Taylor (Med. Jurisprudence, 1873, vol. i., p. 364) mentions a case where the blood had a strong odour of prussic acid, and the mucous membrane of the stomach, even after it had been washed three times with water, also exhaled a strong odour. In another case (Med. Gaz., vol. xxxvi., p. 104), where 20 grains of Scheele’s acid had been taken with ultimate recovery, the vomited matters had no odour, “showing that, if not concealed by other odours, the whole of the acid must have been absorbed.” Many other instances might be quoted where nothing was smelt, and yet the tests revealed prussic acid.
As to the question about the salts of prussic acid, it may be generally said that all poisonous cyanides would smell in the stomach, except, perhaps, mercuric cyanide. See “Properties of the Salts,” p. 73. Possibly the formation of mercuric cyanide may have accounted for the absence of odour in some of the above cases, as I do not find that mercury was tested for, though its compounds are common medicines. Otherwise it is hardly possible that hydrocyanic poisoning should have been effected, and the acid be still there, without its very characteristic odour being perceptible to an observer with an acute olfactory sense. I have entered at some length into the question of odour, as much importance has been attached to it in the trials, and I still consider it as one of the most delicate and positive of tests.
II. Silver Test.—When silver nitrate is added to a solution containing HCN or a cyanide acidulated with nitric acid, a white precipitate falls of silver cyanide, soluble in ammonia, insoluble in dilute, but soluble in hot concentrated nitric acid, and not blackened by light. This reaction is rendered quantitative according to Liebig’s volumetric method. The original solution is made slightly alkaline by potash, and a standard solution containing 1·7 grammes of silver nitrate per litre (1 cub. centimetre = ·0017 grm. AgNO3) is added until a permanent white turbidity is produced, seen best over a sheet of black paper or a black book. Then each cub. centimetre used is equivalent to a double quantity or ·00054 grammes of HCN. Formic acid, or chlorides, do not interfere; in fact, it is advantageous to have a little chloride present.
The silver cyanide may be also estimated gravimetrically by adding excess of silver nitrate, collecting the precipitate on a weighed filter, washing, drying, and weighing. Silver cyanide corresponds to two-tenths of its weight of HCN (134 gives 27). If chlorides be present, the mixed precipitate of silver cyanide and chloride is weighed, treated with dilute hydrochloric acid, and weighed again. The HCN is thus displaced, and passes into the filtrate; the silver precipitate, now all as chloride, is weighed again: then the increase of weight multiplied by 27 and divided by 9·5 (the difference between the equivalent weights of silver chloride and cyanide) is equal to the weight of HCN present. But the volumetric process is quite as accurate, and more expeditious.
In poisoning cases advantage is taken of the opacity of silver cyanide thus: A drop of moderately dilute silver nitrate is placed on a watch-glass over the substance, which may be gently warmed, taking care that the steam condensed does not cause the drop to fall. If HCN be present, the drop will become opaque-white from formation of silver cyanide. 1/100 grain of HCN, equal to ¼ grain of B.P. acid, will give this reaction (Taylor). If there is only a small amount, and the action is gradual, the drop on drying in the air may exhibit crystals of silver cyanide, recognizable under the microscope as minute prisms obliquely truncated. Of course the silver nitrate itself may give crystals, but they will be very soluble in water.
Cyanide of silver is decomposed by (1) hydrochloric acid, giving silver chloride; (2) dilute sulphuric acid and zinc, giving silver; (3) sulphuretted hydrogen, giving silver sulphide; in each case HCN is liberated and may be distilled off: then the other tests may be applied.
If sulphuretted hydrogen be present, it will give a black with silver nitrate. The liquid should in this case be previously shaken with just enough carbonate of lead to remove the sulphuretted hydrogen. The latter, however, does not interfere with the Prussian blue or sulphur tests.
When sufficient in quantity, the cyanide of silver, thoroughly dried in a water-bath, may be transferred to a small bulb-tube and heated, the end being closed with the finger. It breaks up into cyanogen gas, silver, and paracyanide of silver, a peculiar glow and effervescence occurring as it decomposes. The cyanogen will have the characteristic bitter almond odour, and, on removing the finger, will burn with a flame violet on the margin and rosy in the centre.
III. Prussian Blue (Scheele).—Add to the solution or distillate caustic potash in excess, then a drop or two of fresh ferrous sulphate (protosulphate of iron), and a little ferric chloride (perchloride of iron—the tinct. ferri perchlor. of the Pharmacopœia will do), warm gently for a few minutes, add dilute hydrochloric acid in slight excess: if much HCN be present, a deep blue precipitate (Prussian blue) will remain; if only a trace, the liquid will be greenish, and on standing till the next day a blue deposit will form.[22] This is the only blue iron precipitate which is insoluble in dilute hydrochloric acid.[23]
Remarks.—Sulphuretted hydrogen does not interfere with this test, as the black ferrous sulphide dissolves in hydrochloric acid. The amount of iron salts added should have some relation to the amount of HCN present, an idea of which will have been attained by the silver test. Moderate excess of potash must be present all the time till the hydrochloric acid is added. A large amount of iron salt is objectionable, as the yellow colour interferes with the final green tint with traces of HCN. The test may be made quantitative by imitating the tint with a weak standard solution of potassium ferrocyanide treated with a drop of hydrochloric acid and a drop of ferric chloride, on the same principle as “Nesslerizing” (see Wanklyn’s Water Analysis). Finally the precipitate of Prussian blue should be preserved to exhibit at the trial, as this is the most positive, though not the most delicate, test.