_In solution_, this substance may be detected by what are called the liquid tests. A solution of arsenic in water is colorless, almost tasteless, and has a very slightly acid reaction. If a few drops be evaporated on a glass slide and examined by the microscope, numerous minute and mostly imperfect octahedral crystals, or an amorphous deposit, will be seen, presenting triangular surfaces by reflected light.
1. The _ammonio-nitrate of silver_ (prepared by adding a few drops of liquor ammoniæ to a solution of nitrate of silver, till the brown oxide of silver at first precipitated is nearly redissolved) throws down with arsenious acid a rich yellow deposit of arsenite of silver.
2. The _ammonio-sulphate of copper_ (formed by adding liquor ammoniæ to a solution of sulphate of copper till the bluish-white oxide of copper is almost redissolved) produces a pale green precipitate, arsenite of copper, or Scheele’s green. Care must be taken not to add too much of the test in the first instance, otherwise its blue may overpower the green of the precipitate.
3. _Sulphuretted hydrogen water_ precipitates a yellow deposit of sulphide of arsenic. It is better, however, to use pure and well-washed sulphuretted hydrogen generated in the usual manner. Care must be taken that the liquid is not alkaline, or no precipitate will be produced, even though arsenic be present. For this reason yellow sulphide of ammonium will not precipitate arsenic until acidulated with pure hydrochloric or some such acid. The precipitate should be collected, carefully washed, and dried. It should then be mixed with black flux or dry ferrocyanide of potassium and reduced, as already described.
These tests are so delicate, especially the first, that they will detect the 8000th part of a grain of arsenic in solution; they should be employed successively. There are several other processes which require to be noticed, namely, Marsh’s process, the process known as Reinsch’s process, Fresenius’s process, and the Destillation process.
_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.