Laboratory work No. 1.
Standardization of NaOH alkali solution
This work serves as an example of the standardization of a secondary standard solution: determination of the exact concentration of an alkali solutionNaOH based on titrated hydrochloric acid solutionHCl with indicator methyl orange or phenolphthalein.
Reagents . Hydrochloric acid HCl, 0.05000 mol / dm3 solution. Sodium hydroxide NaOH. Methyl orange, 0.1% solution. Phenolphthalein, alcohol 0.1% solution.
Dishes . Conical flasks (100, 250 cm3). Pipette (10.0 cm3). Burette (25 cm3). Titration stand. Glasses (50, 100 cm3). Funnel. Graduated cylinder (50 cm3). Filter paper.
Determination progress. Titration is carried out by direct (1) or inverse (2) method. Place the previously washed burette on the stand and rinse it 2-3 times with small portions (3-5 cm3) of the prepared titrant solution: hydrochloric acid HCl solution (1) or alkali solution NaOH (2). Fill the burette with the titrant solution according to Titration technique.
After achieving a change in the color of the solution from one drop of the titrant solution, make a reading on the burette and record the data. Conduct two parallel experiments until similar results are obtained.
After completing the work in the work journal, record the operations you performed. Enter all the data obtained in the table.
Mathematical processing of measurement results. From the data obtained, determine the concentration of the NaOH solution:
where C (NaOH) is the molar concentration of the equivalent of sodium hydroxide solution, mol / dm3; С (HCl) - molar concentration of the equivalent of hydrochloric acid solution, mol / dm3; Val (solution HCl) - aliquot volume of HCl solution, cm3, V i(NaOH solution) - alkali solution NaOH, used for titration of an aliquot volume of HCl solution, cm3.
An example of a report
for laboratory work
Laboratory work No. 1.
Standardization of NaOH alkali solution
Purpose of work. Study of the method of standardization of an alkali solution NaOH (secondary standard) by pipetting.
Determination progress. Titration is carried out by direct (1) or inverse (2) method.
1. Into a 100 cm3 conical flask add a selected aliquot of the standardized NaOH solution, 1-2 drops of methyl orange solution. Titrate the resulting mixture with a solution of hydrochloric acid HCl with a molar concentration equivalent of 0.05000 M until the orange-yellow color turns to orange.
2. Into a 100 cm3 conical flask add a selected aliquot of a standard solution of hydrochloric acid HCl with a molar concentration equivalent of 0.05000 M, 1-2 drops of phenolphthalein solution. Titrate the resulting mixture with an alkali solution until a pale pink color appears, stable for 30 seconds.
Read off the burette and record the data. Conduct two parallel experiments until similar results are obtained.
Mathematical processing of measurement results. Calculation of the concentration of NaOH solution according to the obtained data:
Calculating the discrepancy between parallel measurements:
Table 1. Determination of concentration of alkali solution NaOH
C (HCl), mol / cm3 | Aliquot volume, cm3 | Titration results | Discrepancy between parallels. measurements | Determination results, mol / dm3 |
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X 1 = 0.04950 M |
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X 2 = 0.04902 M |
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X cf = 0.04926 M |
When titrating with methyl orange, it is convenient to use bystanders. For their preparation, 20 cm3 of distilled water, 2-3 drops of the indicator are introduced into 2 conical flasks for titration using a measuring cylinder. Then 1-2 drops of 0.1 M HCl solution are introduced into one flask, and 1-2 drops of 0.1 M NaOH solution are added to the other.
Titration results are considered to be convergent if they differ from each other by no more than the scale interval of the burette (as a rule, it is 0.05 cm3 or 0.1 cm3, depending on the type of burette). If the titration results differ by more than the scale interval of the burette, then both results are discarded and the titration is repeated again.
Among the variety of organic substances, there are special compounds that are characterized by color changes in different environments. Before the advent of modern electronic pH meters, indicators were indispensable "tools" for determining the acid-base indicators of the environment, and continue to be used in laboratory practice as auxiliary substances in analytical chemistry, as well as in the absence of the necessary equipment.
What are indicators for?
Initially, the property of these compounds to change color in various media was widely used to visually determine the acid-base properties of substances in solution, which helped to determine not only the nature of the medium, but also to draw a conclusion about the resulting reaction products. Indicator solutions continue to be used in laboratory practice to determine the concentration of substances by titration and allow you to learn how to use improvised methods in the absence of modern pH meters.
There are several dozen of this kind of substances, each of which is sensitive to a rather narrow area: usually it does not exceed 3 points on the scale of information content. Thanks to such a variety of chromophores and their low activity among themselves, scientists have managed to create universal indicators that are widely used in laboratory and industrial conditions.
Most used pH indicators
It is noteworthy that, in addition to the identification property, these compounds have a good dyeing ability, which allows them to be used for dyeing fabrics in the textile industry. Of the large number of color indicators in chemistry, the most famous and used are methyl orange (methyl orange) and phenolphthalein. Most of the other chromophores are currently used in mixture with each other, or for specific syntheses and reactions.
Methyl orange
Many dyes got their name from their basic colors in a neutral environment, which is also inherent in this chromophore. Methyl orange is an azo dye with a grouping - N = N - in its composition, which is responsible for the transition of the indicator color to red in and to yellow in alkaline. The azo compounds themselves are not strong bases, however, the presence of electron donor groups (- OH, - NH 2, - NH (CH 3), - N (CH 3) 2, etc.) increases the basicity of one of the nitrogen atoms, which becomes capable of attaching hydrogen protons according to the donor-acceptor principle. Therefore, with a change in the concentration of H + ions in solution, a change in the color of the acid-base indicator can be observed.
Learn More About Getting Methyl Orange
Methyl orange is obtained by reaction with diazotization of sulfanilic acid C 6 H 4 (SO 3 H) NH 2 followed by coupling with dimethylaniline C 6 H 5 N (CH 3) 2. Sulfanilic acid is dissolved in a sodium alkali solution by adding sodium nitrite NaNO 2, and then it is cooled with ice to carry out the synthesis at temperatures as close as possible to 0 ° C and hydrochloric acid HCl is added. Next, a separate solution of dimethylaniline in HCl is prepared, which is poured cooled into the first solution to obtain a dye. It is additionally made alkaline, and dark orange crystals precipitate from the solution, which are filtered off after several hours and dried in a water bath.
Phenolphthalein
This chromophore got its name from the addition of the names of two reagents that are involved in its synthesis. The color of the indicator is notable for the change in its color in an alkaline medium with the acquisition of a crimson (red-violet, crimson-red) hue, which becomes discolored when the solution is strongly alkalized. Phenolphthalein can take several forms depending on the pH of the medium, and in highly acidic environments it has an orange color.
This chromophore is obtained by condensation of phenol and phthalic anhydride in the presence of zinc chloride ZnCl 2 or concentrated sulfuric acid H 2 SO 4. In the solid state, phenolphthalein molecules are colorless crystals.
Previously, phenolphthalein was actively used in the creation of laxatives, but gradually its use was significantly reduced due to the established cumulative properties.
Litmus
This indicator was one of the first reagents used on solid supports. Litmus is a complex mixture of natural compounds that is obtained from certain types of lichens. It is used not only as but also as a means for determining the pH of the medium. This is one of the first indicators that began to be used by humans in chemical practice: it is used in the form of aqueous solutions or strips of filter paper impregnated with it. Solid litmus is a dark powder with a faint ammoniacal odor. When dissolved in pure water, the indicator color becomes violet, and when acidified, it turns red. In an alkaline medium, litmus turns into blue, which allows it to be used as a universal indicator for the general determination of the indicator of the medium.
It is not possible to accurately establish the mechanism and nature of the reaction occurring when the pH changes in the structures of litmus components, since it can contain up to 15 different compounds, and some of them can be inseparable active substances, which complicates their individual studies of chemical and physical properties.
Universal indicator paper
With the development of science and the appearance of indicator papers, the establishment of indicators of the environment became many times easier, since now it was not necessary to have ready-made liquid reagents for any field research, which scientists and criminologists still successfully use. So, solutions were replaced by universal indicator papers, which, due to their wide spectrum of action, almost completely removed the need to use any other acid-base indicators.
The composition of the impregnated strips may differ from manufacturer to manufacturer, so an approximate list of ingredients may be as follows:
- phenolphthalein (0-3.0 and 8.2-11);
- (di) methyl yellow (2.9-4.0);
- methyl orange (3.1-4.4);
- methyl red (4.2-6.2);
- bromothymol blue (6.0-7.8);
- α ‒ naphtholphthalein (7.3-8.7);
- thymol blue (8.0-9.6);
- cresolphthalein (8.2-9.8).
On the packaging, the standards of the color scale are necessarily given, which make it possible to determine the pH of the medium from 0 to 12 (about 14) with an accuracy of one whole.
Among other things, these compounds can be used together in aqueous and aqueous-alcoholic solutions, which makes the use of such mixtures very convenient. However, some of these substances may be poorly soluble in water, therefore a universal organic solvent must be selected.
Due to their properties, acid-base indicators have found their application in many fields of science, and their diversity has made it possible to create universal mixtures that are sensitive to a wide range of pH indicators.
Consider the following titration cases.
Titration of a strong acid with a strong base
HCl + NaOH ® NaCl + H 2 O
H + + OH - ® H 2 O
At the equivalence point, a salt of a strong acid and a strong base is formed, which does not undergo hydrolysis. The reaction of the medium will be neutral (pH = 7). In this case, litmus can serve as an indicator.
Titration of a weak acid with a strong base
CH 3 COOH + NaOH ® CH 3 COONa + H 2 O
CH 3 COOH + OH - ® CH 3 COO - + H 2 O
The resulting salt of a weak acid and a strong base in solution undergoes hydrolysis:
CH 3 COO - + HOH ® CH 3 COOH + OH -
The equivalence point in this case will be in an alkaline environment, therefore, an indicator that changes color at pH> 7, for example, phenolphthalein, should be used.
Titration of a weak base with a strong acid
NH 4 OH + HCl ® NH 4 Cl + H 2 O
NH 4 OH + H + ® NH 4 + + H 2 O
The resulting salt in solution undergoes hydrolysis:
NH 4 + + HOH ® NH 4 OH + H +
The equivalence point will be in an acidic environment, so methyl orange can be used.
SAMPLE of the design of laboratory work in
Titrimetric analysis
Laboratory work No. ... Date
"Name of the laboratory work"
Primary standard - C E (NaOH) = ………… mol / l
The substance to be determined (titrant) - C E (HCl) =?, T (HCl) =?
Indicator - methyl orange
Titration conditions - (pH of the medium, heating, etc.)
Reaction equation (in molecular and ionic-molecular forms):
The results of the experiment are entered in the table:
Calculations:
PREPARATION AND STANDARDIZATION OF ACID-BASIC TITRANT SOLUTIONS
1. Preparation and standardization of 0.1 M HCl
Using a hydrometer, determine the density of the concentrated hydrochloric acid solution given to you (let's say that r = 1.179 g / ml).
From the table of the density of solutions (appendix table. 3) find the mass fraction of acid in this solution (w = 36%). Calculate what volume of 36% HCl solution you need to take to prepare 250 ml of 0.1 mol / l solution.
The molar mass of the HCl equivalent is 36.46 g / mol, therefore, 250 ml of 0.1 mol / L solution should contain 0.912 g of anhydrous HCl:
m(HCl) = M E S E V = 36.46 0.1 0.25 = 0.912 g
The mass of a 36% HCl solution containing this amount of acid is:
The volume of the original acid solution can be found by the formula:
Measure out the calculated volume (»2.0 ml) of the 36% hydrochloric acid solution with a small graduated cylinder and pour into the large cylinder. Bring the volume of the solution to 250 ml with distilled water, pour it into a 250 ml bottle and mix.
1.2 Preparation of the primary standard substance solution
Sodium tetraborate and HCl react with each other as follows
Na 2 B 4 O 7 + 2HCl + 5H 2 O ® 2NaCl + 4H 3 BO 3
The equivalence factor for Na 2 B 4 O 7 in this reaction is 1/2, M (1/2 Na 2 B 4 0 7 × 10H 2 O) = 190.686 g / mol.
To prepare 100 ml 0.1 M 1/2 Na 2 B 4 O 7, you need to take m = C × V × M = 0.1 × 0.1 × 190.686 = 1.9 g Na 2 B 4 0 7 × 10H 2 O ...
First, about 1.9 g of Na 2 B 4 0 7 × 10H 2 O are weighed using a hand balance. The weight of the sample taken is then specified using an analytical balance and dissolved in about 50 ml of hot water in a 100 ml volumetric flask. After cooling, the solution is brought to the mark with water at ambient temperature and stirred. Calculate the exact concentration of sodium tetraborate in the solution
,
where m is the mass of the weighed portion of sodium tetraborate decahydrate, measured using an analytical balance.
1.3. Titration of a solution of a primary standard substance with a standardized HCl solution
In 3 flasks for titration, measure 10.00 ml of sodium tetraborate solution with a pipette, add 2 drops of 1% methyl orange solution to each flask and titrate with the prepared HCl solution. Titration is carried out until the pure yellow color of the solution acquires an orange tint. For comparison, you can take 10 ml of distilled water and add to it the same amount of indicator as to the analyzed solution. The titration is carried out until a difference in the colors of both solutions appears.
The molar concentration of HCl in solution is
where 
10.00 ml, 
- the average value of the volume (ml) of the HCl solution consumed for titration.
Methyl orange
| Methyl orange | |
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| General | |
|---|---|
| Systematic name | 4- (4 - dimethylaminophenylazo) benzenesulfonate sodium |
| Traditional names | methyl orange |
| Chemical formula | C 14 H 14 N 3 O 3 SNa |
| Physical properties | |
| Molar mass | 327.3359 g / mol |
| Density | 1.28 g / cm³ |
| Thermal properties | |
| Chemical properties | |
| Water solubility | (at 50 ° С) 0.2 g / 100 ml |
| Solubility in ethanol | insoluble g / 100 ml |
Methyl orange(methyl orange helianthin, 4- (4-dimethylaminophenylazo) benzenesulfonate sodium) is a well-known acid-base indicator. Methyl orange is an organic synthetic dye from the group of azo dyes.
Properties
Appearance under normal conditions: orange-yellow leaves or powder, scales. Methyl orange is soluble in water 0.2 g per 100 g, preferably hot.
In solutions with pH 2, it absorbs light at λmax 505 nm.
The color transition in aqueous solutions from red to orange-yellow is observed in the pH range 3, 1 - 4, 4 (in an acidic medium, red, in an alkaline medium, yellow).
The color transition interval is influenced by: temperature, the presence of salts, organic solvents, protein substances and others in the solution. The effect of temperature is most significant for indicators that are weak bases: for example, for methyl orange at room temperature, the color changes within the pH range of 3, 1 - 4, 4, and at 100 ° C - within the range of pH 2, 5 - 3, 7.
Application
Methyl orange
It is used as an acid-base indicator, titrant in the determination of strong oxidants, spectrophotometric determination of oxidants (chromium, bromine).
0.1% aqueous solution is used in analytical chemistry as an indicator.
Changes color from red in an acidic environment (pH 3.1 to 4.4) to orange in neutral and yellow in alkaline.
Receiving
Laboratory synthesis method:
Methyl orange is obtained by diazotizing sulfanilic acid and then combining the resulting material with dimethylaniline.
Execution method:
A weighed portion of sulfanilic acid is dissolved in 25 ml of 2 M sodium hydroxide solution (2 g of NaOH in 25 ml of solution). A weighed portion of sodium nitrite weighing 4 g is then dissolved in the same solution. After that, the solution is cooled with ice and poured into 25 ml of 2 M hydrochloric acid solution cooled with ice (except for external cooling, pieces of ice can be placed in the solution). A weighed portion of dimethylaniline is dissolved in 5 ml of 1 M hydrochloric acid, cooled with ice, and the diazobenzenesulfonic acid solution obtained above is added to the cooled solution. Dye is formed. Add more sodium hydroxide solution to a strongly alkaline reaction. The sodium salt of the dye is released from the solution in the form of orange-brown petal crystals. After a few hours, the dye is filtered off with suction and washed on a funnel with 25 ml of water. Then it is carefully wrung out on filter paper and dried in a porcelain cup in a water bath.
Mnemonic rule
A mnemonic poem is used to memorize the indicator color of methyl orange in alkalis and acids.
