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Compounds of Mercury

mercuric, mercurous, iodide, oxide, obtained, acid and salts

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COMPOUNDS OF MERCURY Mercury forms two well-defined series of salts—the mercurous salts derived from the oxide Hg20, and the mercuric salts from the oxide HgO. Like mercurous oxide the mercurous salts are generally formulated as containing two mercury atoms in each molecule. According to H. B. Baker vaporized mercurous chloride or calomel (q.v.) has the formula Hg2C12.

Mercurous Oxide, is an unstable dark-brown powder formed when caustic potash acts on calomel; it is decomposed by light or on trituration into mercury and mercuric oxide. Mer curic oxide, HgO, occurs in two forms : it is obtained as a bright red crystalline powder (also known as "red precipitate," or as mercurius praecipitatus per se) by heating the metal in air, or by calcining the nitrate, and as an orange-yellow powder by precipi tating a solution of a mercuric salt with potash ; the difference is probably one of subdivision. The yellow form is the most reactive and is transformed into the red when heated to 400°. If the red oxide be heated it becomes black, regaining its colour on cooling, and on further heating to 63o° it decomposes into mercury and oxygen. It is slightly soluble in water, to which it imparts an alkaline reaction and strongly metallic taste. A peroxide is ob tained as a brown solid from mercury and slightly acid 3o% hydrogen peroxide at low temperatures.

Mercurous and mercuric chlorides, known respectively as calo mel (q.v.) and corrosive sublimate (q.v.), are two of the most important salts of mercury. Mercurous bromide, is a yellowish-white powder, insoluble in water. Mercuric bromide, forms white crystals, sparingly soluble in cold water, readily in hot, and prepared by the direct union of its Mercurous iodide, is a yellowish-green powder obtained by heating its components to about 25o°, or by triturating them with a little alcohol ; it is also obtained by precipitating a solution of mercurous nitrate with potassium iodide. It is blackened by exposure to light. Mercuric iodide, exists in two crystalline forms. By mixing solutions of mercuric chloride and potassium iodide under a microscope, yellow rhombic plates are seen to be formed which are transformed very quickly into scarlet quadratic octahedra. On heating to about 126° the red form is transformed

into the yellow modification; on cooling the reverse gradually occurs, and immediately if the yellow iodide be touched. When cooled to the temperature of liquid air red mercuric iodide as sumes a pale yellow colour. Mercuric iodide is insoluble in water, but soluble in absolute alcohol ; and also in potassium iodide solu tion, with the formation of K2HgI4, which may be obtained in lemon-yellow crystals. A strongly alkaline solution of this salt known as Nessler's reagent is specially used for determining traces of ammonia. (See below.) Mercuric iodide dissolves in other iodide solutions to form similar compounds; these solutions are characterized by their exceptionally high specific gravity, and hence are employed in density determinations. (See DENSITY.) It also forms many other double salts. Oxidation with strong nitric acid gives the iodate, Hg(I03)2.

Mercurous Nitrite is obtained in yellow needles by the action of cold dilute nitric acid on mercury. Prolonged action of this acid transforms the salt into mercurous nitrate, a white crystalline salt soluble in water. If the metal be in excess basic salts are obtained. By adding ammonia to a solu tion of mercurous nitrate a black precipitate of variable composi tion, known in pharmacy as mercurius solubilis Hahnemanni, is obtained.

Mercuric Nitrate.—By dissolving mercuric oxide in strong nitric acid there is obtained a heavy liquid which on the addition of more strong nitric acid gives a white precipitate of Water decomposes it to basic salts. By dissolving the oxide in dilute nitric acid, the crystalline basic salt is obtained.

When mercurous or mercuric nitrate is treated with aqueous ethylene thiourea (etu) a stable neutral complex nitrate [Hg,3etu] is produced in rhombic plates which dissolve in water to a neutral solution and resemble the nitrates of the alkaline earths.

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