Mercury in solution can be withdrawn by acidulating the liquid, and then inserting either simply a piece of gold foil, gold wire, or bright copper foil; or else, by a galvanic arrangement, such as iron wire wound round a gold coin, or gold foil attached to a rod of zinc; or, lastly, by the aid of gold or copper electrodes in connection with a battery. By any of these methods, mercury is obtained in the metallic state, and the metal with its film can be placed in a subliming cell, and globules deposited and identified, as before described.
The Precipitating Reagents for mercury are numerous: a solution of stannous chloride, heated with a solution of mercury, or any combination, whether soluble or insoluble, reduces it to the metallic state.
Mercurous Salts in solution yield, with potash, soda, or lime, a black precipitate of mercurous oxide. Mercuric Salts, a bright yellow precipitate of mercuric oxide.
Mercurous Salts yield black precipitates, with sulphides of ammonium and hydrogen. Mercuric Salts give a similar reaction, but, with sulphuretted hydrogen, first a whitish precipitate, passing slowly through red to black.
Mercurous Salts, with solutions of the chlorides, give a white precipitate of calomel; the Mercuric Salts yield no precipitate under similar circumstances. Mercurous Salts, treated with iodide of potassium, give a green mercurous iodide; Mercuric, a scarlet.
§ 853. The Detection of Mercury in Organic Substances and Fluids.—Ludwig’s process, previously described, is found in practice the best. Fluids, such as urine, must be evaporated to dryness, and then treated with hydrochloric acid. Such organs as the liver are cut up and boiled in 20 per cent. HCl. Distinct evidence of mercury in the liver has been obtained on a piece of copper gauze, in a case where a child had been given 2 grains of calomel before death. “Four ounces of the liver were treated with hydrochloric acid and water, and a small piece of pure copper placed in the acid liquid while warm, and kept there for about forty-eight hours. It acquired a slight silvery lustre, and globules of mercury were obtained from it by sublimation.”
To detect the cyanide of mercury may require special treatment, and Vitali[943] recommends the following process:—The fluid is acidified with tartaric acid and neutralised by freshly precipitated CaCO3; a slight excess of hydric sulphide is added, and the flask allowed to rest for twenty-four hours in the cold. Then a further quantity of SH2 is added, and a current of hydrogen passed through the liquid; the effluent gas is first made to bubble through a solution of bismuth nitrate in dilute nitric acid (for the purpose of absorbing SH2), and then through aqueous potash (to absorb HCl); in the first flask the analyst will separate and identify mercury sulphide, while in the last flask there will be potassic cyanide, which will respond to the usual tests.
[943] L’Orosi, xii. 181-196.