Mercuric nitrate-potassium chromate solution

Reagent Mercurynitrate (or acetate)-potassium chromate solution. Several ml of a 5 % aqueous solution of mercuric nitrate or acetate are made acidic with dilute acetic acid and 2 g of sodium acetate are added, followed by several drops of a potassium chromate solution. Should a slight precipitate appear, it can be removed by filtering or centrifuging. The clear yellow reagent is stable. 

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. 

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. 

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