discrete samples, and the method is gradually moving to the principles of continuous analysis of gases and liquids.
Voltammetric polarographic detectors are successfully used in high performance liquid chromatography (HPLC). In this case, the combination of a highly selective separation method with a sensitive detection method leads to a significant expansion of the range of substances determined by the chromatographic method (traces of highly toxic substances, herbicides, drugs, growth stimulants, etc.).
Details of the method can be clarified in the specialized literature ,,,,.
Potentiometry- a method for determining the concentration of substances, based on the measurement of the EMF of reversible galvanic cells.
In practice, two analytical methods are used: direct potentiometry to determine the particle activity, which can be calculated using the Nernst equation from the electromotive force of a galvanic cell, and potentiometric titration , in which a change in the activities of chemicals during the titration process leads to a change in the EMF of a galvanic cell.
The apparatus for carrying out potentiometric titrations and for direct potentiometry is the same. The circuit of potentiometric measurements includes an indicator electrode and a reference electrode with a stable constant potential, as well as a secondary device. The schematic diagram of the method is shown in rice. 3.
1 - indicator electrode; 2 - reference electrode
Rice. 3. Potentiometric cell
The potential of a pair of electrodes is constant. Changing the concentration of the analyte in solution changes the EMF of the circuit. Indicator electrodes usually come in four types, depending on the membrane used, which separates the electrode solution from the test solution: 1) electrodes with a homogeneous membrane of powder or crystalline material; 2) electrodes with a heterogeneous membrane, in which the electrode active substance is distributed, for example, in silicone rubber; 3) electrodes with a liquid membrane, in which the membrane is a solution deposited on a neutral substance, for example, porous glass; 4) glass electrodes with different chemical composition of glass.
Indicator electrodes acquire the potential of the solution in which they are placed. Distinguish two kind indicator electrodes:
1) indifferent electrodes (indestructible during electrolysis);
2) electrodes changing (oxidizing or reducing) during measurements.
Role indifferent electrodes(these are sometimes called electrodes third kind) is to give or add electrons, i.e. be conductors of electricity. Such electrodes can be made of gold, polished platinum, graphite and other materials. Examples of changing electrodes (sometimes referred to as electrodes first kind) can be plates of copper, zinc and other metals, as well as quinhydrone and hydrogen indicator electrodes. The indicator electrodes can also be ion-selective membrane electrodes to determine numerous cations: Li +, Pb +, Cs +, Tl +, NH +, Na +, K +, Ag +, etc. As reference electrodes ( standard electrodes), the potential of which remains constant throughout the measurement, most often used, for example, normal and decinormal calomel (calomel) electrodes with potentials of +0.282 V and +0.334 V, respectively, as well as a saturated silver chloride electrode with a potential of +0.201 V.
In the ideal case, direct potentiometric measurement of the EMF of a galvanic cell can be connected through the Nernst equation with the activity of the particle being determined, or with the concentration, if the corresponding activity coefficients are known:
where E 0 – standard electrode potential, V; R is the gas constant; T is the absolute temperature; F- Faraday number; n is the number of lost or gained electrons; , [rest.] - equilibrium concentrations of the oxidized, reduced forms, respectively, mol / dm 3.
If we substitute the reference values of the constants and go from the natural logarithm to the decimal logarithm, then for a temperature of 25 ° C we get;
The most important indicator in characterizing the state of the OS is the pH value of this environment, the definition of which ( pH-metry ) is now usually carried out using glass indicator (measuring) electrodes. For long-term measurements, special designs of glass electrodes have been developed with additional devices that clean the glass membrane. Glass electrodes covered with a semi-permeable membrane with an electrolyte film also serve as the basis for various types of probes ( sensors ) used in the analysis of water and air under production conditions for a number of contaminants (NH 3 , CO 2 , NO x , SO 2 , H 2 S, etc.).
The process in the field of creating ion-selective electrodes (ISE) allows you to control the ions F - , I - , Br - , Cl - , CN - , SCN - , NO 3 - , NO 2 - , ClO 4 - , S 2- , Na + , K + Ca 2+ , Ag + , Cu 2+ , Cd 2+ , Pb 2+ in concentration ranges from 10–2 to 10–7 mol/l (approximately 1–10–5 mg/ml). ISE control is characterized by rapidity, simplicity and great possibilities for continuous measurements. ISEs have been developed that are selective to a wide class of organic substances, as well as isomers in their mass, surfactants and detergents in the air of the industrial zone and the water management regime of industrial enterprises.
Potentiometry is also used to measure the redox potentials of various redox (O/W) systems in water. As a rule, the measurement results correspond to a mixed potential, since usually several O/W systems coexist simultaneously in water.
It should be noted that the use of sensors based on semiconductor metal oxide chemically selective and ion-selective field-effect transistors (HSPTs, ISPTs) is promising. Selectivity in these systems is achieved by choosing the composition of the membrane and the layer deposited on the gate of the transistor. The system is immersed in the analyzed solution, and the potential difference between the reference electrode and the gate of the transistor modulates the current flowing between its source and drain. Due to the selectivity of the membrane or the deposited layer, the modulated current becomes a function of the activity of the corresponding component of the solution. Semiconductor sensors form the basis of monitors-analyzers of various gases and vapors. The small size of such sensors makes it possible to combine their aggregates in the form of a mosaic on a single substrate, so that an analyzer is obtained that is capable of monitoring a whole range of harmful substances. Signals from individual sensors included in the mosaic can be sequentially and periodically recorded by the measuring center of the analytical system.
The development of microelectronics makes it possible to design compact probe-type analyzers using modern ISEs. At the same time, a circuit processing the response from the environmental control object, and even a display, can be mounted in the probe handle.
In the special literature, you can familiarize yourself with the details of the method,,,.
Coulometric the method of analysis is the measurement of the current of the electrode reaction, into which the test substance enters, entering the coulometric cell with the analyzed flow. The schematic diagram of the coulometric cell is shown in rice. four.
1 – cathode chamber; 2 – anode chamber; 3 - microammeter
Rice. four. Diagram of a coulometric cell
Coulometric analysis is based on measuring the amount of electricity used to quantify a given electrochemical process in a given sample, i.e. provided that the current output is 100%. This is the amount of electricity with the help of a current-time integrator connected in series with the measuring cell, or a coulometer-electrolyzer, in which an electrochemical process is carried out with a 100% current output, accompanied by the release of a substance, the amount of which can be easily and accurately restored.
In accordance with Faraday's law:
m( x)/M(x) = m(k)/M(k),
where m(x), m(k) mass of analyte X and the substance released in the coulometer, respectively; M(x), M(k) is the molar mass of substance equivalents X and the substance released in the coulometer, g/mol.
The calculation can also be made according to the equation describing Faraday's law:
if during the analysis the current strength is measured i, A and time t, s spent on the electrochemical process.
In another modification of this method, called
coulometric titration
, the titrant is generated electrolytically in the analyzed solution at a given current. The consumption of titrant in the analytical reaction is made up for by the charge flowing through the solution during titrant generation until the equivalence point is reached.
One of advantages of coulometric methods is that the titrant standardization process is often not necessary, since the calculations are based on the Faraday constant, i.e. the method is absolute and allows you to estimate the amount of the analyte, and not its concentration. The disadvantage of coulometry with a given potential is the duration of the analysis procedure associated with the need to complete the electrolysis. Computer technology makes it possible to reduce this time by predicting the end of electrolysis by mathematical processing of the current-time curve for the initial stages of electrolysis and by calculating the amount of electricity or the concentration of a substance in a solution. When analyzing multicomponent samples, it can be used scanning coulometry , in which the electrolysis potential is changed continuously or stepwise. For such systems, coulometric titration is preferable to direct coulometry, since 100% current efficiency in titrant generation can be easily achieved by choosing the right titrant reagent and the composition of the working medium. Coulometric titration is applicable to determine from 0.01 to 100 mg of substances (sometimes below 1 μg). The working volume of the samples is usually between 10 and 50 ml. The method is characterized by high accuracy, the relative error does not exceed several tenths of % even with coulometric titration of microgram contents. Under optimal conditions, titration can be performed with a very small total error of 0.01% (rel.). Various acid-base, redox; precipitation and complexometric titration options can be carried out coulometrically.
Coulometric gas analyzers and aqua analyzers (“coulometers”) have been developed and are being produced for the determination of sulfur dioxide and hydrogen sulfide (sulfates and sulfides), ozone (and hydrogen peroxide), chlorine in the air (and active chlorine in water), carbon monoxide and nitrogen dioxide in air (nitrates and nitrites in water). Coulometry is also used as a means of electrochemical detection in liquid chromatography.
Details of the method can be found in the specialized literature.
Conductometric method analysis is based on measuring the electrical conductivity of the solution. The conductometric method of analysis consists in measuring the change in the resistance of an electrolyte solution when a component of the mixture is absorbed. Conductometric installations are used, for example, to determine carbon monoxide and dioxide, gasoline vapors, ammonia and others.
Electrical conductivity is the reciprocal of resistance R, its dimension is CM (Siemens) i.e. æ = 1/ R.
The electrical conductivity of the solution depends on the number of ions per unit volume of the solution, i.e. from concentration FROM, on the mobility of these ions - v. Based on the known relations
where Z is the distance between the electrodes; S- electrode area; k- coefficient of proportionality.
For a specific pair of electrodes with a constant distance between them S/Z= const. Then
,
where k 1 = k(S/Z).
When calculating in conductometry, the concept of "electrical conductivity" æ 0 is used:
In calculations, it is convenient to use the equivalent electrical conductivity, which is equal to:
where P - the number of moles of the equivalent in 1 cm 3 of the solution. The equivalent electrical conductivity l ¥ at infinite dilution is equal to the sum of the mobilities of the cation U and anion v.
The ratio of the equivalent electrical conductivity of a weak electrolyte solution to the equivalent electrical conductivity of this electrolyte at infinite dilution is equal to the degree of dissociation a of this electrolyte:
Despite the non-specificity, this method is quite often, compared to other electrochemical methods, used in environmental monitoring systems. This is explained by the fact that when assessing pollution, for example, of water and the atmosphere, not a stage-by-stage, but an output (final) control of industrial processes is possible. Due to the extremely low electrical conductivity of water, it is often enough to estimate the total content of contaminants, which is what conductometry provides. Typical examples of the use of conductometric methods in environmental monitoring are analyzers of detergents in wastewater, concentrations of synthetic components in irrigation systems, quality (salinity) of drinking water. Conductivity analyzers are used for continuous monitoring of air pollution and precipitation, such as SO 2 and H 2 SO 4 . In addition to direct conductometry can be used to identify certain types of pollution indirect methods, which provide very effective estimates of the content of the substances listed above, which interact before measurement with specially selected reagents and the registered change in electrical conductivity is caused only by the presence of the corresponding products in the reaction. So it is possible to determine nitrogen oxides after their catalytic reduction of doammonia, as well as HCl, HBr and CO 2 after a preliminary reaction with Ba(OH) 2 or NaOH. The described principle of CO 2 determination can also be used for indirect determination of organic substances in water.
In addition to classical conductometry, there is also its high-frequency version ( oscillometry ), in which the indicator electrode system is not in contact with the sample. This principle is often implemented in continuous conductometric analyzers.
Electrochemical methods of analysis are also described in a number of educational and special publications,,,,.
LITERATURE
1. Drugov Yu.S., Rodin A.A.Ecological analytical chemistry.
St. Petersburg: 2002. - 464 p.
2. Pashkevich M.A., Shuisky V.F. Environmental monitoring. Tutorial. St. Petersburg State University. - St. Petersburg, 2002. - 90 p.
3. Cattrall Robert W. chemical sensors. M.: Scientific world, 2000. - 144 p.
4. Turyan Ya.I., Ruvinsky O.E., Zaitsev P.M.Polarographic catalimetry. M.: Chemistry, 1998. - 272 p.
5. Budnikov G.K., Maistrenko V.N., Murinov Yu.I. Voltammetry with modified and ultramicroelectrodes. M.: Nauka, 1994. - 239s.
6. Brainina Kh.Z., Neiman E.Ya., Slepushkin V.V. Inversion electroanalytical methods. M.: 1988. - 240 p.
7. Salikhdzhanova R.F. and etc. Polarographs and their operation in practical analysis and research. M.: Chemistry, 1988. - 192 p.
8. Kaplan B.Ya., Pats R.G., Salikhdzhanova R.F. AC voltammetry. M.: Chemistry, 1985. - 264.
9. Bond A.M. Polarographic methods in analytical chemistry. Moscow: Chemistry, 1983.
10. Efremenko O.A. Potentiometric analysis. Moscow: MMA im. THEM. Sechenov, 1998.
11. Reference guide to the use of ion-selective electrodes. M.: Mir, 1986.
12. Korita I. Ions, electrodes, membranes. M.: Mir, 1983.
13. Nikolsky B.V., Materova E.A. ion-selective electrodes. L.: Chemistry, 1980.
14. Efremenko O.A.coulometric titration. Moscow: MMA im. THEM. Sechenov, 1990.
15. Khudyakova T.A., Koreshkov A.P. Conductometric method of analysis. Textbook for universities. M.: Higher school, 1975. - 207 p.
16. Budnikov G.K., Maistrenko V.N., Vyaselev M.R. Fundamentals of modern electroanalysis. Moscow: Chemistry, 2000.
17. Prokhorova G.V. Introduction to electrochemical methods of analysis. M.: Publishing House of Moscow State University, 1991. - 97 p.
18. Electroanalytical Methods in Environmental Control. / Ed. R. Kalvoda, R. Zyka, K. Shtulik et al. M.: Chemistry, 1990. - 240 p.
19. Plambeck J.Electrochemical methods of analysis. Fundamentals of theory and application./ Per. from English. M.: Mir, 1986.
Introduction
Chapter 1. General concepts. Classification of electrochemical methods of analysis
Chapter 2. Potentiometric methods of analysis (potentiometry)
1 Method principle
3 Potentiometric titration
Chapter 3
1 The principle of the method. Basic concepts
2 Conductometry principle
3 Conductometric titration
Chapter 4. Conductometric analysis (conductometry)
1 The essence of the method
2 Quantitative polarographic analysis
3 Application of polarography
Chapter 5. Amperometric Titration
Chapter 6
1 Method principle
3 Coulometric titration
Conclusion
Bibliography
INTRODUCTION
Electrochemical methods of analysis are a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the medium under study or at the phase boundary and associated with a change in the structure, chemical composition or concentration of the analyte.
Electrochemical methods of analysis are divided into five main groups: potentiometry, voltammetry, coulometry, conductometry and amperometry.
The use of these methods in quantitative analysis is based on the dependence of the values of the measured parameters during the course of an electrochemical process on the separated substance in the analyzed solution participating in this electrochemical process. These parameters include the difference in electrical potentials, the amount of electricity. Electrochemical processes are processes that are simultaneously accompanied by a chemical reaction and a change in the electrical properties of the system, which in such cases can be called an electrochemical system. In analytical practice, an electrochemical system usually contains an electrochemical cell, including a vessel with an electrically conductive analyte solution, in which electrodes are immersed.
There are direct and indirect electrochemical methods. In direct methods, the dependence of the current strength (potential, etc.) on the concentration of the analyte is used. In indirect methods, the current strength (potential and the like) is measured in order to find the end point of titration of the component to be determined with a suitable titrant, that is, the dependence of the measured parameter on the volume of the titrant is used.
CHAPTER 1. GENERAL CONCEPTS. CLASSIFICATION OF ELECTROCHEMICAL METHODS OF ANALYSIS
Electroanalytical chemistry includes electrochemical methods of analysis based on electrode reactions and on the transfer of electricity through solutions.
The use of electrochemical methods in quantitative analysis is based on the use of the dependences of the values of the measured parameters of electrochemical processes (electric potential difference, current, amount of electricity) on the content of the analyte in the analyzed solution participating in this electrochemical process. Electrochemical processes are those processes that are accompanied by the simultaneous occurrence of chemical reactions and a change in the electrical properties of the system, which in such cases can be called an electrochemical system. In analytical practice, an electrochemical system usually contains an electrochemical cell, including a vessel with an electrically conductive analyzed solution, in which electrodes are immersed.
Classification of electrochemical methods of analysis. Electrochemical methods of analysis 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:
a) Methods without the imposition of an 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.
b) Methods with the imposition of an external (extraneous) potential. These methods include:
conductometric analysis - based on measuring the electrical conductivity of solutions as a function of their concentration;
voltammetric analysis - based on the measurement of current as a function of the applied known potential difference and the concentration of the solution;
coulometric analysis - based on measuring the amount of electricity that has passed through the solution, as a function of its concentration;
electrogravimetric analysis - based on measuring the mass of the product of an electrochemical reaction.
Classification according to the method of application of electrochemical methods. There are direct and indirect methods.
a) Direct methods. The electrochemical parameter is measured as a known function of the concentration of the solution and, according to the indication of the corresponding measuring device, the content of the analyte in the solution is found.
b) Indirect methods are titration methods in which the end of the titration is fixed based on the measurement of the electrical parameters of the system.
According to this classification, a distinction is made, for example, between direct conductometry and conductometric titration.
CHAPTER 2. POTENTIOMETRIC METHOD OF ANALYSIS (POTENTIOMETRY)
1 Method principle
Potentiometric analysis (potentiometry) is based on the measurement of 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:
aA+bB↔dD + eE
with the transfer of n electrons, then the Nernst equation for the EMF E of this reaction has the form:
E꞊E˚- RTnFlnaDda Eea(A)a aBb
where, as usual, E ° is the standard EMF of the reaction (the difference in standard electrode potentials), R is the gas constant, T is the absolute temperature at which the reaction proceeds, F is the Faraday number; a(A), a(B), a(D), and n(E) are the activities of the reagents participating in the reaction. Equation (10.1) is valid for the EMF of a reversibly operating galvanic cell.
For room temperature, equation (10.1) can be represented in the form:
E꞊E˚- 0.059nlnaDda Eea(A)a aBb
Under conditions when the activities of the reagents are approximately equal to their concentrations, equation (1) turns into equation (3):
꞊E˚- RTnFlncDdc EecAa aBb
where c(A), c(B), c(E), c(D) are the concentrations of the reagents. For room temperature, this equation can be represented as (4):
꞊E˚- 0.059nlncDdc EecAa aBb
In potentiometric measurements in an electrochemical cell, two electrodes are used: an indicator electrode, the potential of which depends on the concentration of the analyte (potential-determining) substance in the analyzed solution, and a reference electrode, the potential of which remains constant under the conditions of the analysis. Therefore, the EMF value determined by equations (1)-(4) can be calculated as the difference between the real potentials of these two electrodes.
In potentiometry, the following types of electrodes are used: electrodes of the first, second kind, redox, membrane electrodes.
Electrodes of the first kind are electrodes that are reversible in terms of a cation common to the electrode material. There are three types of electrodes of the first kind.
a) Metal M immersed in a solution of a salt of the same metal. A reversible reaction occurs on the surface of such electrodes:
Mn+ + ne = M
The real potential of such an electrode of the first kind depends on the activity a(Mn+) of metal cations and is described by equations (5)-(8).
In general, for any temperature:
꞊E˚+ RTnFln a(Mn+)
For room temperature:
꞊E˚+ 0.059nln a(Mn+)
At low concentrations c(Mn+), when the activity a(Mn+) of metal cations is approximately equal to their concentration:
꞊E˚+ RTnFln c(Mn+)
For room temperature:
b) Gas electrodes, for example, 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:
꞊E˚+ 0.059 lg a(H30+) = 0.059 lg a(H3O+) = -0.059pH
since for the hydrogen electrode the standard potential is assumed to be zero ( £° =0), and according to the electrode reaction: H++e = H the number of electrons participating in this reaction is equal to one: n = 1. c) Amalgam electrodes, which are an amalgam of a metal immersed in a solution containing cations of the same metal. The potential of such electrodes of the first kind depends on the activity of a(Mn+) metal cations in solution and the activity of n(M) of the metal in the amalgam: ꞊E˚+ RTnFlna(Mn+)a(M) Amalgam electrodes are highly reversible. Electrodes of the second kind 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 sparingly soluble salt of the same metal, immersed in a solution containing anions that are part of this sparingly soluble salt. An example is the silver chloride electrode Ag|AgCl, KS1 or the calomel electrode Hg|Hg2Cl2, KS1. The silver chloride electrode consists of a silver wire coated with a slightly water-soluble AgCI salt 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 sparingly soluble mercury(1) chloride Hg2Cl2 - calomel, in contact with an aqueous solution of potassium chloride. A reversible reaction takes place on the calomel electrode: Cl2 + 2е = 2Hg + 2СГ. The real potential of electrodes of the second kind depends on the activity of anions and for a reversibly operating electrode on which the reaction proceeds: Ne = M + An- is described by the Nernst equations (9)-(12). In general, at any acceptable temperature T: ꞊E˚- RTnFln a(An-) For room temperature: ꞊E˚- 0.059nln a(An-) For conditions in which the activity of anions is approximately equal to their concentration c (A "~): E꞊E˚- RTnFln c(An-) For room temperature: ꞊E˚- 0.059nln c(An-) So, for example, the real potentials E1 and E2, respectively, of silver chloride and calomel electrodes at room temperature can be represented as: ꞊E1˚- 0.0591g a(Cl-),꞊E2˚- 0.0591g a(Cl-). Electrodes of the second kind are highly reversible and 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 Pt, Cl2 KC1. Gas electrodes of the second kind are rarely used in quantitative potentiometric analysis. Redox electrodes 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: a) electrodes, the potential of which does not depend on the activity of hydrogen ions, for example, Pt | FeCl3, FeCI2, Pt | K3, K4, etc.; b) electrodes, the potential of which depends on the activity of hydrogen ions, for example, the quinhydrone electrode. On a redox electrode, the potential of which does not depend on the activity of hydrogen ions, a reversible reaction occurs: Ox + ne = Red The real potential of such a redox electrode depends on the activity of the oxidized and reduced forms of a given substance and, for a reversibly operating electrode, is described, depending on the conditions (by analogy with the above potentials), by the Nernst equations (13) - (16): ꞊E˚+ RTnFln a (Ox)a (Red)꞊E˚+ 0.059nlg a (Ox)a (Red)꞊E˚+ RTnFln c(Ox)c (Red)꞊E˚+ 0.059nlg c (Ox) c(Red) 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 are electrodes that are 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 analyte ions, but with different concentrations: a solution (standard) with a precisely known concentration of analyte ions and an analyzed solution with an unknown concentration of analyte ions. Due to the different concentrations of ions in both solutions, ions on different sides of the membrane are sorbed in unequal amounts, and the electric charge arising during the sorption of ions on different sides of the membrane is not the same. As a result, a membrane potential difference arises. The determination of ions using membrane ion-selective electrodes is called ionometry. As mentioned above, in potentiometric measurements, the electrochemical cell includes two electrodes - an indicator electrode and a reference electrode. The magnitude of the EMF generated in the cell is equal to the potential difference of these two electrodes. Since the potential of the reference electrode remains constant under the conditions of the potentiometric determination, the EMF depends only on the potential of the indicator electrode, i.e. on the activities (concentrations) of certain ions in 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. The determination of the concentration of a substance in direct potentiometry is usually carried out by the method of a calibration curve or by the method of standard additions. a) Calibration curve 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 the reference 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 constant by introducing an indifferent electrolyte. Reference solutions are sequentially introduced into the electrochemical (potentiometric) cell. Typically, this cell is a glass beaker in which an indicator electrode and a reference electrode are placed. Measure the EMF of the reference solutions by thoroughly rinsing the electrodes and beaker with distilled water before filling the cell with each reference solution. Based on the data obtained, a calibration graph is built in the EMF-lg c coordinates, where c is the concentration of the analyte in the reference solution. Usually 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 graph, log c(X) is found, where c(X) is the concentration of the analyte in the analyzed solution. b) Standard addition method. A known volume V(X) of the analyzed solution with concentration c(X) is introduced into the electrochemical cell and the EMF of the cell is measured. Then, an accurately measured small volume of the standard solution V(st) with a known, sufficiently large concentration c(st) of the analyte is added to the same solution, and the EMF of the cell is again determined. Calculate the concentration c(X) of the analyte in the analyzed solution according to the formula (10.17): c (X) \u003d c (st) V (st) V X + V (st) where △ E is the difference between the two measured EMF values, n is 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 solutions), anions, metal ions (ionometry). When using direct potentiometry, the choice of a suitable indicator electrode and an accurate measurement of the equilibrium potential play an important role. When determining the pH of solutions, electrodes are used as indicator electrodes, the potential of which depends on the concentration of hydrogen ions: glass, hydrogen, quinhydrone, and some others. The most commonly used membrane glass electrode is reversible with respect to hydrogen ions. The potential of such a glass electrode is determined by the concentration of hydrogen ions, so the EMF of the circuit, including the glass electrode as an indicator, is described at room temperature by the equation: K + 0.059 pH, where the constant K depends on the material of the membrane, the nature of the reference electrode. The glass electrode allows you to determine the pH in the range pH = 0-10 (more often - in the range pH = 2-10) and has a high reversibility and stability in operation. Quinhydrone electrode, often used in the past, is 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 of composition C6H402 C6H4(OH)2 (dark green powder, slightly soluble in water). Schematic designation of quinhydrone electrode: Pt | quinhydrone, HC1. A redox reaction occurs on the quinhydrone electrode: C6H402 + 2H+ + 2e \u003d C6H4 (OH) 2 The potential of the quinhydrone electrode at room temperature is described by the formula E°-0.059 pH. Quinhydrone electrode allows you to measure the pH of solutions in the range 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, Quinhydrone electrode cannot be used in the presence of strong oxidizing and reducing agents. Membrane ion-selective electrodes are used, as already noted above, in ionometry as indicator ones for determining various cations (Li+, Na+, K+ Mg2t, Ca2+, Cd2+, Fe2+, Ni2+, etc.) and anions (F-, Cl-, Br -, I-, S2-, etc.). The advantages of direct potentiometry include the simplicity and speed of measurements, measurements require small volumes of solutions. 3Potentiometric titration Potentiometric titration - a method for determining the volume of titrant spent on titration of the analyte in the analyzed solution by measuring the EMF (during the titration process) using a galvanic circuit composed of an indicator electrode and a reference electrode. In potentiometric titration, the analyzed solution, located in the electrochemical cell, is titrated 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. The change in the potential of the indicator electrode during the titration process is measured depending on the volume of the added titrant. Based on the data obtained, a potentiometric titration curve is built, and the volume of the consumed titrant in the fuel cell is determined from this curve. Potentiometric titration does not require the use of indicators that change color near the fuel cell. Application of potentiometric titration. The method is universal, it can be used to indicate the end of the titration in all types of titration: acid-base, redox, compleximetric, precipitation, titration in non-aqueous media. Glass, mercury, ion-selective, platinum, silver electrodes are used as indicator ones, and calomel, silver chloride, glass electrodes are used as reference electrodes. The method has high accuracy, high sensitivity: it 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. The founder of conductometric analysis is the German physicist and physical chemist F.V.G. Kohlrausch (1840-1910), who for the first time in 1885 proposed an equation establishing a relationship between the electrical conductivity of solutions of strong electrolytes and their concentration. AT mid 40s. 20th century developed a method of high-frequency conductometric titration. From the beginning of the 60s. 20th century began to use conductometric detectors in liquid chromatography. 1 The principle of the method. Basic concepts Conductometric analysis (conductometry) is based on the use of the relationship between the electrical conductivity (electrical conductivity) of electrolyte solutions and their concentration. The electrical conductivity of electrolyte solutions - conductors of the second kind - is judged on the basis of measuring their electrical resistance in an electrochemical cell, which is a glass vessel (glass) with two electrodes soldered into it, between which the test electrolyte solution is located. An alternating current is passed through the cell. Electrodes are most often made of metallic platinum, which is coated with a layer of spongy platinum by electrochemical deposition from solutions of platinum compounds to increase the surface of the electrodes (platinum platinum electrodes). To avoid complications associated with the processes of electrolysis and polarization, conductometric measurements are carried out in an alternating electric field. The electrical resistance R of the electrolyte solution layer between the electrodes, as well as the electrical resistance of the conductors of the first kind, is directly proportional to the length (thickness) l of this layer and inversely proportional to the area S of the electrode surface: R= ρ lS lkS where the coefficient of proportionality p is called the electrical resistivity, and the reciprocal value k \u003d 1 / p is the electrical conductivity (electrical conductivity). Since the electrical resistance R is measured in ohms, and the thickness l of the electrolyte solution layer is in cm, the area S of the electrode surface is in cm2, then the electrical conductivity k is measured in units of Ohm-1 cm-1, or, since Ohm-1 is Siemens (Cm), then - in units of Cm cm-1. In physical terms, specific electrical conductivity is the electrical conductivity of an electrolyte layer located between the sides of a cube with a side length of 1 cm, numerically equal to the current passing through an electrolyte solution layer with a cross-sectional area of 1 cm2 at an applied electric potential gradient of 1 V/cm. The specific electrical conductivity depends on the nature of the electrolyte and solvent, on the concentration of the solution, and on the temperature. With an increase in the concentration of the electrolyte solution, its electrical conductivity first increases, then passes through a maximum, after which it decreases. This character of the change in electrical conductivity is due to the following reasons. Initially, with an increase in electrolyte concentration, the number of ions - current-carrying particles - increases for both strong and weak electrolytes. Therefore, the electrical conductivity of the solution (the electric current passing through it) increases. Then, as the concentration of the solution increases, its viscosity increases (reducing the speed of movement of ions) and electrostatic interactions between ions, which prevents the increase in electric current and, at sufficiently high concentrations, contributes to its decrease. In solutions of weak electrolytes, with increasing concentration, the degree of dissociation of electrolyte molecules decreases, which leads to a decrease in the number of ions - current-conducting particles - and to a decrease in electrical conductivity. In solutions of strong electrolytes at high concentrations, the formation of ion associates (ionic twins, tees, etc.) is possible, which also favors a drop in electrical conductivity. The specific electrical conductivity of electrolyte solutions increases with increasing temperature due to a decrease in the viscosity of the solutions, which leads to an increase in the speed of movement of ions, and for weak electrolytes, also to an increase in the degree of their ionization (dissociation into ions). Therefore, quantitative conductometric measurements must be carried out at a constant temperature, thermostating the conductometric cell. In addition to specific electrical conductivity, conductometry uses the equivalent electrical conductivity X and the molar electrical conductivity p. In physical terms, the equivalent electrical conductivity X is the electrical conductivity of a layer of an electrolyte solution 1 cm thick, located between identical electrodes with such an area that the volume of the electrolyte solution enclosed between them contains 1 g-eq of the solute. In this case, the molar mass of identical particles with a unit charge number (“charge”) is taken as the molar mass of the equivalent, for example, H+, Br - , 12Са2+, 13Fe3+, etc. The equivalent electrical conductivity increases with decreasing concentration of the electrolyte solution. The maximum value of the equivalent electrical conductivity is reached at the infinite dilution of the solution. Equivalent electrical conductivity, like specific conductivity, increases with increasing temperature. The equivalent electrical conductivity X is related to the electrical conductivity to the relation (20): λ= 1000 kc In direct conductometry, the concentration of a substance in the analyzed solution is determined from the results of measurements of the specific electrical conductivity of this solution. When processing measurement data, two methods are used: the calculation method and the calibration curve method. Calculation method. In accordance with equation (10.20), the molar concentration of the equivalent c of the electrolyte in solution can be calculated if the specific electrical conductivity k and the equivalent electrical conductivity are known : c = 1000 kλ The electrical conductivity is determined experimentally on the basis of measuring the electrical resistance of a temperature-controlled conductometric cell. Equivalent electrical conductivity of a solution λ is equal to the sum of the cation mobilities λ+ and anion X λ -:
λ = λ + + λ-
If the mobilities of the cation and anion are known, then the concentration can be calculated using formula (24): c = 1000 kλ + + λ- This is done when determining the concentration of a sparingly soluble electrolyte in its saturated solution (calcium sulfate, barium sulfate; silver halides, etc.) by direct conductometry. Calibration curve method. A series of standard solutions are prepared, each of which contains a precisely known concentration of the analyte, and their electrical conductivity is measured at a constant temperature in a thermostatically controlled conductometric cell. Based on the data obtained, a calibration graph is built, plotting the concentration of reference solutions along the abscissa axis, and the values of electrical conductivity along the ordinate axis. In accordance with equation (24), the constructed graph in a relatively small range of concentrations is usually a straight line. In a wide range of concentrations, when the mobilities of the cation and anion included in Eq. (24) can change noticeably, deviations from the linear dependence are observed. Then, strictly under the same conditions, the specific electrical conductivity k(X) of the electrolyte being determined is measured in the analyzed solution with an unknown concentration c(X) and the desired value c(X) is found from the graph. So determine, for example, the content of barium in barite water - a saturated solution of barium hydroxide. Application of direct conductometry. The method of direct conductometry is characterized by simplicity and high sensitivity. However, the method is not selective. Direct conductometry is of limited use in analysis. It is used to determine the solubility of sparingly soluble electrolytes, to control the quality of distilled water and liquid food products (milk, drinks, etc.), to determine the total salt content in mineral, sea, river water, and in some other cases. 3 Conductometric titration In conductometric titration, the progress of the titration is monitored by a change in the electrical conductivity of the analyzed solution located in the conductometric cell between two inert electrodes (usually made of platinum platinum). Based on the data obtained, a conductometric titration curve is drawn, which reflects the dependence of the electrical conductivity of the titrated solution on the volume of the added titrant. The end point of the titration is most often found by extrapolating sections of the titration curve in the region of its slope change. This does not require the use of indicators that change color near the TE. In conductometric titration, various types of reactions are used: acid-base, redox, precipitation, complex formation processes. Application of conductometric titration. The conductometric titration method has a number of advantages. Titration can be carried out in cloudy, colored, opaque media. The sensitivity of the method is rather high - up to ~10~* mol/l; the determination error is from 0.1 to 2%. Analysis can be automated. The disadvantages of the method include low selectivity. The concept of high-frequency (radio-frequency) conductometric titration. The progress of the titration is followed by a modified alternating current conductometric technique, in which the frequency of the alternating current can reach the order of a million oscillations per second. Typically, electrodes are placed (overlaid) on the outside of the titration vessel (conductivity cell) so that they do not come into contact with the solution to be titrated. Based on the measurement results, a conductometric titration curve is plotted. The end point of the titration is found by extrapolating the segments of the titration curve in the region of its slope change. CHAPTER 4. CONDUCTOMETRIC ANALYSIS (CONDUCTOMETRY) 4.1 Essence of the method Polarographic analysis (polarography) is based on the use of the following relationships between the electrical parameters of an electrochemical (in this case, polarographic) cell, to which an external potential is applied, and the properties of the analyzed solution contained in it. a) Qualitative polarographic analysis uses the relationship between the value of the external electric potential applied to the microelectrode, at which reduction (or oxidation) of the analyte on the microelectrode is observed under given conditions, and the nature of the reducing (or oxidizing) substance. b) In quantitative polarographic analysis, the relationship between the magnitude of the diffusion electric current and the concentration of the analyte (reducing or oxidizing) substance in the analyzed solution is used. Electrical parameters - the magnitude of the applied electric potential and the magnitude of the diffusion current - are determined by analyzing the obtained polarization, or current-voltage, curves, which graphically reflect the dependence of the electric current in the polarographic cell on the magnitude of the applied potential of the microelectrode. Therefore, polarography is sometimes called direct voltammetry. The classical polarographic method of analysis using a mercury dripping (dropping) electrode was developed and proposed in 1922 by the Czech scientist Jaroslav Hejrovsky (1890-1967), although the mercury dripping electrode itself was used by the Czech physicist B. Kucera as early as 1903. In 1925 J. Geyrovsky and M. Shikata designed the first polarograph, which made it possible to automatically record polarization curves. Subsequently, various modifications of the polarographic method were developed. The value of the average diffusion current iD is determined by the Ilkovich equation (25): where K is the coefficient of proportionality, c is the concentration (mmol/l) of the polarographically active substance-depolarizer; iD is measured in microamps as the difference between the limiting current and the residual current. The coefficient of proportionality K in the Ilkovich equation depends on a number of parameters and is equal to K=607nD12m23τ16 where n is the number of electrons involved in the electrode redox reaction; D is the diffusion coefficient of the reducing substance (cm2/s); m is the mass of mercury flowing out of the capillary per second (mg); t is the time of formation (in seconds) of a drop of mercury at a half-wave potential (usually it is 3-5 s). Since the diffusion coefficient D depends on temperature, the coefficient of proportionality K in the Ilkovich equation also changes with temperature. For aqueous solutions in the temperature range of 20–50 °C, the diffusion coefficient of polarographically active substances-depolarizers increases by about 3% with a one degree increase in temperature, which leads to an increase in the average diffusion current iD by ~1–2%. Therefore, polarography is carried out at a constant temperature, thermostating the polarographic cell, usually at 25 ± 0.5 °C. The mass of mercury t and the dropping time t depend on the characteristics of the mercury dropping electrode and the height of the mercury column in the capillary and in the reservoir associated with the capillary. The glass capillary of a mercury dropping microelectrode usually has an outer diameter of 3-7 mm, an inner diameter of 0.03 to 0.05 mm, and a length of 6-15 cm. The height of the mercury column from the lower end of the capillary to the upper level of the mercury surface in the tank is 40-80 cm; The content of the indifferent electrolyte in the analyzed polarographic solution should be approximately 100 times higher than the content of the depolarizing substance to be determined, and the ions of the background electrolyte should not be discharged under the conditions of polarography until the discharge of the polarographically active substance. Polarography is carried out using water as a solvent, water-organic mixtures (water - ethanol, water - acetone, water - dimethylformamide, etc.) and non-aqueous media (ethanol, acetone, dimethylformamide, dimethyl sulfoxide, etc.). Prior to polarography, an inert gas current (nitrogen, argon, etc.) is passed through the analyzed solution to remove dissolved oxygen, which also gives a polarographic wave due to reduction according to the scheme: 2H+ + 2e = H202 H202 + 2H+ + 2e = 2H20 Sometimes, in the case of alkaline solutions, instead of passing an inert gas current, a small amount of an active reducing agent, sodium sulfite, metol, is added to the analyzed solution, which bind dissolved oxygen by reacting with it. 4.2 Quantitative polarographic analysis It follows from the foregoing that quantitative polarographic analysis is based on measuring the diffusion current iD as a function of the concentration of the polarographically active depolarizing substance to be determined in the polarographic solution. When analyzing the obtained polarograms, the concentration of the analyte is found by the methods of a calibration curve, standard additions, standard solutions. a) The calibration curve method is most often used. This method prepares a series of standard solutions, each of which contains a precisely known concentration of the analyte. Polarography of each solution is carried out (after an inert gas current is blown through it) under the same conditions, polarograms are obtained and the values of E12 (the same for all solutions) and the diffusion current iD (different for all solutions) are found. Based on the data obtained, a calibration graph is built in iD-c coordinates, which is usually a straight line in accordance with the Ilkovich equation. Then polarography of the analyzed solution with an unknown concentration c(X) of the analyte is carried out, a polarogram is obtained, the value of the diffusion current iD (X) is measured, and the concentration c(X) is found from the calibration curve. b) Standard addition method. A polarogram of the analyzed solution is obtained with an unknown concentration c(X) of the analyte and the magnitude of the diffusion current is found, i.e. height h of the polarogram. Then, a precisely known amount of the analyte is added to the analyzed solution, increasing its concentration by the value of c(st), polarography is carried out again and a new value of the diffusion current is found - the height of the polarogram h + △ h. In accordance with the Ilkovich equation (25), we can write: h = Kc(X), △ h = Kc(st), where h △ h = c(X)c(st) and c(X) = h △ hc(st) c) Method of standard solutions. Polarography of two solutions is carried out under the same conditions: an analyzed solution with an unknown concentration c(X) and a standard solution with a precisely known concentration c(st) of the analyte. On the obtained polarograms, the heights of the polarographic waves h(X) and h(st) are found, corresponding to the diffusion current at concentrations c(X) and c(st), respectively. According to the Ilkovich equation (25), we have: (X) = Kc(X), h(st) = Kc(st), The standard solution is prepared so that its concentration is as close as possible to the concentration of the solution to be determined. Under this condition, the determination error is minimized. 3 Application of polarography Application of the method. Polarography is used to determine small amounts of inorganic and organic substances. Thousands of techniques for quantitative polarographic analysis have been developed. Methods for the polarographic determination of almost all metal cations, a number of anions (bromate, iodate, nitrate, permanganate ions), organic compounds of various classes containing diazo groups, carbonyl, peroxide, epoxy groups, double carbon-carbon bonds, as well as bonds carbon-halogen, nitrogen-oxygen, sulfur-sulfur. The method is pharmacopoeial, used to determine salicylic acid, norsulfazol, vitamin B alkaloids, folic acid, kellin in powder and tablets, nicotinamide, pyridoxine hydrochloride, arsenic preparations, cardiac glycosides, as well as oxygen and various impurities in pharmaceutical preparations. The method has high sensitivity (up to 10"5-10T6 mol/l); selectivity; relatively good reproducibility of results (up to ~2%); a wide range of applications; allows the analysis of mixtures of substances without their separation, colored solutions, small volumes of solutions (polarographic cells can be as small as 1 ml); conduct analysis in the solution flow; automate the analysis." The disadvantages of the method include the toxicity of mercury, its rather easy oxidizability in the presence of oxidizing substances, and the relative complexity of the equipment used. Other variants of the polarographic method. In addition to the classical polarography described above, which uses a dropping mercury microelectrode with a uniformly increasing electric potential on it at a constant electric current, other versions of the polarographic method have been developed - derivative, differential, pulse, oscillographic polarography; alternating current polarography - also in different versions. CHAPTER 5. AMPEROMETRIC TITRATION The essence of the method. Amperometric titration (potentiostatic polarization titration) is a kind of voltammetric method (along with polarography). It is based on measuring the amount of current between the electrodes of an electrochemical cell, to which a certain voltage is applied, as a function of the volume of added titrant. In accordance with the Ilkovich equation (25): the diffusion current iD in the polarographic cell is the greater, the higher the concentration c of the polarographically active substance. If, when a titrant is added to the analyzed titrated solution located in a polarographic cell, the concentration of such a substance decreases or increases, then the diffusion current also decreases or increases accordingly. The equivalence point is fixed by a sharp change in the decrease or increase in the diffusion current, which corresponds to the end of the reaction of the titrated substance with the titrant. A distinction is made between amperometric titration with one polarizable electrode, also called current limit titration, polarographic or polarimetric titration, and amperometric titration with two identical polarizable electrodes, or titration "until the current is completely stopped", biamperometric titration. Amperometric titration with one polarizable electrode. It is based on measuring the current in a polarographic cell as a function of the amount of added titrant at a constant external potential on the microelectrode, slightly higher than the half-wave potential on the current-voltage curve of the titrated substance X or titrant T. Usually, the selected external potential corresponds to the region of the limiting current on the polarogram X or T The titration is carried out on an installation consisting of a direct current source with adjustable voltage, to which a galvanometer and a polarographic cell for titration are connected in series. The working (indicator) electrode of the cell can be a dropping mercury electrode, a fixed or rotating platinum or graphite electrode. When using solid electrodes, it is necessary to stir the solution during the titration. As a reference electrode, chlorine-silver or calomel electrodes are used. The background is, depending on the conditions, various polarographically inactive electrolytes at a given potential (HN03, H2S04, NH4NO3, etc.). First, current-voltage curves (polarograms) are obtained for X and T under the same conditions under which the amperometric titration is supposed to be carried out. Based on the consideration of these curves, the potential value is selected at which the limiting current of the polarographically active X or T is reached. The selected potential value is maintained constant throughout the entire titration process. The titrant concentration T used for amperometric titration should be approximately 10 times the concentration X; in this case, it is practically not necessary to introduce a correction for the dilution of the solution during the titration. Otherwise, all the conditions that are required to obtain polarograms are observed. The requirements for temperature control are less stringent than with direct polarography, since the end of the titration is determined not by the absolute value of the diffusion current, but by a sharp change in its value. The analyzed solution containing X is introduced into the polarographic cell, and the titrant T is added in small portions, each time measuring the current i. The value of the current i depends on the concentration of the polarographically active substance. At the equivalence point, the value of i changes sharply. Based on the results of amperometric titration, titration curves are built. The amperometric titration curve is a graphical representation of the change in the magnitude of the current / as a function of the volume V of the added titrant. The titration curve is constructed in the coordinates current i - the volume V of the added titrant T (or the degree of titration). Depending on the nature of the substance X being titrated and the titrant T, amperometric titration curves can be of various types. Biamperometric titration is carried out with vigorous stirring of the solution on an installation consisting of a direct current source with a potentiometer, from which an adjustable potential difference (0.05-0.25 V) is fed through a sensitive microammeter to the electrodes of the electrochemical cell. Before the titration, the titrated solution is introduced into the latter and the titrant is added in portions until the current stops abruptly or appears, as judged by the reading of the microammeter. The platinum electrodes used in the electrochemical cell are periodically cleaned by immersing them for ~30 minutes in boiling concentrated nitric acid containing ferric chloride additives, followed by washing the electrodes with water. Biamperometric titration - pharmacopoeial method; used in iodometry, nitritometry, aquametry, titration in non-aqueous media. CHAPTER 6. COULOMETRIC ANALYSIS (COULOMETRY) 1 Principles of the method electrochemical conductometry titration coulometry Coulometric analysis (coulometry) is based on using the relationship between the mass m of a substance that has reacted during electrolysis in an electrochemical cell and the amount of electricity Q that has passed through the electrochemical cell during the electrolysis of only this substance. In accordance with the combined law of electrolysis M Faraday, the mass m (in grams) is related to the amount of electricity Q (in coulombs) by the relation (27) where M is the molar mass of the substance that reacted during electrolysis, g/mol; n is the number of electrons involved in the electrode reaction; 96487 C/mol - Faraday number. The amount of electricity Q (in C) that passed through the electrochemical cell during electrolysis is equal to the product of electric current i (in A) and the time of electrolysis τ ( in c): If the amount of electricity Q is measured, then according to (27) it is possible to calculate the mass m. This is true in the case when the entire amount of electricity Q that passed through the electrochemical cell during electrolysis was spent only on the electrolysis of this substance; side processes should be excluded. In other words, the current output (efficiency) should be 100%. Since, in accordance with the combined law of electrolysis by M. Faraday, in order to determine the mass m (g) of the substance reacted during electrolysis, it is necessary to measure the amount of electricity Q spent on the electrochemical transformation of the substance being determined, in coulombs, the method is called coulometry. The main task of coulometric measurements is to determine the amount of electricity Q as accurately as possible. Coulometric analysis is carried out either in amperostatic (galvanostatic) mode, i.e. at a 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 the electrolysis time t (s), direct current / (A) as accurately as possible and calculate the value of Q using the formula (10.28). In the second case, the value of Q is determined either by calculation or by chemical coulometers. There are direct coulometry and indirect coulometry (coulometric titration). The essence of the method. Direct coulometry at direct current is rarely used. More often, coulometry is used at a controlled constant potential of the working electrode or direct potentiostatic coulometry. In direct potentiostatic coulometry, the directly determined substance is subjected to electrolysis. The amount of electricity spent on the electrolysis of this substance is measured, and the mass m of the substance to be determined is calculated using the equation. In the process of electrolysis, the potential of the working electrode is maintained constant, E=const, for which devices - potentiostats are usually used. The constant value of the potential E is preliminarily chosen based on the consideration of the current-voltage (polarization) curve plotted in the coordinates current i - potential E (as is done in polarography), obtained under the same conditions in which electrolysis will be carried out. Usually, the value of the potential E is chosen, corresponding to the area of \u200b\u200bthe limiting current for the substance being determined and slightly exceeding its half-wave potential E12 (by -0.05-0.2 V). At this potential value, as in polarography, the supporting electrolyte should not be subjected to electrolysis. 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 an exponential law from the initial value i0 at time t = O to the value i at time t: where the coefficient k 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. Methods for determining the amount of electricity passed through a solution in direct potepciostatic coulometry. The Q value can be determined by calculation methods or by using a chemical coulometer. a) Calculation of the value of Q from the area under the curve of dependence of i on m. To determine Q without a noticeable error, the method requires the almost complete completion of the electrolysis process, i.e. long time. In practice, as noted above, the area is measured at a value of m corresponding to 0.001i0 (0.1% of i0). b) Calculation of the value of Q based on the dependence of In / on m. In accordance, we have: Q = 0∞i0e-k τ d τ =i00∞e-k τ d τ =i0k Because the ∞i0e-k τ d τ = - k-1 e-k∞-e-k0= k-10-1=k-1 Application of direct coulometry. The method has high selectivity, sensitivity (up to 10~8-10~9 g or up to ~10~5 mol/l), reproducibility (up to ~1-2%), and makes it possible to determine the content of microimpurities. The disadvantages of the method include high complexity and duration of the analysis, the need for expensive equipment. Direct coulometry can be used to determine - in cathodic reduction - metal ions, organic nitro and halogen derivatives; during anodic oxidation - chloride-, bromide-, iodide-, thiocyanate anions, metal ions in lower oxidation states when they are transferred to higher oxidation states, for example: As (IH) -> As (V), Cr (II) - > Cr(III), Fe(II) -» Fe(III), T1(I) -> Tl(III), etc. In pharmaceutical analysis, direct coulometry is used to determine ascorbic and picric acids, novocaine, oxyquinoline, and in some other cases. As noted above, direct coulometry is rather laborious and lengthy. In addition, in some cases, side processes begin to noticeably proceed even before the completion of the main electrochemical reaction, which reduces the current efficiency and can lead to significant errors in the analysis. Therefore, indirect coulometry is often used - coulometric titration. 3 Coulometric titration The essence of the method. During coulometric titration, the analyte X, which is in solution in an 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, is fixed either visually by the indicator method, introducing into the solution the appropriate indicator that changes color near the TE, or using instrumental methods - potentiometrically, amperometrically, photometrically. Thus, in coulometric titration, the titrant is not added from the buret to the solution being titrated. The role of the titrant is played by substance T, which is continuously generated during the electrode reaction on the generator electrode. Obviously, there is an analogy between ordinary titration, when the titrant is introduced from the outside into the titrated solution and, as it is added, reacts with the analyte, and the generation of substance T, which, as it is formed, also reacts with the analyte. Therefore, the method under consideration was called "coulometric titration". Coulometric titration is carried out in amperostatic (galvanostatic) or potentiostatic mode. Most often, coulometric titration is carried out in an amperostatic mode, maintaining a constant electric current throughout the entire electrolysis time. Instead of the volume of added titrant in coulometric titration, the time t and current i of electrolysis are measured. The process of formation of substance T in a coulometric cell during electrolysis is called titrant generation. Coulometric titration at direct current. During coulometric titration in amperostatic mode (at direct current), the time t during which electrolysis was carried out is measured, and the amount of electricity Q consumed during electrolysis is calculated by the formula, after which the mass of the analyte X is found by the ratio. So, for example, the standardization of a solution of hydrochloric acid HC1 by coulometric titration is carried out by titrating hydrogen ions H30+ of a standardized solution containing HC1 with hydroxide ions OH- generated on a platinum cathode during the electrolysis of water: H20 + 2e = 20H- + H2 The resulting titrant - hydroxide ions - reacts with H30+ ions in solution: H30+ + OH- = 2H20 The titration is carried out in the presence of the phenolphthalein indicator and is stopped when a light pink color of the solution appears. Knowing the value of direct current i (in amperes) and the time t (in seconds) spent on titration, calculate the amount of electricity Q (in coulombs) using formula (28) and using formula (27) - the mass (in grams) of the reacted HC1 contained in an aliquot of the standardized HCl solution added to the coulometric cell (generator vessel). Conditions for coulometric titration. From the foregoing, it follows that the conditions for conducting coulometric titration should provide 100% current efficiency. To do this, at least the following requirements must be met. a) The auxiliary reagent, from which the titrant is generated on the working electrode, must be present in the solution in a large excess relative 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 supporting electrolyte, for example, hydrogen ions: H+ + 2e = H2 b) The value of direct current i=const during electrolysis must be less than the value of the diffusion current of the auxiliary reagent in order to avoid the reaction with the participation of background electrolyte ions. c) It is necessary to determine as accurately as possible the amount of electricity consumed during electrolysis, for which it is necessary to accurately record the beginning and end of the countdown and the magnitude of the electrolysis current. Coulometric titration at constant potential. Potentiostatic mode in coulometric titration is used less frequently. Coulometric titration in the potentiostatic mode is carried out at a constant potential value corresponding to the potential of the substance discharge at the working electrode, for example, during cathodic reduction of metal cations M "* on a 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 longer any potential-determining metal cations in the solution. Application of coulometric titration. In coulometric titration, all types of reactions of titrimetric analysis can be used: acid-base, redox, precipitation, complex formation reactions. So, small amounts of acids can be determined by coulometric acid-base titration with electrogenerated OH- ions formed during the electrolysis of water on the cathode: H20 + 2e = 20N "+ H2 Bases can also be titrated with H+ hydrogen ions generated at the anode during water electrolysis: H20-4e = 4H+ + 02 With redox bromometric coulometric titration, arsenic (III), antimony (III), iodides, hydrazine, phenols and other organic substances can be determined. Bromine electrogenerated at the anode acts as a titrant: VG -2e = Vg2 Precipitation coulometric titration can determine halide ions and organic sulfur-containing compounds by electrogenerated silver cations Ag+, zinc cations Zn2+ by electrogenerated ferrocyanide ions, etc. The complexometric coulometric titration of metal cations can be carried out with EDTA anions electrogenerated on a mercury(II) complexonate cathode. Coulometric titration has high accuracy, a wide range of applications in quantitative analysis, allows you to determine small amounts of substances, low-resistant compounds (since they react immediately after their formation), for example, copper (1), silver (H), tin (P), 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. CONCLUSION Electrochemical methods of analysis are based on the processes occurring on the electrodes or the interelectrode space. Electrochemical methods of analysis are among the oldest physical and chemical methods of analysis (some were described at the end of the 19th century). Their advantage is high accuracy and comparative simplicity of both the equipment and the analysis technique. High accuracy is determined by very precise laws used in electrochemical methods of analysis, for example, Faraday's law. The great convenience is that they use electrical influences, and that the result of this influence (response) is obtained in the form of an electrical signal. This provides high speed and accuracy of counting, opens up wide possibilities for automation. Electrochemical methods of analysis are distinguished by good sensitivity and selectivity; in some cases, they can be attributed to microanalysis, since sometimes less than 1 ml of solution is sufficient for analysis. Their instrument is an electrochemical cell, which is a vessel with an electrolyte solution, in which at least two electrodes are immersed. Depending on the problem being solved, the shape and material of the vessel, the number and nature of electrodes, solution, analysis conditions (applied voltage (current) and recorded analytical signal, temperature, mixing, inert gas purge, etc.) can be different. The substance to be determined can be included both in the composition of the electrolyte filling the cell and in the composition of one of the electrodes. Electrochemical methods of analysis play an important role in the modern world. Nowadays, care for the environment is especially important. Using these methods, it is possible to determine the content of a huge amount of various organic and inorganic substances. Now they are more effective for identifying hazardous substances.
1. Electrochemical methods of analysis
2. Potentiometry. Potentiometric titration
3. Conductometry. Conductometric titration
4.Coulometry. Coulometric titration
5. List of used literature
Electrochemical methods of analysis
Classification of electrochemical methods of analysis
Electrochemical methods are based on measuring the electrical parameters of electrochemical phenomena that occur in the test solution. Such a measurement is carried out using an electrochemical cell, which is a vessel with an investigated solution, in which electrodes are placed. Electrochemical processes in a solution are accompanied by the appearance or change in the potential difference between the electrodes or a change in the magnitude of the current passing through the solution.
Electrochemical methods are classified depending on the type of phenomena measured during the analysis. In general, there are two groups of electrochemical methods:
1. Methods without superimposing an extraneous potential, based on measuring 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. In potentiometric methods, the dependence of the equilibrium potential of the electrodes on the concentration of ions involved in the electrochemical reaction on the electrodes is used.
2. Methods with the imposition of an extraneous potential, based on the measurement of: a) the electrical conductivity of solutions - conductometry; b) the amount of electricity that has passed through the solution - coulometry; c) the dependence of the current on the applied potential - voltammetry; d) the time required for the passage of an electrochemical reaction - chronoelectrochemical methods(chronovoltammetry, chronoconductometry). In the methods of this group, an extraneous potential is applied to the electrodes of the electrochemical cell.
The main element of instruments for electrochemical analysis is an electrochemical cell. In methods without the imposition of an extraneous potential, it is 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, two electrodes are in contact with the analyzed solution - 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 superimposed extraneous potential, electrochemical cell, so named because electrolysis occurs on the electrodes of the cell under the action of the applied potential - the oxidation or reduction of a substance. Conductometric analysis uses a conductometric cell in which the electrical conductivity of a solution is measured. According to the method of application, electrochemical methods can be classified into direct methods, 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 fixed using electrochemical measurements. In accordance with this classification, there are potentiometry and potentiometric titration, conductometry and conductometric titration, etc.
Instruments for electrochemical determinations, in addition to the 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 by pointer instruments (voltmeter or microammeter), oscilloscopes, automatic recording potentiometers. If the electrical signal from the cell is very weak, then it is amplified with the help of radio amplifiers. In devices of methods with superimposed extraneous potential, an important part is the devices for supplying the cell with the appropriate potential of a stabilized direct or alternating current (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 stability of the instrument.
Potentiometry
Potentiometry is based on measuring the difference in electrical potentials that arise between dissimilar electrodes immersed in a solution with a substance to be determined. An electric potential arises at the electrodes when an oxidation-reduction (electrochemical) reaction passes through them. Redox reactions proceed between an oxidizing agent and a reducing agent with the formation of redox pairs, the potential E of which is determined by the Nernst equation by the concentrations of the components of the pairs [ox] and [rec]:
Potentiometric measurements are carried out by lowering two electrodes into the solution - an indicator electrode that reacts to the concentration of the ions being determined, and a standard or reference electrode, relative to which the indicator potential is measured. Several types of indicator and standard electrodes are used.
Electrodes of the first kind are reversible with respect to the metal ions of which the electrode consists. When such an electrode is lowered into a solution containing metal cations, an electrode pair is formed
/M .Electrodes of the second kind are sensitive to anions and represent a metal M coated with a layer of its insoluble salt MA with an anion
to which the electrode is sensitive. When such an electrode comes into contact with a solution containing the indicated anion, a potential E arises, the value of which depends on the product of the solubility of the salt and the concentration of the anion in the solution.Electrodes of the second kind are silver chloride and calomel. Saturated silver chloride and calomel electrodes maintain a constant potential and are used as reference electrodes, against which the potential of the indicator electrode is measured.
Inert electrodes- a plate or wire made of difficult-to-oxidize metals - platinum, gold, palladium. They are used to measure E in solutions containing a redox pair (for example,
/).Membrane electrodes different types have a membrane on which the membrane potential E arises. The value of E depends on the difference in concentrations of the same ion on different sides of the membrane. The simplest and most widely used membrane electrode is the glass electrode.
Mixing insoluble salts such as AgBr, AgCl, AgI and others with some plastics (rubber, polyethylene, polystyrene) led to the creation ion-selective electrodes on the
, , selectively adsorbing these ions from the solution due to the Panet-Fajans-Han rule. Since the concentration of the ions to be determined outside the electrode differs from that inside the electrode, the equilibria on the membrane surfaces differ, which leads to the appearance of a membrane potential.To carry out potentiometric determinations, an electrochemical cell is assembled from an indicator reference electrode, which is lowered into the analyzed solution and connected to a potentiometer. The electrodes used in potentiometry have a large internal resistance (500-1000 MΩ), so there are types of potentiometers that are complex electronic high-resistance voltmeters. To measure the EMF of the electrode system in potentiometers, a compensation circuit is used to reduce the current in the cell circuit.
Most often, potentiometers are used for direct measurements of pH, concentrations of other ions pNa, pK, pNH₄, pCl and mV. Measurements are made using appropriate ion-selective electrodes.
A glass electrode and a silver chloride reference electrode are used to measure pH. Before carrying out analyzes, it is necessary to check the calibration of pH meters using standard buffer solutions, the fix channels of which are applied to the device.
pH meters, in addition to direct determinations of pH, pNa, pK, pNH₄, pCl and others, allow potentiometric titration of the ion to be determined.
Potentiometric titration
Potentiometric titration is carried out in cases where chemical indicators cannot be used or in the absence of a suitable indicator.
In potentiometric titration, potentiometer electrodes dipped into the titrated solution are used as indicators. In this case, electrodes sensitive to the titratable ions are used. In the process of titration, the concentration of ions changes, which is recorded on the scale of the measuring probe of the potentiometer. Having recorded the readings of the potentiometer in units of pH or mV, they build a graph of their dependence on the volume of titrant (titration curve), determine the equivalence point and the volume of titrant used for titration. Based on the data obtained, a potentiometric titration curve is built.
The potentiometric titration curve has a form similar to the titration curve in titrimetric analysis. The equivalence point is determined from the titration curve, which is in the middle of the titration jump. To do this, draw tangents to sections of the titration curve and determine the equivalence point in the middle of the tangent of the titration jump. The change in ∆рН/∆V acquires the greatest value at the equivalence point.
Ryazan State Technological College
Course work
by discipline
« Technical measurements and their metrological support»
Theme of the course work: "Electrochemical methods for studying the composition of matter"
performed:
group student №158
Kharlamova Anastasia Igorevna
checked:
coursework supervisor
Chekurova Natalya Vladimirovna
Ryazan 2011
CONTENT
INTRODUCTION 2
- THEORETICAL PART 3
- 1.1 General characteristics of physical and chemical methods of analysis 3
- 1.3
Classification of electrochemical methods of analysis 5
- CONCLUSION 21
REFERENCES 22
INTRODUCTION
Modern branches of production and social life of people pose their own specific tasks for physical and chemical methods of analysis for product quality control. One of the main physical and chemical methods of analysis are electrochemical methods of analysis.
These methods can quickly and fairly 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 product quality results 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 designed for electrochemical analysis is relatively cheap, readily available, 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 course work is to study electrochemical methods for studying the composition of a substance.
To achieve this goal, the following tasks were formulated:
- consider electrochemical methods of analysis, their classification and importance in the product quality control system;
- To study the method of potentiometric titration;
- Determine the acidity of the jam.
- THEORETICAL PART
- 1.1
General characteristics of physicochemical methods of analysis
The main working tools of analytical chemistry are physical and chemical methods of analysis. They are based on the registration of analytical signals, the appearance of which depends on the physicochemical properties of the substance, its nature and content in the analyzed product.
Modern branches of production and social life of people pose their own specific tasks for physical and chemical methods of analysis for product quality control.
In physicochemical methods of quantitative analysis, 3 groups are distinguished:
Figure 1 - Classification of physical and chemical methods of quantitative analysis
1) Optical methods are based on the interaction of electromagnetic radiation with matter. These include: polarimetry, spectrometry, refractometry, photocolometry, etc.
2) Electrochemical methods are based on the study of processes occurring on the electrode surface or in the near-electrode space. This group of methods includes: conductometry, voltammetry, potentiometry and others.
3) Chromatographic methods are based on the distribution of one of several substances between two, as they say, phases (for example, between a solid and a gas, between two liquids, etc.), and one of the phases is constantly moving, that is, it is mobile. There are gas-liquid, liquid and ion methods for assessing the quality of food products.
Chromatographic and electrochemical methods of analysis have been widely used in product quality control.
1.2Characterization of electrochemical methods
Electrochemical methods are based on measuring the electrical parameters of electrochemical phenomena that occur in the test solution. Electrochemical methods are classified depending on the type of phenomena measured during the analysis. In general, two groups of electrochemical methods are distinguished (Figure 2):
Figure 2 - classification of electrochemical methods of analysis, depending on the type of phenomena measured during the analysis
Methods without superimposing an extraneous potential, based on measuring 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. In potentiometric methods, the dependence of the equilibrium potential of the electrodes on the concentration of ions involved in the electrochemical reaction on the electrodes is used.
Imposed extraneous potential methods based on measurement:
a) electrical conductivity of solutions - conductometry;
b) the amount of electricity passed through the solution - coulometry;
c) the dependence of the magnitude of the current on the applied potential - volt-amperometry;
d) the time required for the passage of an electrochemical reaction - chronoelectrochemical methods (chronovoltammetry, chronoconductometry). In the methods of this group, an extraneous potential is applied to the electrodes of the electrochemical cell.
The main element of instruments for electrochemical analysis is an electrochemical cell. In methods without the imposition of an extraneous potential, it is a galvanic cell in which an electric current arises due to the occurrence of chemical redox reactions. In a cell of the type of a galvanic cell, two electrodes are in contact with the analyzed solution - 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 measurement of the potential difference is carried out with special devices - potentiometers.
- 1.3
Classification of electrochemical methods of analysis
Figure 3-Basic Electrochemical Analysis Methods
Some electrochemical methods are divided into two types of analysis: direct and indirect (Figure 4)
Figure 4- types of electrochemical analysis
- conductometric method.
In the conductometric method, two types of direct analysis are distinguished - conductometry and indirect - conductometric titration (Figure 4)
Figure 5 – Methods of conductometric analysis.
Conductometry is based on measuring the electrical conductivity of a solution. The analysis is carried out using conductometers - devices that measure the resistance of solutions. The value of the resistance R determines the electrical conductivity of the solutions L, which is opposite to it in magnitude.
Direct conductometry is used to determine the concentration of a solution from a calibration curve. To compile a calibration graph, the electrical conductivity of a series of solutions with a known concentration is measured and a calibration graph of the dependence of electrical conductivity on concentration is built. Then the electrical conductivity of the analyzed solution is measured and its concentration is determined from the graph.
Conductometric titrations are most commonly used. At the same time, the analyzed solution is placed in the cell with electrodes, the cell is placed on a magnetic stirrer and titrated with the appropriate titrant. The titrant is added in equal portions. After adding each portion of the titrant, the electrical conductivity of the solution is measured and a graph is plotted between the electrical conductivity and the volume of the titrant. When a titrant is added, the electrical conductivity of the solution changes, i.e. there is an inflection of the titration curve. The electrical conductivity of the solution depends on the mobility of the ions: the higher the mobility of the ions, the greater the electrical conductivity of the solution.
Conductometric titration has several advantages. It can be carried out in turbid and colored environments, in the absence of chemical indicators. The method is highly sensitive and allows the analysis of dilute solutions of substances (up to mol/dm). Conductometric titration analyzes mixtures of substances, because differences in the mobility of various ions are significant and they can be differentially titrated in the presence of each other.
- Potentiometric method of analysis
- The potentiometric method is a method of qualitative and quantitative analysis based on the measurement of potentials that occur between the test solution and an electrode immersed in it.
Figure 6 - Methods of potentiometric titration
Potentiometry is based on measuring the difference in electrical potentials that arise between dissimilar electrodes immersed in a solution with a substance to be determined. An electric potential arises at the electrodes when a redox (electrochemical) reaction passes through them. Redox reactions proceed between an oxidizing agent and a reducing agent with the formation of redox pairs, the potential E of which is determined by the Nernst equation by the concentrations of the components of the pairs.
Potentiometric measurements are carried out by lowering two electrodes into the solution - an indicator electrode that reacts to the concentration of the ions being determined, and a reference electrode, relative to which the indicator potential is measured. Several types of indicator electrodes and reference electrodes are used.
The electrodes of the first kind are reversible with respect to the metal ions of which the electrode consists. When such an electrode is lowered into a solution containing metal cations, an electrode pair is formed.
Electrodes of the second kind are sensitive to anions and are a metal coated with a layer of its insoluble salt with an anion, to which the electrode is sensitive. Upon contact of such an electrode with a solution containing the indicated anion, a potential E arises, the value of which depends on the product of the solubility of the salt and the concentration of the anion in the solution.
Electrodes of the second kind are silver chloride and calomel.Saturated silver chloride and calomel electrodes maintain a constant potential and are used as reference electrodes, against which the potential of the indicator electrode is measured.
Inert electrodes - a plate or wire made of hard-to-oxidize metals - platinum, gold, palladium. They are used to measure E in solutions containing a redox pair.
Membrane electrodes of various types have a membrane on which the membrane potential E arises. The value of E depends on the difference in concentrations of the same ion on different sides of the membrane. The simplest and most widely used membrane electrode is the glass electrode.
The electrodes used in potentiometry have a high internal resistance (500-1000 MΩ), so the existing types of potentiometers are complex electronic high-resistance voltmeters. To measure the EMF of the electrode system in potentiometers, a compensation circuit is used to reduce the current in the cell circuit.
Most often, potentiometers are used for direct measurements of pH, concentrations of other ions pNa, pK, pNH, pCl and mV. Measurements are made using appropriate ion-selective electrodes.
To measure pH, which characterizes the concentration of hydrogen ions in solutions, drinking water, food products and raw materials, environmental objects and production systems for continuous monitoring of technological processes, including in aggressive environments, special devices are used, which are called pH meters (Figure 6). They are a glass electrode and a reference electrode - silver chloride. Before carrying out analyzes, it is necessary to check the calibration of pH meters using standard buffer solutions, the fix channels of which are applied to the device.
Figure 7- pH meter
The action of the pH meter is based on measuring the value of the EMF of the electrode system, the indicators of which are proportional to the activity of hydrogen ions in the solution - pH (its hydrogen index). To control and adjust the modes of the pH meter, a remote control connected to the electronic conversion unit is used. pH meters, in addition to direct determinations of pH, pNa, pK, pNH, pCl and others, allow potentiometric titration of the ion to be determined
Measurement errors of pH meters:
1) measurement errors of EMF, temperature.
2) calibration error, which includes the BR error together with the instrument error;
3) random component of the measurement error.
In addition to the instrumental error, there is an error in the measurement technique.
Two main settings are made during calibration - the gain and offset of the inverting amplifier are set.
etc.................
1. Electrochemical methods of analysis are based on the use of the electrochemical properties of the analyzed substances. These include the following methods.
An electrogravimetric method based on accurate measurement of the mass of the analyte or its constituents, which are released on the electrodes when a direct electric current passes through the analyzed solution.
A conductometric method based on measuring the electrical conductivity of solutions, which changes as a result of ongoing chemical reactions and depends on the properties of the electrolyte, its temperature and the concentration of the solute.
A potentiometric method based on measuring the potential of an electrode immersed in a solution of the test substance. The electrode potential depends on the concentration of the corresponding ions in the solution under constant measurement conditions, which are carried out using potentiometers.
Polarographic method based on the use of the phenomenon of concentration polarization that occurs on an electrode with a small surface when an electric current is passed through the analyzed electrolyte solution.
A coulometric method based on measuring the amount of electricity used for the electrolysis of a certain amount of a substance. The method is based on Faraday's law.
2. Optical methods of analysis are based on the use of the optical properties of the compounds under study. These include the following methods.
Emission spectral analysis based on the observation of line spectra emitted by vapors of substances when they are heated in a gas burner flame, spark or electric arc. The method makes it possible to determine the elemental composition of substances.
Absorption spectral analysis in the ultraviolet, visible and infrared regions of the spectrum. There are spectrophotometric and photocolorimetric methods. The spectrophotometric method of analysis is based on measuring the absorption of light (monochromatic radiation) of a certain wavelength, which corresponds to the maximum absorption curve of a substance. The photocolorimetric method of analysis is based on measuring light absorption or determining the absorption spectrum in photocolorimeter devices in the visible part of the spectrum.
Refractometry based on the measurement of the refractive index.
Polarimetry based on the measurement of the rotation of the plane of polarization.
Nephelometry based on the use of the phenomena of reflection or scattering of light by uncolored particles suspended in a solution. The method makes it possible to determine very small amounts of a substance in solution in the form of a suspension.
Turbidimetry based on the use of the phenomena of reflection or scattering of light by colored particles that are in suspension. The light absorbed by the solution or passed through it is measured in the same way as in the photocolorimetry of colored solutions.
Luminescent or fluorescent analysis based on the fluorescence of substances that. exposed to ultraviolet light. This measures the intensity of emitted or visible light.
The design of the devices provides for equalizing the intensity of two light fluxes using an adjusting diaphragm. With the same illumination of both photocells, the currents from them in the galvanometer circuit are mutually compensated and the galvanometer needle is set to Zero. When one photocell is darkened by a cuvette with a colored solution, the galvanometer needle will deviate by an amount proportional to the concentration of the solution. The zero position of the galvanometer pointer is restored by darkening the second photocell with a calibration diaphragm. The shape and design of diaphragms can be varied. For example, FEK-56 photoelectric colorimeters use a cat-eye sliding diaphragm. The cat's eye diaphragm consists of sickle-shaped segments that move and move apart, thereby changing the diameter of the holes through which light passes.
The diaphragm, located in the right beam of light of the colorimeter, changes its area and the intensity of the light flux incident on the right photocell when the drum associated with it rotates. The sliding diaphragm, located in the left beam, serves to attenuate the intensity of the light flux incident on the left photocell. The right light beam is measuring, the left one is compensation.
CHROMATOGRAPHIC METHODS. CLASSIFICATION OF METHODS
The separation and analysis of substances by chromatographic methods are based on the distribution of substances between two phases, one of which is stationary (stationary), and the other is mobile, moving along the first. Separation occurs if the stationary phase exhibits different sorption capacity for ions or molecules of the mixture being separated. Typically, the stationary phase is a sorbent with a developed surface, and the mobile phase is a liquid or gas flow.
Chromatographic methods are classified according to several parameters: a) according to the mechanism of separation of the components of the analyzed mixture (adsorption, distribution, ion exchange, sedimentation, etc.); b) according to the state of aggregation of the mobile phase (gas, liquid); c) according to the type of the stationary phase and its geometric arrangement (column, thin-layer, paper chromatography); d) according to the method of moving the mixture to be separated in the column (eluent, frontal, displacement).
In the simplest version, chromatography is carried out on columns in which a sorbent is placed, which serves as a stationary phase. A solution containing a mixture of substances to be separated is passed through a column. The components of the analyzed mixture move through the stationary phase together with the mobile phase under the action of gravity or under pressure. The separation is carried out due to the movement of the components of the mixture at different speeds due to their interaction with the sorbent. As a result, substances are distributed on the sorbent, forming adsorption layers called zones. Depending on the purpose of separation or analysis, there may be different options for subsequent processing. The most common method is eluted. A suitable solvent is passed through a column with substances adsorbed on it - an eluent that elutes one or more adsorbed components from the column; they can then be determined in the resulting solution - the eluate. It is possible to pass a developer reagent through the column, due to which the sorbed substances become visible, i.e. the sorbent layer with the retained substance acquires a certain color. A developed chromatogram is obtained, which makes it possible to draw conclusions about the composition of the mixture without additional qualitative reactions.
Key parameters in chromatographic methods: retention characteristics, efficiency and degree of separation.
Retention volume and retention time are the volume of eluent and the time required to remove a given substance from the column. These values depend on the properties of the sorbent, the rate of movement of the mobile phase and its volume, and also on the distribution coefficient Kp:
Kr \u003d S TV / Szh,
where Сtv is the total concentration of the dissolved substance in the stationary phase; Cg is the concentration of the substance in the mobile phase. By measuring the relative retention values, the components to be separated can be identified.
To evaluate the efficiency of separation on a column, the concept of theoretical plates is introduced. The sorbent layer in the column is conventionally divided into a number of contiguous narrow horizontal layers, each of which is called a theoretical plate. In each layer, an equilibrium is established between the stationary and mobile phases. The greater the number of theoretical plates, the higher the separation efficiency. Another value characterizing the separation efficiency is the height equivalent to the theoretical plate, which is the ratio H= L/N, where L is the length of the column; N is the number of theoretical plates.
The degree of separation of two components 1 and 2 is determined by the separation criterion R, which depends on the retention time (ti) and the width of the zones occupied by the components on the sorbent (∆ti):
R1,2=2 (t2‑t1)/(∆t2+∆t1)
The components are separated if R2,1≥1 and are not separated if R2,1=0.
In the course of chemical methods of analysis, they study ion-exchange chromatography and chromatography on paper, other chromatographic methods - in the course of physicochemical methods of analysis.
ION EXCHANGE CHROMATOGRAPHY
Ion-exchange chromatography is based on the reversible stoichiometric exchange of ions of the analyzed solution for mobile ions - counterions of sorbents, called ion exchangers (or ion exchangers). Natural or synthetic resins are used as ion exchangers - solid, water-insoluble macromolecular acids and their salts containing active groups in their composition. Ion exchangers are divided into cation exchangers RSO 3 -H + (where R is a complex organic radical), capable of exchanging a hydrogen ion for cations, and anion exchangers RNHz + OH-, capable of exchanging an OH group - for anions. Cation exchange scheme:
RSO3‑H+ + M+ ↔ RSO 3 -M+ + H+
Anion exchange scheme:
RNH3 + OH - + A- ↔ RNH 3 + A- + OH-
The technique for performing ion exchange is most often columnar. In the dynamic version, the column is filled with an ion exchanger and the analyzed solution is passed through it at a certain rate.
For the purposes of qualitative analysis, methods have been developed for the isolation and detection of all the most important inorganic ions and many organic compounds, and partial and complete analysis of a mixture of cations and anions has been developed.
Sorption of ions depends on the nature and structure of the ion exchanger, the nature of the analyzed substances, the conditions of the experiment (temperature, pH, etc.). For most practical calculations, it can be assumed that the equilibrium between the ion exchanger and the solution obeys the law of mass action.
CHROMATOGRAPHY ON PAPER
Chromatography on paper does not require expensive equipment and is extremely simple to perform. This method combines separation with simultaneous detection or identification of substances. Paper holds water in the pores - an immobile solvent. The substances deposited on the chromatographic paper pass into the mobile phase and, moving at different speeds through the capillaries of the paper, are separated. The ability of substances to separate is estimated by the coefficient Rf, which is the ratio of the displacement of the zone of the substance h to the displacement of the solvent front H: Rf = h/H
The numerical values of Rf depend on the nature of the mobile and stationary phases, the distribution coefficient, and the type of chromatographic paper. Experimental conditions are essential for efficient separation.
BASIC CONCEPTS OF TITROMETRY. TITRATION METHODS
Titrimetric methods of analysis are based on recording the mass of the reagent consumed for the reaction with the analyte. The reagent (titrant) is added to the analyzed solution either in solid form (powder, tablets, paper impregnated with the reagent), or most often in the form of a solution with a precisely known concentration of the reagent. It is possible to measure the mass of the consumed titrant by weighing the vessel with the test solution and the added reagent (gravimetric titration), or the volume of the titrant used for titration. In the latter case, the mass of the titrant is expressed in terms of its volume according to the formulas
m=TV and m=CnVE/1000,
where T is the titer of the titrant solution; g/cm 3 ; V is the volume of the titrant solution, cm3; Cn is the normal concentration of the titrant solution, mol/dm3; E is the titrant equivalent.
The titrant is added to a precisely measured volume of the analyzed solution in small portions. After adding each new portion of the titrant, an equilibrium is established in the system described by the chemical reaction equation, for example
where A is the analyzed substance; B-titrant; ha, t are stoichiometric coefficients. As the reaction proceeds, the equilibrium concentrations of the analyte and titrant decrease, while the equilibrium concentrations of the reaction products increase. When an amount of titrant equivalent to the amount of the titrated substance is consumed, the reaction will end. This moment is called the equivalence point. In practice, the end point of the reaction is fixed, which, with some degree of approximation, corresponds to the equivalence point. In chemical methods of analysis, it is fixed visually by a noticeable analytical effect (change in the color of the solution, precipitation) caused by any of the starting compounds, reaction products, or substances specially introduced into the system - indicators. In physicochemical methods of analysis, the end point is determined by a sharp change in the measured physical parameter - pH, potential, electrical conductivity, etc.
In titrimetry, there are direct, back and indirect titrations.
In the direct titration method, the component A to be determined directly reacts with the standard solution B. If such a reaction is impossible for some reason, then back or indirect titration is used. To do this, an auxiliary reagent is added to the analyzed substance - a secondary standard that reacts with the component being determined. In the back titration method, B is taken in excess, and the unreacted residue is titrated with a secondary standard. In indirect titration methods, the reaction product reacts with the standard solution (substituent titration).
Titration methods
In some cases, a so-called reverse titration is carried out, in which a standard solution of a reagent is titrated with the analyzed solution. It is usually used when the analyte is unstable in air. When analyzing mixtures of substances, it is possible to combine different titration methods.
The process of any measurement consists in comparing the selected parameter of the object with a similar parameter of the standard. In titrimetric analyses, solutions with precisely known concentrations (titer, normality) of the component to be determined serve as standards. Such solutions are called standard (titrated). They can be prepared in several ways: 1) by accurately weighing the starting material; 2) by an approximate sample with subsequent determination of the concentration according to the primary standard; 3) dilution of a pre-prepared solution with a known concentration; 4) by fixed channel; 5) ion exchange.
In the first method, only chemically pure stable compounds can be used as starting materials, the composition of which strictly corresponds to the chemical formula, as well as easily cleaned substances. In the second method, it is necessary to have a primary standard - a chemically pure compound of a precisely known composition that meets the following requirements.
2. Stability in air, standard solutions should not change the titer during storage.
3. Large molecular weight to minimize weighing errors.
4. Good solubility, fast reaction with a solution of a substance whose concentration is being determined.
5. The equivalent point must be defined accurately and clearly.
Establishing titers of solutions - standardization - can
be carried out by gravimetric and volumetric methods. In the latter, the title is installed faster, so it is mainly used. An accurately weighed primary standard (single-weight method) or a primary standard solution (pipetting method) is titrated with the standardized solution. The correctness of the titer setting is checked by calculating the systematic error of the titer setting.
For each titrimetric method, methods have been developed for standardizing the titrants used, and recommendations are given for choosing primary standards. It must be remembered that the characteristics of standard solutions must be determined with the required accuracy. The titer, molarity and normality are determined to the fourth significant figure, not counting the zeros after the decimal point (for example, ТNaon = 0.004014 g/cm 3 ; Skmno 4 = 0.04995 n).
CLASSIFICATION OF TITRIMETRY METHODS
Titrimetric methods are divided into four large groups according to the type of reaction underlying the method. In each of these groups, there are particular methods associated with the use of a particular titrant. As follows from the table, the largest group consists of redox titration methods. This includes (in addition to those indicated in the table) also chromatometry (standard solution - K2Cr2O7), cerimetry (standard solutions containing Ce 4+), bromatometry (KBrO 3), vanadatometry (NH 4 VOz), ascorbinometry (standard solution - ascorbic acid) and etc. In the group of complexometric methods, complexometry (titrant - EDTA, or Trilon B, or complex III) has the greatest use so far, but the number of complexones used in analytical practice is constantly increasing. The methods of precipitation titration, on the contrary, tend to be gradually eliminated from practice. The reason is obviously that although the precipitation reactions are very numerous, in many cases it is difficult to fix the end point of the titration. Methods of argentometry, rhodanometry and mercurymetry are well developed, but they are suitable for determining a small number of ions, besides, silver is a valuable metal, and mercury salts are poisonous. A determination method based on the precipitation of sparingly soluble sulfates is proposed.
Acid-base titrations are becoming more and more popular. This is due to the ever-expanding use in practice of non-aqueous solvents that change the acid-base properties of substances.
Advantages of titrimetric methods of analysis: the speed of determination and the simplicity of the equipment used, which is especially convenient when performing serial analyses. The sensitivity threshold of these methods is about 10~3 mol/dm 3 , or 0.10%; accuracy ~0.5% (rel.). These figures depend on the sensitivity of the indicators used and the concentration of the reacting solutions.
ACCURACY AND SCOPE OF COLORIMETRIC DETERMINATIONS
Colorimetric methods are often used for the analysis of small quantities. The determination is carried out quickly, and such quantities of a substance are determined with greater accuracy, which are practically impossible to detect by gravimetric and titrimetric analysis methods, since to obtain the required concentration in the solution it would be necessary to take too much of the substance under study.
Colorimetric methods are used to solve problems of technological control, so that on the basis of their data it is possible to regulate the technological chemical process; in sanitary and hygienic analysis for the determination of ammonia, fluorine, nitrites and nitrates, iron salts in water, vitamins in food, in clinical laboratories for the quantitative determination of iodine, nitrogen, bilirubin and cholesterol in the blood and bile, hemoglobin in the blood, etc. .
AIR ANALYSIS
The main source of pollution of the air basin of cities are the harmful components contained in the products! combustion. These include: ash, solid fuel particles, mechanical impurities; oxides of sulfur, nitrogen, lead; carbon monoxide; products of incomplete combustion. In most modern production processes, technological cycles do not clean up emissions. According to M.A. Styrikovich, in the world, annual emissions of solid substances amount to 100, 5O2–150, CO-300, nitrogen oxides - 50 million tons. When burning solid and liquid fuels, aromatic carcinogenic hydrocarbons are formed! one of which - 3.4 - benzpyrene C2oH1 2, present in soil, air and water (maximum permissible concentration 0.00015 mg / dm 3).
The main air emissions from the chemical industry:
nitric acid - N0, N02, N43
hydrochloric acid - HC1, C1 2 sulfuric acid obtained
nitrous method - N0, N02, ZO 2, 8Oz, H 2 5O, Re 2 Oz (dust)
by contact method - 5O 2, 5Oz, H 2 5O4, Re 2 Oz (dust) of phosphorus and
phosphoric acid - P 2 Ob, HzPO4, HP, Ca5F (PO4) s (dust)
acetic acid - CH3CHO, CH3COOH
complex fertilizers - N0, N02, NHz, HP, H 2 5O4, P 2 Oa, HNOz, fertilizer dust
calcium chloride - HC1, H 2 5O4, CaC1 2 (dust) liquid chlorine - HC1, C1 2, Hg
methanol - CH 3 OH, caprolactam CO - N0, N02, 5O 2, H 2 5, acetylene CO - C2H 2 carbon black of artificial fibers - H 2 5, C5 2, etc.
To reduce air pollution, you need to create conditions for the complete combustion of fuel, which you achieve by burning at a high temperature. In this case, it increases the content of nitrogen oxides, which are more toxic than CO. Therefore, new ways of burning are being sought. In one of them, proposed by A.K. Vnukov, use a flameless combustion furnace with full premix burners to suppress the formation of nitrogen oxides. The gas-air mixture is burned in a layer of crushed refractory, which contains heat-receiving surfaces that reduce the temperature in the furnace. It is also possible to reduce air pollution by directing polluted air or products of incomplete combustion into the furnaces of furnace boilers. Replacing the air supplied to the furnaces with polluted air allows, in addition to everything, to reduce fuel consumption by -10%.
The analysis of gas mixtures is carried out by various methods.
The organoleptic method is based on the determination of impurities by color and smell by a person and gives only an approximate idea of the composition of the mixture. The smell is hydrogen sulfide, chlorine, ammonia, sulfur dioxide, phosphorus oxides, hydrocarbons and many organic substances. Colored gases - fluorine, chlorine, nitrogen dioxide.
Qualitative analysis can be carried out using filter papers impregnated with the appropriate reagent. They change their color in the presence of certain gases.
Indication with liquid or porous absorbers. Air is passed through vessels with a special liquid or through porous absorbers (pumice, alumogel, silica gel) treated with reagents. A change in color or cloudiness of solutions indicates impurities in the air. In the general analysis of gas mixtures, the qualitative and quantitative composition is determined.
Gravimetric analysis is based on the release of a constituent part of the gas in the form of a precipitate by carrying out chemical reactions. The precipitate is washed, filtered, dried (or calcined), weighed. An increase in the mass of the solution after passing the analyzed gas through it also makes it possible to judge the content of impurities.
It is possible to determine the constituent part of a mixture of gases by titration with special reagents using the reaction of neutralization, oxidation - reduction, precipitation, complexation.
To accurately determine the concentration of any component in a gas mixture, it is important to take a sample for analysis correctly. If the determined component of air is a gas or vapor, then it is passed through an absorbing liquid, where the substance is dissolved. If the substance to be determined is a liquid, then solid absorbers are used, as a result of which the particles are enlarged and adsorbed. Solid impurities and dust are retained by solid absorbing media (AFA filters, etc.). Large volumes of gases are taken with calibrated gas meters. Currently, devices for automatic sampling are being produced. Below are the maximum permissible concentrations (MPC) for various substances in the air of the working area (GOST 12.1.005–76).
| Substances | MPC. mg/mM |
| Acetone | 200 |
| Solvent gasoline (in terms of C) | 300 |
| Fuel gasoline (in terms of C) | 100 |
| Mercury metal | 0,01 |
| Lead and its inorganic compounds | 0,01 |
| Sulfuric acid | 1 |
| carbon monoxide | 20 |
| Caustic alkali solutions (in terms of MaOH) | 0,5 |
| Dust containing silicon dioxide, more than 70% | 1 |
| Formaldehyde (aerosols) | 0,5 |
| Phenol (in vapor) | 0,3 |
In the study of atmospheric air, the most reliable data are obtained if sampling is short. The duration of sampling for most harmful substances is set at 20–30 minutes. It is known that the concentration of a harmful substance in this case is averaged and 3 times less real than when sampling for 2-5 minutes. There are specific recommendations for taking an air sample, taking into account the distance to the source of air pollution. For example, in the study of atmospheric air for races "; standing 3 km from the source of pollution, the sample is taken for 4-5 minutes with a liquid "Richter absorber model 7 R with an aspiration rate of 20 dm 3 / min, and at a distance of up to 10 km - 2-3 minutes with a Richter absorber 10 R at a speed of 50 dm 3 / min.
The sample must contain such an amount of the test substance in the air that it is sufficient for determination by the selected method. Too much air leads to averaging of the analysis results, and if the volume is insufficient, the accuracy of the analysis is reduced.
SOIL ANALYSIS
The task of chemical analysis of soils is to obtain their chemical characteristics for solving theoretical and practical issues of agriculture, determining the genesis and properties of soils and agrotechnical measures to increase their fertility.
The extraction of the studied compounds from the soil for their chemical analysis is carried out using various extracts (water, saline, acidic or alkaline). In some cases, the soil is decomposed by fusing small portions with carbonates, treatment with hydrofluoric (hydrofluoric) acid, or wet burning with other acids (HC1 + HNO 3 , HNYO 3 + H 2 5O 4). Most analyzes are carried out with soil samples in an air-dry state, ground in a mortar and sieved through a sieve with holes of 1 mm in diameter.
To do this, a soil sample of 500–1000 g is spread in a thin layer on a sheet of paper and dried in air in a clean and dry room. Large pieces of soil are crushed by hand and roots, stones, etc. are removed. Organic residues are conveniently removed with an electrified glass rod to which they adhere. A part of the sample is weighed on a technical scale for the subsequent selection of an average sample. Some analyzes require soil samples that have just been taken from the field without prior drying, such as when determining nitrates. It is better to take an average sample by quartering. The sifted soil is stored in corked jars, cardboard boxes or paper bags.
To prepare an aqueous extract, 100 g of soil is transferred to a wide-mouthed flask for 750–1000 cm 3, five times the volume of distilled water free of CO is poured in. The flask is closed with a cork and shaken for 5 minutes. In the study of saline soils, shaking is carried out for 2 hours, followed by settling for a day or only shaking for 6 hours. The extract is filtered through a funnel with a diameter of 15 cm and a large folded filter placed in it. The filtrate should be clear.
Water extract gives an idea of the content of water-soluble organic and mineral substances in the soil, consisting mainly of simple salts. Salts that are soluble in water can be harmful. According to the degree of harmfulness, they are arranged in the following order: Na 2 CO 3 > ManCO 3 > NaC1> CaC1 2 > Ma 2 5O 4 > MdC1 2 > Me5O 4. The content of Ma 2 CO 3 (even 0.005 vol. shares,%) causes the death of plants in saline soil. In acid waterlogged and peat-bog soils, the excessive content of water-soluble compounds of iron (II), manganese, and aluminum is harmful to plants. The analysis of water extracts in identifying the cause of soil salinization is supplemented by an analysis of groundwater. The table shows the classification of soils according to the content of toxic salts.
WATER ANALYSIS
The protection of water from pollution is the most important task, since it is associated with providing the population with clean drinking water. To develop effective measures for wastewater treatment, it is necessary to know exactly what kind of pollution is found in branch waters that enter a particular reservoir, and in what quantities. These problems are solved by water analysis.
Industrial water is used in various chemical industries. Water should not cause corrosion of boilers, equipment, pipes, contain an excess of suspended solids clogging the pipes of the cooling system; it regulates the content of salts that form scale.
Determination of the so-called chemical oxygen demand (COD), i.e. oxidizability of water, serves as a measure of the content of organic substances in water.
Theoretically, COD is the mass of oxygen (or oxidizing agent in terms of oxygen) in mg/dm 3 required for the complete oxidation of organic substances contained in the sample, and carbon, hydrogen, sulfur, phosphorus are oxidized to oxides, and nitrogen is converted into ammonium salt. Oxygen, which is part of the oxidized substances, is involved in the oxidation process, and hydrogen is involved in the formation of the ammonium salt. The applied methods for determining COD give results close to COD theory.
One of the most common types of water pollutants are phenols. They are contained in the wastewater of coke production, are part of the products of cellulose splitting, and are used as raw materials in the production of many artificial materials, dyes, etc. Phenols are poisonous to most microorganisms, fish and mammals.
The lack of oxygen associated with water pollution causes the death of aerobic microorganisms, which leads to the death of fish. Organic impurities affect the color and transparency of water, its smell and taste. Water used in the food industry must be free from any organic impurities.
Analyze natural and waste water, determining their alkalinity, acidity, total content of nitrogen and nitrogen-containing substances, metals, non-metallic elements, etc. Water sampling from reservoirs and reservoirs with running water is carried out according to special instructions.
It consists in the quick choice of the optimal method of analysis and its successful implementation in solving the analytical problem facing it. The choice of the optimal method of analysis is carried out by sequential consideration of the conditions of the analytical problem. 1. Type of analysis: a) industrial, medical, environmental, judicial, etc.; b) marking, express, arbitration; c) static or...
Ammonia Single once a month from each unit no more than 0.03% Photocolorimetric method M.I. No. 213-A ΔMVI= ±21% 1.7 Production wastes, their use In the production of nitric acid by the combined method, "tail" gases purified in catalytic treatment reactors, ventilation emissions of harmful substances, and waste water are formed as production wastes. After...
