Sartori The War Gases Chemistry and analysis

330 ARSENIC COMPOUNDS

sulphuric acid and i ml. fuming nitric acid in a narrow-mouthed flask of Jena glass. After cooling and adding 50 ml. water, the solution is evaporated and the above treatment repeated. 10 ml. water, 2 gm. potassium iodide and sufficient water to dissolve the precipitate are then added in succession to the cooled solution.

After allowing to stand-for about \ hour, the iodine liberated is titrated without using any indicator.

Ewins's Method* 0-1-0-2 gm. of the substance is mixed in a 300 ml. Kjeldahl flask with 10 gm. potassium sulphate, 0-2-0-3 gm. starch and 20 ml. concentrated sulphuric acid. This mixture is then heated by means of a Bunsen burner, first moderately for 10-15 minutes, then more vigorously for about 4 hours, until decomposition is complete. The liquid is cooled, transferred to a 350 ml. flask and made alkaline to litmus paper with sodium hydroxide. It is then cooled to 30° to 40° and sulphuric acid added drop by drop until the solution is faintly acid. A saturated solution of sodium bicarbonate is then added until the solution is again alkaline, 5-10 ml. being added in excess. The arsenious acid formed is then titrated with iodine solution using starch as indicator.

Robertson's Method? This method consists synoptically of the following phases :

(a)Decomposition of the substance with sulphuric-nitric acid mixture.

(b)Elimination of the nitrous compounds with ammonium sulphate.

(c)Titration of the arsenite formed with iodine.

0-2 gm. of the substance is weighed into an Erlenmeyer flask and heated for about an hour with 5 ml. concentrated sulphuric acid and i ml. fuming nitric acid. After cooling the flask cautiously, a further 10-15 drops of fuming nitric acid are added and the flask again heated for 5 minutes to ensure complete decomposition, i gm. solid ammonium sulphate is then added and the contents of the flask agitated well until all the nitrogen has been evolved. They are then cooled and diluted with 60-70 ml. water, i gm. potassium iodide is then added and a fewfragments of porous plate. A pear-shaped bulb of glass is placed in the mouth of the flask and the liquid concentrated to 4p ml. The iodine which is liberated is then decolorised with N/ioo thiosulphate and the solution diluted to 100-120 ml. with cold water. The whole is then transferred to a 500 ml. flask containing 50 ml.

1EWINS, /. Chem. Soc., 1916, 109, 1355.

2ROBERTSON, /. Am. Chem. Soc., 1921, 43, 182.

4 N sodium carbonate solution and the remaining acid neutralised with a slight excess of sodium bicarbonate. Starch solution is added and the arsenite present titrated with iodine.

Rogers's Method.1 This method is based on the decomposition of the arsenical compound with nitric acid and ammonium persulphate and the titration of the iodine liberated on addition of potassium iodide.

About 0-5 gm. of the substance is weighed accurately into a 500 ml. flask and 10 ml. water and 5 ml. nitric acid are added. The mixture is then heated, ammonium persulphate being added until the solution becomes clear. If the liquid persistently remains yellow, showing that the substance is refractory, it is boiled for several minutes with a few ml. water and several gm. of ammonium persulphate.

The solution is then diluted with 100 ml. water, treated with about 5 nilof a saturated solution of acid sodium ammonium phosphate and then an excess (about 40 ml.) of magnesia mixture added. A precipitate forms and is dissolved in dilute nitric acid ; the solution is then heated to boiling, an excess of ammonia added, and it is then allowed to stand for about 2 hours. The precipitate is filtered off, washed with dilute ammonia and dissolved in 70 ml. dilute hydrochloric acid (3:2).

To the acid solution 3 gm. of potassium iodide in 6 ml. water are added and 70 ml. water. The liberated iodine is then titrated with sodium thiosulphate.

Direct methods of estimation have been proposed for certain of the war gases. Some of these are described below.

DETERMINATION OF THE ALIPHATIC ARSINES

Jurecev 2 suggested the following method for the determination of the aliphatic arsines present in vapour form in the air.

A known volume of the air is passed through a U tube filled with dry, fine-grained silica gel. The silica gel with the substance absorbed on it is transferred to a nickel or silver crucible and covered with a layer of magnesium oxide which is well pressed down. 6 gm. of a mixture of equal parts of sodium peroxide and sodium carbonate are added and pressed down, and this is finally covered with a layer of sodium carbonate. The crucible is heated with a small flame for about 15 minutes until the bottom is dull red. It is then allowed to cool and placed in a beaker, hot water is added and the beaker warmed on the water-bath. The solution is neutralised with dilute sulphuric acid and warmed

again on the water-bath to decompose the hydrogen peroxide present. The silica gel is filtered off and washed with hot water, then the liquid is allowed to cool, made up to a convenient volume and the arsenic determined in an aliquot by the colorimetric 1 method using mercuric chloride paper (see p. 326).

DETERMINATION OF METHYL DICHLOROARSINE

For this determination, the following method has been recommended by Uehlinger and Cook 2 :

5 gm. of the methyl dichloroarsine are treated with 200 ml. water and the hydrochloric acid formed by the hydrolysis neutralised to litmus. Sodium bicarbonate is then added and the solution titrated with a decinormal iodine solution.

DETERMINATION OF ft CHLOROVINYL DICHLOROARSINE

This determination is usually carried out by the method of Lewis and Perkins,3 in which the ft chlorovinyl dichloroarsine is decomposed by 15% sodium hydroxide solution at a temperature below 37° C. Acetylene is evolved quantitatively.

FIG. 20.

0-2-0-4 gm. of the substance is weighed into a flask B (Fig. 20) of 50 ml. capacity, and 5 ml. of 15% sodium hydroxide solution are introduced from the burette A, the liquid then being warmed to about 37° C. The decomposition of the ft chlorovinyl dichloroarsine is complete after 15 minutes' agitation and then the volume of acetylene formed is read off in the burette C. From this the quantity of ft chlorovinyl dichloroarsine in the sample

1K. UHL, Z. angew. Chem., 1937, 50, 164.

2UEHLINGER and COOK, /. Ind. Eng. Chem., 1919, 11, 105.

3LEWIS and PERKINS, Ind. Eng. Chem., 1923, 15, 290.

ARSENIC COMPOUNDS: DETERMINATION 333

can be calculated. The U tube contains 15% sodium hydroxide solution which ensures the complete decomposition of the sample.

DETERMINATION OF CHLOROVINYL ARSINES

In order to determine the amount of each constituent in a mixture of the chlorovinyl arsines, the method proposed by Brinton1 may be employed. This utilises the following reactions :

(1) Cold water hydrolyses

3 chlorine atoms in arsenic trichloride,

(2)By prolonged heating with alcoholic soda all four compounds are attacked with the formation of 3 moleculesof sodium chloride from each.

(3)A solution of sodium bromate in dilute hydrochloric acid oxidises the arsenic trichloride and /J chlorovinyl dichloroarsine to the pentavalent state.

(4)Moderate boiling with 15% sodium hydroxide causes the attack of both £ chlorovinyl dichloroarsine and /?/?' dichlorovinyl chloroarsine, but not trichlorovinyl arsine, with formation of sodium arsenite which may be titrated with sodium bromate after acidification.

DETERMINATION OF PHENYL DICHLOROARSINE

Fleury's z method may be employed for this determination ; it consists in hydrolysing the sample with water and titrating the oxide formed with iodine solution. The following reaction takes place :

/OH

CsH5-AsCla + I2 + 3 H,0 = C6 Hs AsO + 2 HI + 2 HC1

A sample of phenyl dichloroarsine is weighed accurately, treated with water and alcohol and then titrated, without adding any sodium bicarbonate, with a decinormal solution of iodine, until the yellow colour is permanent. The number of ml. of iodine solution employed multiplied by 0-01115 gives the amount of phenyl dichloroarsine (in gm.) in the sample.

DETERMINATION OF DIPHENYL CHLOROARSINE

Fleury's method, depending on the same principle as the preceding estimation, is most frequently employed for the

334 ARSENIC COMPOUNDS

determination of diphenyl chloroarsine. The titration must be carried out in benzene or chloroform solution, however, and not in aqueous alcoholic solution, and in presence of sodium bicarbonate, which accelerates the velocity of the reaction and also dissolves the diphenyl arsenic acid which is formed :

(C6H5)2AsCl + I2 + 2H2O = (C6H6)2AsOOH + zHI + HC1.

The sample (0-2-0-4 gm.) is weighed accurately and dissolved in 10-15 ml. chloroform or benzene, 20 ml. of a saturated solution of sodium bicarbonate are added and the liquid is then titrated with N/io iodine solution, being shaken vigorously after each addition of iodine. The end of the titration is shown by the appearance of a violet colouration in the solvent. The number of ml. of N/io iodine solution employed multiplied by 0-0132 gives the amount of diphenyl chloroarsine present in the sample, in gm.

In order to determine the amount of diphenyl chloroarsine in air, Sieverts1 recommends the following method (cp. note,

P- 327)-

A sample of the air is taken in a glass flask of 10-15 litres capacity and washed three times with 30 ml. benzene ; the benzene solutions are evaporated together on the water-bath to a volume of 10-20 ml. and then titrated with a N/i,ooo solution of iodine as described above. The number of ml. of iodine employed multiplied by 0-132 gives the amount of diphenyl

consists in passing a known volume of the aerosol (e.g., 50 litres) through a wash-bottle with a porous partition containing ether. The solvent is then evaporated off on the water-bath, the residue decomposed by one of the methods described on p. 329 et seq., and the arsenic determined colorimetrically by means of mercuric chloride paper (see p. 326).

DETERMINATION OF DIPHENYL CYANOARSINE

Diphenyl cyanoarsine may be estimated in the same manner as diphenyl chloroarsine, by titration with iodine (Sieverts) :

(C6H8)2AsCN + 2H2O + I2 = (C6H5)2AsOOH + 2HI +HCN.

A sample of the air to be analysed is introduced into a glass

ARSENIC COMPOUNDS: DETERMINATION 335

flask, as in the case of diphenyl chloroarsine, and washed with alcohol (not benzene, for in the latter solvent diphenyl cyanoarsine does not react with iodine). The alcoholic solution is diluted with an equal volume of water and about 5 ml. benzene are added. The diphenyl cyanoarsine reacts quantitatively with iodine in the aqueous alcoholic solution, while the excess of iodine passes into the benzene layer.

The number of ml. of N/i,ooo iodine employed multiplied by 0-127 gives the quantity of diphenyl cyanoarsine in the sample of air taken, in mgm.

It is advisable to carry out a blank determination. DETERMINATION OF PHENARSAZINE CHLORIDE

A knownvolume of air containing the substance to be examined, in the form of an aerosol, is passed through a wash-bottle with a porous partition, of the type recommended by Kolliker,1 containing ether.

The solvent is removed on the water-bath, the residue is oxidised by means of a mixture of concentrated sulphuric acid, concentrated nitric acid and hydrogen peroxide, according to Winterstein's method,2 and the arsenic is determined in the solution obtained by the colorimetric method with mercuric chloride paper (see p. 326).

DETERMINATION OF ARSENIC TRICHLORIDE IN PHENYL

DlCHLOROARSINE

Fleury recommendsthe following method for the determination of the arsenic chloride present in a sample of phenyl dichloroarsine.

A known amount of the sample (equivalent to about 30 ml. N/io iodine solution) is dissolved in 15-20 ml. 95% alcohol and titrated directly with iodine without addition of sodium bicarbonate. An excess of a saturated solution of sodium bicarbonate is then added ; if after this the solution absorbs more iodine, the presence of arsenic trichloride is indicated, and from the number of ml. of iodine solution absorbed in these conditions, the quantity of arsenic trichloride present in the sample may be calculated.

According to Delepine,3 this method may also be employed for the determination of the arsenic trichloride present in the aliphatic arsines.

TABLE XIII. Table of Conversion for Gas Concentrations : parts per

336