If arsenic be present, the distillate contains the arsenic as chloride, and can be at once subjected with great facility to the usual tests for the presence of that metal. This mode of proceeding both facilitates and expedites the ordinary methods of testing, as it separates the arsenic present from the complex organic mixtures with which it is associated, and presents it in a comparatively pure form for identification. The process also admits of the residue left in the retort being examined for lead and the other metallic poisons.
Before the following processes are applied, some of the sediment from the contents of the stomach, or the vomited matters, may be collected and well washed. If this is boiled in distilled water and filtered, the following tests, known as “the liquid tests for arsenic,” may be applied to the filtrate:
1. Ammonia Nitrate of Silver, prepared by adding a weak solution of ammonia to a strong solution of nitrate of silver, gives with arsenic a yellow precipitate of arsenite of silver soluble in nitric, citric, acetic, and tartaric acids, and ammonia.
2. Ammonia-Sulphate of Copper, prepared by adding ammonia to a dilute solution of sulphate of copper, gives with arsenic a green precipitate of arsenite of copper. This precipitate is soluble in the mineral and vegetable acids and ammonia, but is not affected by soda or potash. The precipitate, dried and heated in a reduction tube, yields octahedral crystals of arsenious acid.
3. Sulphuretted Hydrogen.—The suspected liquid should be first slightly acidulated with pure hydrochloric acid before the sulphuretted hydrogen gas is passed into it, when, if arsenic be present, a yellow precipitate is formed, known to be such by the following tests:
(1) Insoluble in water, ether, alcohol, the vegetable acids, and dilute hydrochloric acid, but decomposed by strong nitric and nitro-hydrochloric acids.
(2) Dissolved, if no organic matter present, forming a colourless solution, when potash, soda, or ammonia is added.
(3) The yellow precipitate dried and heated with soda and cyanide of potassium yields a sublimate of metallic arsenic.
N.B.—None of the above tests should be applied in the presence of organic matter. The soluble salts of cadmium and per-salts of tin give yellow-coloured precipitates with sulphuretted hydrogen.
(4) If stannous chloride dissolved in strong hydrochloric acid be added to a solution of arsenic in hydrochloric acid, metallic arsenic is thrown down as a precipitate. This is a fairly delicate test.