Mercury thiocyanate

Colourless, light-sensitive powder. Insoluble in H2O, soluble in KSCN solution, precipitating Hg. Decomposes on heating, forming a foamy mass.

A slightly acid solution of Hg2(NO3)2, freed of mercuric ions using metallic Hg, is treated with less than the stoichiometric amount (~75%) of KSCN solution. A dark-grey salt precipitates first; after standing for several days (repeated stirring), it becomes completely white. It is washed several times with boiling H2O.