Preparation of mercury(II) cyanide
To a solution of 7.3 g of pure potassium cyanide in 100 ml of water is added, in small portions, and with good stirring, 14.4 g of mercury(II) oxide. When all but a small amount of the oxide remains undissolved, the solution is filtered. To the clear filtrate, immersed in ice water, one or two drops of phenolphthalein indicator are added and then 3M sulfuric acid is added by drops from a burette until the alkali is just neutralized. About 22 ml of the acid should be required. The mixture is evaporated to dryness on the water bath in the hood, and the residue of product and potassium sulfate is treated with hot absolute methanol in three portions of 50 ml, 30 ml, and 30 ml respectively. The filtered methanolic extracts are combined and evaporated to dryness on the steam bath yielding 14-16 g of mercury(II) cyanide.
Inorganic laboratory preparations, by G. G. Schlessinger, 47, 1962