Treatise on Poisons by Sir Robert Christison

sulphate of copper, soon becomes covered with a red powdery crust, which is metallic copper; and ere long the solution is changed in colour from blue to greenish-yellow. The action is simple; the iron merely displaces the copper in the solution, in which a sulphate of iron is consequently formed. This test is characteristic, and even of considerable delicacy. At the same time other substances may cause a reddish encrustation on iron by simply rusting it, so that the test cannot be relied on alone. The four preceding reagents taken together are amply sufficient to prove the existence of copper in a solution. Three other tests, however, may be here briefly alluded to. Caustic potass in a solution not too diluted causes a fine azure-blue precipitate, the hydrated peroxide of copper. Oxide of arsenic, with the previous addition of a few drops of ammonia, causes a fine apple-green or grass-green precipitate, the arsenite of copper. This test, which is both delicate and characteristic, has been already fully considered under the head of Arsenic. The process by fluid reagents, as hitherto laid down, merely proves the presence of copper, but does not indicate the acid with which the oxide is combined. In order to determine whether it is sulphuric acid, the fluid must also be tested with nitrate of baryta followed by nitric acid: a heavy white precipitate is thus produced, which the excess of nitric acid does not redissolve.