The first point to be noticed is, whether any odour of the acid can be perceived in the substance under examination. In any case, the contents of the stomach or finely-divided tissues should be mixed with water, and examined as to the reaction with test paper. If the mixture be found to be alkaline, it must be neutralised by the addition of tartaric acid; if, on the contrary, it be acid, carbonate of soda must be carefully added to neutralisation. A state of neutrality is always necessary previous to distillation, for the following reasons:
An alkaline state of the liquid would, on the one hand, prevent, or, at all events, retard, the evolution of the hydrocyanic acid; whilst, on the other, the existence of any free acid would decompose any cyanide which might be present, and thus give rise to an evolution of hydrocyanic acid not existing as such in the mixture.
The organic mixture is then placed in a flask, and the contents distilled at as low a temperature as possible by the aid of a water bath.
Should hydrocyanic acid be present, the distillate will yield all the characteristic reactions of the dilute acid.
1. Nitrate of silver will give a curdy-white precipitate, insoluble in cold but soluble in boiling nitric acid. A portion of the precipitate, on the addition of some liquor potassæ, sulphate of iron, ferric chloride and hydrochloric acid, forms Prussian blue. In this test, which may be taken as quite conclusive, the hydrochloric acid decomposes the cyanide of silver; and on the addition of the sulphate of iron, Prussian blue is formed.
2. If a portion of the dry precipitate formed by the nitrate of silver be heated in a test tube, cyanogen gas will be evolved, known by its characteristic odour of peach blossoms, and by its burning at the mouth of the tube with a rose-coloured flame.
3. To the solution containing hydrocyanic acid add a few drops of potassium nitrite, two or three drops of ferric chloride solution and dilute sulphuric acid until a yellow tint is obtained; heat to boiling, cool, precipitate excess of iron with ammonia, filter, and add one or two drops of a very dilute solution of colourless ammonium sulphide. A very minute quantity of hydrocyanic acid gives a violet-red colour, changing to blue, green, and finally yellow.
4. If a solution of starch be tinged blue with iodine, the colour will be discharged by a minute quantity of hydrocyanic acid.
Vapour Tests.—There are three tests for the presence of hydrocyanic acid when present in organic mixtures, which have the advantage of being applicable without the addition of anything extraneous to the mixture to be tested. They are all dependent on the volatile nature of hydrocyanic acid, and may be applied as follows, the suspected mixture being divided into three portions:
1. Iron or Prussian Blue Test.—The liquid mixture to be tested is placed in a small beaker glass, and covered with a glass plate the centre of which is smeared with a mixture of potash and proto-sulphate of iron. The whole is now left undisturbed for some time. The glass is eventually removed, and the mixture of potash and iron treated with hydrochloric acid, which, should hydrocyanic acid be present, will cause the development of Prussian blue.