Marsh’s Test or Process.—This process is founded on the decomposition of arsenious acid by nascent hydrogen, and the formation of arsenuretted hydrogen gas, which possesses the following properties: It burns with a bluish-white flame, and white smoke (arsenious acid), possessing a slight garlic odor. If a piece of glass or porcelain be held in the flame a blackish metallic stain will be deposited upon it, consisting of metallic arsenic. This stain might be confounded with one produced by antimony under similar circumstances. But the antimonuretted hydrogen gas does not burn with the odor of the arsenuretted hydrogen; while the antimonial stain is sooty, and has not a metallic lustre. The arsenical stain is further readily dissolved by a solution of chloride of lime (bleaching powder), whilst the antimonial stain is not affected. To the stain may be added a few drops of strong nitric acid; it will dissolve the arsenic; if this be allowed to evaporate, and the acid be neutralized, a few drops of nitrate of silver solution will give a brick-red precipitate of arseniate of silver.
Marsh’s test is thus accomplished: a flask furnished with a cork through which pass a funnel and a long bent tube drawn to a point, is prepared so that the funnel reaches almost to its bottom. Several pieces of pure zinc are introduced, and then some sulphuric acid is poured through the funnel. In this way hydrogen gas is produced and escapes by the bent tube. When all the air has been expelled the gas should be ignited as it escapes, and a piece of cool porcelain held over the flame. If there be no deposit it is plain that neither the zinc nor the sulphuric acid, contains arsenic. A portion of the suspected fluid is next introduced into the flask by the funnel, and the issuing gas again tested. Should a stain giving the characters alluded to above be produced, the fluid contains arsenic in some form or other.
Reinsch’s Process.—The suspected liquid is boiled with from one-sixth to one-eighth of its bulk of pure hydrochloric acid, and a bright slip of copper introduced. If arsenic be present the copper will be coated with it in the form of an iron-grey deposit. Next after removing the copper, washing it with distilled water, and drying it between folds of blotting paper, cut it into slips, and introduce it into a reduction-tube and apply heat; arsenious acid will be sublimed and deposited on the sides of the tube, in the form of minute octahedral crystals. These may be dissolved in water and tested in the usual way. Before resorting to this test, the acid must be examined to make sure of its purity. This is easily effected by boiling the copper with a mixture of the hydrochloric acid and distilled water before adding the suspected liquid. In conducting the analysis in the case of Smethurst (Cent. Crim. Court, Aug. 1859) Taylor and Odling found that all the varieties of copper in common use for Reinsch’s process contained arsenic. A copper of ascertained purity must therefore be used.
Arsenic in Organic Matters.—The following process, which has been introduced by Dr. Taylor, is a very convenient one. The suspected matters are to be thoroughly dried in a water-bath or otherwise, taking care not to use too great a heat. They are then to be introduced into a flask fitted with a long bent tube; to the dried material is to be added a quantity of strong hydrochloric acid, proved free from arsenic, sufficient to drench it, and the whole allowed to digest for some hours. At the end of that time heat is to be applied to the flask by means of a sand-bath, and a receiver fitted to the bent tube. The receiver should contain a little water, and both it and the bent tube should be kept cool. As distillation goes on the arsenic passes over in the form of chloride of arsenic, and is collected in the receiver beyond. A second portion of hydrochloric acid may be used to remove any last traces of arsenic in the organic material. The arsenic may be recovered from the chloride by boiling with pure polished copper, as in Reinsch’s process.
The process of Fresenius and Von Babo given at page [31] is especially adapted for the recovery of arsenic from organic admixture.
Several cases of poisoning by arsenuretted hydrogen are on record, some proving fatal.