Mercurous chromate

Mercurous chromate, molecular formula, properties, and uses, 6 562t Mercury (Hg), 16 31-58. See also Hg entries... 

Mercuric Chromate, HgCr04, is formed by heating an equivalent mixture of mercuric oxide and chromic anhydride with a little water. It yields dark red rhombic prisms, which are decomposed by water with formation of the basic salt, SHgO.CrOa. Many basic salts have been described, but it has been shown from an investigation of the system, HgO—CrOg—HgO, that the above is the only one which exists as a separate chemical entity. It may also be produced by precipitation of... 

If the chromium is in solution as a chromate or dichromate, as is the case after fusion as described above, it may either be reduced to the trivalent condition and precipitated as hydroxide, or directly precipitated as an insoluble chromate. In the absence of sulphates, barium chromate is precipitated by the addition of barium acetate at the boiling-point to a solution made faintly acid -with acetic acid and containing a little alcohol. After ignition the precipitate is w eighed as barium chromate. If chlorides and sulphates are present only in small amount, the chromate may be thrown down by mercurous nitrate, the mercurous chromate then being converted by ignition to the sesqui-... 

The conversion of carbon monoxide to carbon dioxide is also brought about by the action of other oxidising agents as silver oxide, potassium permanganate, iodic acid, chromic acid, mercuric chromate, etc. 

Methyl mercuric Chromate. See Methyl mercuric Salts under M... 

Observations on soln. of sparingly soluble chromates and dichromates have been made. Thus, R. Abegg and R. J. Cox studied the hydrolysis of mercuric chromate ... 

M. Groger reported potassium mercurous chromate, K2Cr04.Hg2Gr04, to be formed by the action of a soln. of mercurous nitrate, containing as little free nitric acid as possible, on a sat. soln. of potassium chromate. After shaking for a couple of weeks, the solid is separated by a suction filter, and dried on a porous tile, "flie brownish-yellow product decomposes when melted and when treated with water it furnishes basic mercurous chromate. 

According to A. Geuther, and A. J. Cox, if equimolar parts of chromium trioxide and yellow mercuric oxide be boiled with water, red crystals of mercuric chromate,... 

C. Freese, and A. J. Cox also obtained it by treating a soln. of mercuric nitrate with potassium dichromate or chromate. The conditions of equilibrium are illustrated by Fig. 51. N. A. E. Millon found that the same product is obtained with mercuric chloride, but A. Caillot could not prepare it with mercuric bromide. A. J. Cox, and A. Geuther obtained it by the hydrolysis of mercuric chromate and A. Geuther, by pouring a cold solution of mercuric carbonate in cone, nitric acid into cold water—the precipitate is at first yellow, but soon becomes scarlet-red. B. Skormin also prepared this salt. 

J. L. Gay Lussao and L. J. Thenard found that mercuric chromate is decomposed with incandescence when heated with potassium. 

S. Lowenthal described mercuric amidochromate, Hg3NH2(Cr04)2 and M. Groger said that potassium mercuric chromate is not produced by treating mercuric nitrate or chloride as in the analogous process for the mercurous salt, but rather yields a basic chromate which adsorbs potassium chromate. 

Yellow mercuric chromate results from the reaction of neutral or acetic acid solutions of mercury nitrate or acetate with potassium chromate. However, large quantities of alkali acetate prevent this precipitation. The interference is due to the formation of complex mercury acetate with consequent reduction of the Hg+ concentration to such an extent that the solubility product of HgCr04 is not reached. However, the slight concentration of Hg+2 in the yellow solution is adequate to react with solid AgCN to produce water-soluble, undissociated Hg(CN)a and Ag+ ions. The latter combine with the CrO - ions present to 3rield water-insoluble red-brown silver chromate. 

Walker (2003) states that a sketchbook belonging to Bouguereau dated to 1869 lists a set of pigments available to the artist which included a purple red material with composition given as mercuric chromate. 

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