GOST 15483.3-78

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  ГОСТ 15483.3-78

GOST 15483.3−78 Tin. Methods for determination of arsenic (with Amendments No. 1, 2, 3)

GOST 15483.3−78
(CT CMEA 4812−84)

Group B59

INTERSTATE STANDARD

TIN

Methods for determination of arsenic

Tin. Methods for determination of arsenic

AXTU 1709

Date of introduction 1980−01−01

INFORMATION DATA

1. DEVELOPED AND INTRODUCED by the Ministry of nonferrous metallurgy of the USSR

DEVELOPERS

B. C. Baev, T. P. Almanova, G. M. Vlasov, C. B. Meshkova, L. V. Mishchenko, L. D. Savilov, R. D. Kresnicka

2. APPROVED AND put INTO EFFECT by Decision of the USSR State Committee for standards from 13.12.78 N 3300

3. The standard complies ST SEV 4812−84 in part to photometric methods

4. REPLACE GOST 15483.3−70

5. REFERENCE NORMATIVE AND TECHNICAL DOCUMENTS

6. Limitation of actions taken by Protocol No. 4−93 of the Interstate Council for standardization, Metrology and certification (ICS 4−94)

7. REVISED (April 1999) with Amendments No. 1, 2, 3, approved in August 1984, October 1985, June 1989 (IUS 12−84, 1−86, 10−89)


This standard establishes photometric methods for determination of arsenic (in mass fraction of arsenic from 0.001 to 0.25%), a volumetric method for the determination of arsenic (in mass fraction from 0.001 to 1%) and visual colorimetric method for the determination of arsenic (if arsenic mass fraction of from 0.00001 to 0.0001%).

The standard complies ST SEV 4812 in part to photometric methods.

(Changed edition, Rev. N 2, 3).

1. GENERAL REQUIREMENTS

1.1. General requirements for methods of analysis and security requirements — according to GOST 15483.0.

(Changed edition, Rev. N 1).

2. PHOTOMETRIC METHOD WITH THE SEPARATION OF THE ARSENIC IN THE FORM OF ARSINE

2.1. The essence of the method

The method is based on dissolving the sample in hydrochloric, nitric and sulphuric acids, the Stripping of arsenic in the form of arsine hydrochloric acid solution and subsequent measurement of optical density of arsenic and molybdenum heteroalicyclic on the spectrophotometer at a wavelength of 840 nm or photoelectrocolorimeter at a wavelength range of from 620 to 660 nm.

(Changed edition, Rev. N 2).

2.2. Apparatus, reagents and solutions

Photoelectrocolorimeter or spectrophotometer.

Installation for the distillation of arsenic (see drawing) consisting of a conical, distillation flasks with 1 shlifoval connection with a capacity of 100 cm; a safety tube 2 with a diameter of 3 mm and a length of 300 mm; connecting pipe 3 with a diameter of 3 mm (the length of the vertical lap 150 and 250 mm, horizontal 80 mm); glass tube 4 50 mm length, 3 mm in diameter with an extended portion with a diameter of 20 mm (this portion of the tube filled with cotton wool, soaked in a solution of acetate of lead; the darkened cotton wool was replaced with fresh); tube 5 with a diameter of 3 mm and a length of 150 mm, ending in the bottom of a capillary with a diameter of 0.5 mm; the tubes of the receiver 6 spout length 150 mm and diameter in the upper part 15 mm (capacity of the lower narrowed end 1.5 cm); glass ring 7 with a height of 8 mm; a rubber connecting pipe 8.

Installation for the distillation of arsenic

Hydrochloric acid according to GOST 3118 and diluted 1:1.

Nitric acid according to GOST 4461.

Sulfuric acid according to GOST 4204, solutions of 0.5 mol/DM3 mol/DM, 1:1 and 1:4.

Sodium hydroxide according to GOST 4328, solution 1 mol/DM, before use, filtered.

A mixture of hydrochloric and nitric acids to dissolve the 3:1 ratio, freshly prepared.

Potassium iodide according to GOST 4232, a solution with a mass fraction of 15%.

Zinc metal granulated, arsenic -.

Mercury chloride (corrosive sublimate) solution with a mass fraction of 1.5%.

Potassium permanganate according to GOST 20490, solution 0,02 mol/DM.

Ammonium molybdate according to GOST 3765, a solution of 1 g of salt dissolved in 100 cmof solution sulfuric acid 3 mol/DM.

Hydrazine sulfate according to GOST 5841, a solution with a mass fraction of 0.25%, freshly prepared.

The reaction mixture; cook before eating: mix 50 cmof molybdenic acid ammonium solution and 10 cmof a solution of hydrazine sulphate and dilute with water to 100 cm.

Lead acetate according to GOST 1027, a solution with a mass fraction of 5%.

Cotton wool, soaked in a solution of acetate of lead and dried.

Arsenic trioxide.

The standard solutions of arsenic.

Solution a: 0,0332 g of arsenic trioxide dissolved in 2 cmof sodium hydroxide solution, poured 20 cmof water cm 3of sulfuric acid of 0.5 mol/DM, is transferred into a volumetric flask with a capacity of 250 cm, was adjusted to the mark with water and mix.

1 cmof the solution contains 1·10g of arsenic.

Solution B is prepared on the day of application: 10,0 smsolution And transferred to a volumetric flask with a capacity of 100 cm, is diluted to the mark with water and mix.

1 cmof a solution contains 1·10g of arsenic.

(Redrafted From

M. N 1, 2, 3).

2.3. Analysis

2.3.1. A portion of tin by weight, are given in table.1a, is placed in a beaker with a capacity of 100 cm, flow 10 cm ina freshly prepared mixture of acids, cover the watch glass and heated until complete dissolution of the sample. After bleaching solution to pour another 2 cmof a mixture of acids, 6 cmof sulphuric acid diluted 1:1, and evaporated to release vapors of sulphuric anhydride. Cool, wash the side of the Cup with water, stirred and again evaporated to release vapors of sulphuric anhydride. Viscous mass of tetravalent tin sulfate is dissolved in 10 cmof water and, if the mass fraction of arsenic exceeding 0.005%, transferred to a volumetric flask with a capacity of 100 cm, wash the beaker and bring to the mark with sulfuric acid diluted 1:4, mix.

Table 1A

Aliquot part of the solution, selected in accordance with table.1A, is transferred into a distillation flask. If you use the entire solution or aliquote parts by volume of 10 cmadd 5 cmof water when using aliquote parts by volume of 5 cmadd 5 cmof sulfuric acid solution (1:4) and 5 cmof water. To the solution in the flask poured 15 cmof a hydrochloric acid solution, 2 cmof a solution of potassium iodide and allowed to stand for 20 min.

At the same time prepare the absorption solution in the tube of the instrument is introduced 0.4 cm3 mol/DMsolution of sulfuric acid, 2cmof a solution of mercuric chloride and two drops of solution of potassium permanganate.

After 20 min the flask of the device is added 5 g of zinc and quickly cover the tube with the outlet tube, the external end of which is drawn into the capillary. This capillary tube is pre-inserted into the vial containing the absorption liquid and evolved gas for 1 h was passed through the absorber. As bleaching of the absorption of the solution was added dropwise a solution of potassium permanganate to obtain the initial color of the solution.

At the end of the distillation device is dismantled, left in the capillary absorption of liquid, which should keep the pink color. From the receiver solution was transferred into a flask with a capacity of 100 cm, in three steps, wash the receiver and the capillary 4 cmof the reaction mixture and 35 cmof water. The flask with the solution was placed in boiling water for 15 min, then cooled, transferred to a volumetric flask with a capacity of 50 cm, adjusted to the mark with water and mix.

Optical density of the solution measured on a spectrophotometer at a wavelength of 840 nm or photocolorimeter, using a filter with a transmission region from 620 to 660 nm with cuvettes and the optimum thickness of the layer. As a solution comparison solution is used in the reference experiment (see section 2.3.2).

The mass of arsenic in the solution found by the calibration schedule.

(Changed edition, Rev. N 1

, 2, 3).

2.3.2. To build a calibration curve in seven of the eight flasks with a capacity of 100 cmpour 1,0; 2,0; 3,0; 4,0; 5,0; 6,0 and 7.0 cmstandard solution B, which corresponds to 0,01; 0,02; 0,03; 0,04; 0,05; 0,06 and 0.07 mg of arsenic. Under stirring poured to a solution of 0.4 cm3 mol/DMsolution of sulfuric acid, 2cmof a solution of mercuric chloride, two drops of potassium permanganate and water up to 40 cm.

The beakers with the solutions kept in a boiling water bath for 5 minutes, cool in running water, pour 4 cmof the reaction mixture and again heated in the boiling water for 15 min. Then the solutions in the flasks cooled, transferred to volumetric flasks with a capacity of 50 cm, adjusted to the mark with water, mix and measure the optical density of the solutions in p. 2.3.1. Solution comparison is the solution not containing a standard solution of arsenic.

On the found values of optical density and known concentration of arsenic build the calibration graph.

(Changed edition, Rev. N 2

).

2.4. Processing of the results

2.4.1. Mass fraction of arsenic () in percent is calculated by the formula

,

where is the mass of arsenic, was found in the calibration schedule g;

 — weight of tin, corresponding aliquote part of the solution,

2.4.2. Allowable absolute discrepancies in the results of parallel measurements at a confidence probability of 0.95 should not exceed the values given in table.1.

Table 1

(Changed edition, Rev. N 2).

3. VISUAL COLORIMETRIC METHOD

3.1. The essence of the method

The method is based on the visual colorimetrically molybdenum blue formed by the interaction of pentavalent arsenic with ammonium molybdate in the presence of the reducing agent is hydrazine sulfate. Arsenic is removed by a pre-Stripping in the form of arsine.

3.2. Apparatus, reagents and solutions

Apparatus for distillation of arsenic (see drawing).

Cylinders for colorimetrically with ground stoppers.

Hydrochloric acid of high purity according to GOST 14261 and diluted 1:1.

Nitric acid of high purity according to GOST 11125.

Sulfuric acid of high purity according to GOST 14262, solutions of 1 mol/DM3 and mol/DM.

Sodium hydroxide according to GOST 4328, 1 mole/DMsolution (filtered before use).

A mixture of hydrochloric and nitric acids to dissolve the 3:1 ratio.

Potassium iodide according to GOST 4232, a solution with a mass fraction of 15%.

Zinc metal granulated, arsenic -.

Mercury chloride (corrosive sublimate) solution with a mass fraction of 1.5%.

Potassium permanganate according to GOST 20490, solution 0,02 mol/DM.

Isoamyl alcohol according to GOST 5830.

Ammonium molybdate according to GOST 3765, twice recrystallized: 70 g of salt is dissolved in 200 cmof hot water and filtered twice through the same filter. The solution was poured 200 cmof ethanol, allowed to stand for 1 h and then filtered through a funnel with vacuum. The obtained salt re-dissolve and repeat the crystallization, and then washed on the funnel 3−4 times with water, with alcohol and air dried.

1 g of salt dissolved in 100 cmof solution sulfuric acid 1 mol/DM.

The technical rectified ethyl alcohol according to GOST 18300.

Hydrazine sulfate according to GOST 5841, a solution with a mass fraction of 0.25%.

A mixture of molybdenum; prepare before use: mix 10 cmof a solution of hydrazine sulfate, 10 cmof molybdenic acid ammonium solution and dilute with water to 100 cm.

Lead acetate according to GOST 1027, a solution with a mass fraction of 5%.

Cotton wool, soaked in a solution of acetate of lead with a mass fraction of 5% and dried.

Arsenious anhydride.

The standard solutions of arsenic.

Solutions A and B with the mass concentration of arsenic 1·10and 1·10g/cm, respectively, prepared as in step 2.2, using sulfuric acid of high purity.

Solution: 10 cmof a solution is quantitatively transferred to a volumetric flask with a capacity of 100 cm, adjusted to the mark with water and mix.

1 cmof the solution contains 0.001 mg arsenic.

Distilled water according to GOST 6709, double-distilled.

(Redrafted From

M. N 1, 3).

3.3. Analysis

3.3.1. A portion of tin weighing 1 g is placed in a beaker with a capacity of 100 cmand carry out the decomposition of the sample as described in claim 2.3.1. Sulfate tetravalent tin is dissolved in 10 cmwater and transfer the solution into the distillation flask of the apparatus, washing the beaker with 5 cmof water in three steps. The solution was poured 15 cmof hydrochloric acid, diluted 1:1,2 cmof potassium iodide and allowed to stand for 20 min.

At the same time prepare the absorption solution in the tube of the instrument is introduced 0.2 cmsolution of 3 mol/DMsulphuric acid 1 cmof a solution of mercuric chloride and two drops of solution of potassium permanganate.

After 20 min the flask of the device is added 5 g of zinc, quickly close the flask with a stopper, provided with a discharge tube with a capillary at the end, which is pre-dipped in the vial containing the absorption liquid. The emitted gas for 1 h was passed through the absorber. If the solution is colorless, add dropwise a solution of potassium permanganate to obtain the initial color of the solution.

At the same time prepare the calibration solutions. In cylinders with ground stoppers with a capacity of 25 cm, having the same diameter, take 0,1; 0,2; 0,4; 0,6; 0,8 and 1,0 cmstandard solution, pour water to a volume of 2 cm, 0.2 cm3 mol/DMsolution of sulphuric acid 1 cmof a solution of mercuric chloride and one drop of potassium permanganate.

The cylinders are lowered into a boiling water bath for 5 min and then cooled in running water.

After 1 hour dismantle the device. Leave the capillary absorption of liquid (the latter should keep the pink color). The liquid is transferred from the receiver into the cylinder with a glass stopper, wash with three-receiver and the capillary 5 cmof a mixture of molybdenum and 2 cmof water.

All cylinders with calibration solutions pour 5 cmof molybdenum compounds. The cylinders with the subjects and the calibration solutions (absolutely colorless) was placed for 15 min in boiling water, then cooled in running water.

All cylinders automatic pipette poured to 1 cmisoamyl alcohol, stoppered and vigorously shaken for 15 s.

After the separation of the liquids of the blue color of the alcohol layers of the test and calibration solutions are compared in reflected light on a white background, watching from a distance. Through all stages of the analysis in parallel with the samples carried out two test experience for contamination of reagents that are taken into account when processing the results of the analysis.

(Amended And

ZM. N 1, 3).

3.4. Processing of the results

3.4.1. Mass fraction of arsenic () in percent is calculated by the formula

,

where is the mass of arsenic in the sample solution, was found when compared with the calibration solutions, g;

 — the mass of arsenic in solution in the reference experiment, was found when compared with the calibration solutions, g;

 — weight of tin,

3.4.2. Allowable absolute discrepancies in the results of parallel measurements at a confidence probability of 0.95 should not exceed the values given in table.2.

Table 2

4. PHOTOMETRIC METHOD WITH THE SEPARATION OF THE ARSENIC IN THE FORM OF CHLORIDE

4.1. The essence of the method

The method is based on dissolving the sample in a mixture of hydrochloric, nitric and sulphuric acids, distillation of the arsenic chloride with subsequent measurement of optical density of arsenic and molybdenum heteroalicyclic on the spectrophotometer at a wavelength of 840 nm or photoelectrocolorimeter at a wavelength range of from 620 to 660 nm.

4.2. Apparatus, reagents and solutions

Spectrophotometer or photoelectrocolorimeter with all accessories.

Apparatus for distillation of arsenic.

Hydrochloric acid according to GOST 3118.

Nitric acid according to GOST 4461.

A mixture of hydrochloric and nitric acids in the ratio 3:1, freshly prepared.

Sulfuric acid according to GOST 4204, solutions of 3 mol/DMand 1:1.

Hydrazine sulfate according to GOST 5841, a solution with a mass fraction of 0.3%, freshly prepared.

Potassium bromide according to GOST 4160.

Sodium hydroxide according to GOST 4328, a solution with a mass fraction of 10%; store in a plastic container.

Hydrogen peroxide according to GOST 10929, solution with a mass fraction of 1%.

The mass is porous, containing no arsenic, and oxidizing substances.

The technical rectified ethyl alcohol according to GOST 18300.

Ammonium molybdate according to GOST 3765, if necessary, cleaned: 70 g of salt are dissolved in 400 cmof hot water and the solution is filtered through a dense filter. To the solution was added to 250 cm.of ethanol and after 2 h, the crystals are sucked off on a Buchner funnel. The ammonium molybdate obtained is again dissolved and the crystallization repeated. The salt crystals dry on air.

Ammonium molybdate solution: 1 g of salt dissolved in 100 cmof solution sulfuric acid 3 mol/DM.

Reaction mixture: 50 cmof molybdate ammonium solution diluted with water to 450 cm, add 5 cmof hydrazine sulfate solution, adjusted with water to 500 cmand mixed. The mixture is prepared before use.

Arsenic trioxide.

Standard solutions of arsenic.

Solution a: 0,0332 g of arsenic trioxide dissolved in a beaker with a capacity of 100 cmin 2 cmof sodium hydroxide solution, poured 20 cmof water, 3 cmof sulfuric acid solution of 3 mol/DM, is transferred into a volumetric flask with a capacity of 250 cm, was adjusted to the mark with water and mix.

1 cmof solution A contains 0.1 mg of arsenic.

Solution B: 10.0 cmsolution And transferred to a volumetric flask with a capacity of 100 cm, adjusted to the mark with water and mix. Solution B is prepared before use.

1 cmof a solution contains 0.01 mg of arsenic.

(Modified, edited by

I, ISM. N 3).

4.3. Analysis

4.3.1. A portion of tin weighing 1 g is placed in a beaker with a capacity of 100 cm, is dissolved in 10 cmof a mixture of acids, close the glass of the watch glass. After the bleaching solution pour another 2 cmof a mixture of acids, 10 cmof sulfuric acid solution (1:1), remove watch glass and evaporate to copious fumes of sulphuric anhydride. After cooling, the sulfate salt is dissolved in 10 cmof water and the contents of the beaker is transferred to a distillation flask, rinsing the beaker with water. The solution volume was adjusted with water to about 50 cm, add a few pieces of porous mass, 3 g of hydrazine sulfate, 1 g of potassium bromide and attach the flask to the distillation apparatus. Into a flask through a funnel poured 75 cmof hydrochloric acid and distilled 2/3 of the original volume. The distillate is collected in a tall glass, which is poured 25 cmof hydrogen peroxide solution. Then the distillate is transferred to a volumetric flask with a capacity of 250 cm, add 15 cmof nitric acid was adjusted to the mark with water and mix.

Depending on the mass fraction of arsenic is taken in a tall glass with a capacity of 150 cmaliquot part of the solution in accordance with the table.3.

Table 3

Aliquot part of the solution is evaporated on a boiling water bath to dryness. The glass is transferred to a drying Cabinet and dried at a temperature of 130 °C for 1 h. To the cooled residue add 30 cmof the reaction mixture and heated in a boiling water bath for 10 min. the Solution was cooled, transferred to a volumetric flask with a capacity of 50 cm, bring the reaction mixture up to the mark and mix. Measure optical density of solution on a photoelectrocolorimeter at a wavelength range of from 620 to 660 nm, or spectrophotometer at a wavelength of 840 nm. Solution comparison is the solution of the reference experiment. The mass of arsenic in the solution found by the calibration schedule.

4.3.2. To build a calibration curve in eight of the nine cups with a capacity of 50 cmis taken 0,4; 1,0; 2,0; 4,0; 6,0; 8,0; 10,0 and 12.0 cmstandard solution B, which corresponds to 0,004; 0,01; 0,02; 0,04; 0,06; 0,08; 0,1 and 0.12 mg of arsenic. All the glasses are poured at 5 cmof nitric acid and evaporate on a water bath to dryness, then placed in a drying Cabinet and maintained at a temperature of 130 °C for 1 h. Next do as stated in point 4.3.1. Solution comparison is the solution not containing a standard solution of arsenic.

According to the obtained values of optical density and corresponding mass fractions of arsenic build the calibration graph.

4.4. Processing of the results

4.4.1. Mass fraction of arsenic () in percent is calculated by the formula

,

where is the mass of arsenic in the solution was found in the calibration schedule g;

 — weight of tin, corresponding aliquote part of the solution,

4.4.2. Allowable absolute discrepancies in the results of parallel measurements at a confidence probability of 0.95 should not exceed the values given in table.1.

Sec. 4. (Added, Rev. N 2).

5. BICHROMATICALLY VOLUMETRIC METHOD

5.1. The essence of the method

The method is based on dissolving the sample in acid, the recovery of arsenic to the elementary hypophosphite sodium in hydrochloric acid solution and reverse titration of the excess of Mohr salt by potassium dichromate in the presence phenylanthranilic acid as indicator.

5.2. Reagents and solutions

Hydrochloric acid according to GOST 3118 and diluted 1:3.

Sulfuric acid according to GOST 4204, and diluted 1:9.

Hydrogen peroxide according to GOST 10929.

Sodium posterolaterally (hipofosfit sodium) according to GOST 200.

Ammonium chloride according to GOST 3773, a solution with a mass fraction of 5%.

Sodium bicarbonate according to GOST 4201.

Orthophosphoric acid according to GOST 6552.

A mixture of acids: 150 cmof concentrated sulfuric acid poured to 500 cmof water, cooled, poured 150 cmof phosphoric acid and add water to 1 DM.

Iron carbonyl.

Salt Mora (double sulphate salt of protoxide of iron and ammonium) according to GOST 4208, a solution concentration of 0.02 mol/l; prepared by dissolving 38.5 g of salt Mora in 5 DMwater containing 250 cmof concentrated sulfuric acid.

Diphenylamine in the NTD, the solution is prepared as follows: 1 g of diphenylamine moistened with 3−5 cmof water and dissolved in 100 cmof concentrated sulfuric acid.

Phenylantranilic acid solution; prepared as follows: 0.1 g sodium bicarbonate are dissolved in 20−40 cmof water, add 0.1 g phenylanthranilic acid, heated, cooled and diluted with water to 100 cm.

The washing liquid: to 100 cmof hydrochloric acid diluted 1:3, add 1 g of sodium hypophosphite.

Potassium dichromate according to GOST 422, the solution concentration of 0.003 mol/DM.

The installation of the mass concentration of the solution of potassium dichromate.

3−4 flasks with a capacity of 250 cmwith Bunsen valves and placed 0.05 g of pure metallic iron, 0.5 g of sodium bicarbonate and dissolved in 40−50 cmof sulphuric acid, diluted 1:9. After dissolution of iron, the solution is slightly heated and cooled, without removing the tube. The solution was then quickly diluted with water to 200 cm, 25 cm pour themixture of acids, two drops of diphenylamine solution and titrated solution of potassium dichromate until a stable blue-violet staining solution.

The molar concentration () of potassium dichromate is calculated by the formula

,

where  — the weight of the portion of metallic iron equal to 0.05 g;

 — the volume of potassium dichromate solution consumed for titration, cm;

is the molar equivalent weight of iron equal 55,85 g/mol.

The mass concentration of the solution () is calculated by the formula

,

where is the molar concentration of potassium dichromate;

0,015 — amount of arsenic, corresponding to 1 cmof potassium dichromate solution of a concentration of 0.17 mol/DM.

5.3. Analysis

A portion of tin mass of 0.5−5 g is placed in a conical flask with a capacity of 250 cm. For the decomposition of sample poured 20 cmof concentrated sulfuric acid or 40 cmof concentrated hydrochloric acid. The decomposition of the sample in hydrochloric acid was added to a solution of hydrogen peroxide, avoiding the lack of oxidizer. After dissolution, the sample solution is boiled to remove excess oxidant, cooled, poured 50 cmof water and mix thoroughly, pour an equal volume of concentrated hydrochloric acid, 2.3 g of sodium hypophosphite, close flask with reflux condenser and heated to boiling. Weak boiling is maintained for 30 min, after which the solution is kept warm on the stove until the complete coagulation of the precipitate. Solution and the precipitate was cooled in running water and filtered off the precipitate through a pad of filtrowanie mass, washing the flask and the precipitate is 5−6 times the flushing fluid and then 8−10 times with a solution of ammonium chloride with a mass fraction of 5%.

The precipitate of arsenic with filtrowanie mass quantitatively transferred to a flask, where he conducted the deposition, pour 50 cmof sulphuric acid, diluted 1:9, and dissolved with vigorous shaking in an excess of potassium dichromate, priliva it from burette of 10−30 cm(depending on the arsenic content). After dissolution of the arsenic from the burette pour the salt solution Mora to the disappearance of the yellow colour of potassium dichromate and some excess (20−30 cm). Next, add 3−4 drops of the solution phenylanthranilic acid and titrate the excess of Mohr salt solution of potassium dichromate of 0.003 mol/DMbefore the appearance of pink colouration of the solution

.

5.4. Processing of the results

5.4.1. Mass fraction of arsenic () in percent is calculated by the formula

,

where is the total volume of potassium dichromate solution consumed for the dissolution of arsenic and titrate the excess of Mohr salt, cm;

 — the volume of potassium dichromate solution consumed for titration of the same amount of salt Mora, which was added to the analyzed solution, cm.

Note. The amount of potassium dichromate consumed for titration of the Mohr salt, set as follows: the burette is poured into a flask with a capacity of 250 cmsame amount of salt Mora, which was added to the analyzed solution (20−30 cm), add 50 cmof sulphuric acid, diluted 1:9, 3−4 drops of solution phenylanthranilic acid and titrated solution of potassium dichromate until the appearance of pink colouration of the solution;


 — mass concentration of a solution of potassium dichromate for arsenic, g/cm;

 — the weight of the portion of the sample

,

5.4.2. Allowable absolute differences of the results of the parallel definitions should not exceed the values given in table.4.

Table 4

Sec. 5. (Added, Rev. N 3).