Sartori The War Gases Chemistry and analysis

solution. The solution is evaporated on the water-bath with a little ammonium sulphide and the residue is taken up in dilute hydrochloric acid, filtered, and the nitrate then treated with a few drops of a very dilute solution of ferric chloride. In the presence of hydrocyanic acid the liquid is coloured blood-red, or merely pink if only traces of hydrocyanic acid are present, owing to the formation of ferric thiocyanate. When a weak colouration is obtained, it may be rendered more distinct by adding a little ethyl ether to the solution ; the ferric thiocyanate then passes into the ether, producing a more intense colouration.1

Guignard's Sodium Pierate Paper.2 Sodium picrate paper on exposure to gaseous hydrocyanic acid assumes a blood-red colour by the formation of sodium isopurpurate.3

These papers are prepared by immersing strips of filter paper first in a 0-1% aqueous solution of sodium carbonate, allowing them to dry, and then immersing them in a 0-1% aqueous picric acid solution. The Guignard reaction is sensitive (0-05 mgm. hydrocyanic acid gives a colouration in 12 hours), but not specific, for various reducing substances, as aldehydes, acetone, hydrogen sulphide and sulphur dioxide also respond to it.

Benzidine Acetate Reaction. If a solution of benzidine acetate is added to a dilute solution of hydrocyanic acid in presence of copper acetate, an intense blue colouration is produced. This reaction, which was suggested by Pertusi and Gastaldi,4 is more conveniently applied by means of papers prepared as follows 5 :

Two separate solutions are prepared :

(a)2-86 gm. copper acetate in i litre of water.

(6)475 ml. of a solution of benzidine acetate, saturated at ordinary temperature, together with 525 ml. water.

The two solutions are mixed in equal parts just before use and strips of filter paper are immersed in the mixture. The two solutions may be kept separately for a long period if stored in a

dark place, but the mixture undergoes alteration in the course of about a fortnight.6

According to Smolczyk,7 on exposing a benzidine acetate paper in an atmosphere containing 0-0001% by volume of hydrocyanic acid (i-i mgm. per cu. m. at 20° C.), the change of colour to blue takes place in I minute.

1 JOFINOVA, /. Obscei Khim., Ser. A., 1935, 5, 34.

In order to carry out this determination, not more than o-i gm. free hydrocyanic acid is treated with a few ml. of sodium hydroxide solution and 0-5 gm. sodium bicarbonate and made up to a volume of 50 ml. with water. The titration is carried out with decinormal silver nitrate solution, shaking until finally a slight permanent opalescence remains.

i ml. N/io AgNO3 = 0-005404 gm. HCN.

In order to detect the end-point with greater ease it is better to add a little sodium iodide to the solution. This reacts with the silver nitrate only when all the hydrocyanic acid has been converted into the double cyanide.

(2) Fordos and Gelis's Method. The method suggested by Fordos and Gelis x depends on the reaction between hydrocyanic acid and iodine in presence of alkali bicarbonate, which has been mentioned already:

HCN + I2 = HI + CNI.

The solution for examination should not contain more than 0-05 gm. hydrocyanic acid. It is treated with a few ml. of sodium hydroxide solution and 0-5 gm. sodium bicarbonate, and is then titrated with a decinormal solution of iodine until a yellow colour persists. In this case it is unnecessary to use starch as indicator :

i ml. N/io iodine solution = 0-001351 gm. HCN.

Several analytical procedures have been based on this reaction, e.g., those of Seil,2 Page,3 etc. The following is especially suitable for the analysis of hydrocyanic acid used in the mixtures of gases used for disinfestation.

About 200 ml. distilled water are placed in a i-litre Woulfe's bottle having two necks. One of these necks is fitted with a delivery tube reaching almost to the bottom of the bottle for the entry of the gas mixture, while the other is connected with an aspirator. 5 ml. 5% sodium bicarbonate solution, 5 ml. i% starch solution and 10 ml. standard iodine solution are added to ths water in the bottle, the concentration of the iodine solution being chosen according to the amount of hydrocyanic acid presumed to be present in the gas. Usually, in analysing disinfestation mixtures, an iodine solution containing 0-94014 gm. of iodine per litre is employed, 10 ml. of this solution corresponding to o-ooi gm. hydrocyanic acid.

1 FORDOS and GELIS, /. prakt. Chem., 1853, 59,255. * G. SEIL, Ind. Eng. Chem., 1926, 18, 142.

3 PAGE and GLOYNS, /. Soc. Chem. Ind., 1936, 55, 209.

From this equation it is seen that each molecule of cyanogen chloride reacts with two molecules of sodium hydroxide. If, for instance, N ml. normal sodium hydroxide were used and n ml. normal sulphuric acid were necessary to neutralise the excess, the quantity of cyanogen chloride present in the sample is given by the following :

CNC1 = (N - n)0-0306.

QUANTITATIVE DETERMINATION OF CYANOGEN BROMIDE

(1) The Hydriodic Acid Method. The determination of cyanogen bromide by this method depends on the estimation of the iodine liberated in the reaction between hydriodic acid and cyanogen bromide 1 :

CNBr + 2HI = I2 + HBr + HCN.

In carrying out this determination, a weighed quantity of cyanogen bromide is treated with an excess of hydriodic acid solution, which is prepared by dissolving 10 gm. potassium iodide in 100 ml. of a 5% solution of acetic acid. The iodine liberated is titrated with a decinormal solution of sodium thiosulphate.

(2) The Ammonia Method. This method depends on the decomposition of cyanogen bromide by ammonia :

2NH3 + CNBr = NH4Br + NH2CN,

and the titration of the ammonium bromide formed with silver nitrate.2

The sample to be analysed is weighed accurately into a flask, about 100 ml. aqueous ammonia are added and the flask stoppered and allowed to stand in the cold for about 20 minutes. After warming for a short time on the water-bath, the solution is diluted with 300 ml. water and brought to the boil to free it from the excess of ammonia. It is then slightly acidified with nitric acid and titrated with silver nitrate solution.

The decomposition of cyanogen bromide may be carried out with sodium or potassium hydroxide instead of ammonia.

QUANTITATIVE DETERMINATION OF CYANOGEN IODIDE

The determination of cyanogen iodide may be carried out by utilising its reaction with sodium arsenite 3 :

CNI + Na3AsO3 + H2O = HCN + HI + Na3AsO4.

1 CHATTAWAY, /. Chem. Soc., 1902, 81, 196.

2 F. OBERHAUSER and I. SCHORMULLER, Ber., 1929, 62, 1439. 8 GUTMANN, Ber., 1909, 42, 3624 ; Z. anal. Chem., 1925, 246.

210 CYANOGEN DERIVATIVES

The sample is treated with an exactly measured quantity of standard arsenious acid solution in excess and an excess of a 15% solution of sodium hydroxide. The liquid is warmed on the water-bath, and while still hot a slow current of carbon dioxide is passed through until the hydrocyanic acid is completely eliminated. The solution is then allowed to cool and made up to a convenient volume. The excess of arsenious acid is then determined in an aliquot of this by addition of sodium bicarbonate and titration with a standard solution of iodine.

212 SULPHUR COMPOUNDS

LABORATORY PREPARATION

In the laboratory, perchloromethyl mercaptan is easily prepared by the method recommended by Helfrich,1 based on the reaction of chlorine with carbon disulphide.

75 gm. carbon disulphide and 0-3 gm. iodine are placed in a flask of 200-300 ml. capacity fitted with a reflux condenser. While the flask is being externally cooled with water, a current of dry chlorine is passed in for several hours. In order to accelerate the reaction, it is as well to expose the contents of the flask to diffused daylight. When the volume of the liquid has been approximately doubled (and its weight has reached about 150-160 gm.) the current of chlorine is stopped and the mixture allowed to stand overnight. It is then warmed on the water-bath to drive off the excess of carbon disulphide and also the carbon tetrachloride, which is formed as a secondary product of the reaction. The sulphur chloride is decomposed by adding an equal volume of water to the residue and shaking vigorously. This is then distilled first in steam and then under reduced pressure (50 mm.).

PHYSICAL AND CHEMICAL PROPERTIES

Perchloromethyl mercaptan is an oily, clear yellow liquid with an unpleasant odour, which boils at ordinary pressure at 148° to 149° C. with decomposition. At 50 mm. pressure it distils unaltered at 73° C. Its specific gravity is 1-722 at o° C., its vapour density is 6-414, and its volatility is 18,000 mgm. per cu. m. at 20°C.

When perchloromethyl mercaptan is heated with water in a closed tube to 160° C., it decomposes into carbon dioxide, hydrochloric acid and sulphur.2 This decomposition also takes place to a very limited extent by the action of atmospheric humidity.

In the presence of oxidising agents, the sulphur atom is oxidised and the perchloromethyl mercaptan is converted into trichloromethyl sulphonic chloride, CC13—SO2—Cl.

Reducing agents act on perchloromethyl mercaptan in various ways. For instance, nascent hydrogen (from zinc and hydrochloric acid) reduces it to methane, iron and hydrochloric acid converts it into carbon tetrachloride (Helfrich), while with stannous chloride, thiophosgene is obtained.

1 HELFRICH, /. Am. Chem. Soc., 1921, 43, 591 ; AUTENRIETH, Ber., 1925, 58, 2152.

* RATHKB, Ann., 1873, 167, 201.

This last reaction also takes place with some other reducing agents, such as copper and silver powder.

When treated in the cold with chlorine, perchloromethyl mercaptan is converted into sulphur chloride and carbon tetrachloride 1:

/CO.

In contact with iron, perchloromethyl mercaptan decomposes even at ordinary temperatures.2

Perchloromethyl mercaptan has an irritant action on the eyes. The minimum concentration capable of causing this irritation is 10 mgm. per cu. m. of air. The limit of insupportability

Thiophosgene was prepared by Kolbe in l843,3 and was employed as a war gas by the Austrians and the French in the war of 1914-18 (" Lacrimite ").

PREPARATION

It may be obtained by passing a mixture of carbon tetrachloride and hydrogen sulphide through a red-hot tube, or by passing perchloromethyl mercaptan over silver powder 4 :

CC14S + 2Ag= CC12S + aAgCl.

In the laboratory, it is preferable to prepare thiophosgene by the reduction of perchloromethyl mercaptan with tin and hydrochloric acid.5

60 gm. of granulated tin and 200 ml. hydrochloric acid (density 1-19) are placed in a round-bottomed flask of about i litre capacity fitted with a tap-funnel and a reflux condenser. It is warmed gently to accelerate solution of the tin and meanwhile 60-70 gm. perchloromethyl mercaptan are added gradually

1 JAMES, /. Chem. Soc., 1887, 51, 273.

8 FRANKLAND and coll., /. Soc. Chem. Ind., 1920, 39, 256.

3KOLBE, Ann., 1843, 45, 44.

4RATHKE, Ann., 1873, 167, 204.

6 KLASON, Ber., 1887, 20, 2380.