2. Sulphuretted hydrogen and ammonium sulphide give a chocolate-coloured precipitate.
3. Ferrocyanide of potassium gives a port-wine colour, or reddish-brown precipitate.
4. If a bright steel needle be introduced into an acid solution of copper, the metal is deposited on the needle.
5. If a piece of zinc bound with platinum wire be placed in a solution of a copper salt, the metal is deposited on the platinum; it is turned violet on exposure to the vapour from sulphuric acid mixed with potassium bromide.
Detection of Copper in Organic Liquids
A.—The finely-divided tissue, or the contents of the stomach, diluted with water, are thrown on a filter, and the insoluble portion set aside for further treatment. (See B.)
The filtrate and washings may now be concentrated, acidified with sulphuric acid, and a polished needle inserted in the liquid; and should no immediate deposition of metallic copper occur, it may be allowed to remain for several hours. The colour of the metallic deposit is highly characteristic of copper. As a corroborative proof, the concentrated liquid may be placed in a platinum capsule with some fragments of zinc, when the copper will be deposited on the platinum capsule at the parts in contact with the zinc; the liquid poured off, and the excess of zinc adhering to the platinum removed by dilute hydrochloric acid. The copper may now be dissolved off the platinum by nitric acid, the excess of acid driven off by heat, and the solution subjected to the wet tests given above.
B.—The insoluble portion from A is incinerated in a porcelain crucible. The ash thus obtained is digested in hydrochloric acid with the aid of heat, and evaporated nearly to dryness. The residue, dissolved in distilled water, may be tested as under ‘A.’
ZINC
The sulphate and the chloride of zinc are alone important. Poisoning by the chloride of zinc has been described ([p. 265]).