(1) BLACK MODIFICATION: HgCl2 + H2S = HgS + 2HCI Hydrogen sulfide is introduced into a mercury (II) solution in
1-2N HCl, absolutely free of oxidizing agents. The transient white
to brownish precipitate reacts with additional H3S to yield black HgS, e.g., Hg3S2CI2 + H2S = 3 HgS + 2 HC1. (2) RED MODIFICATION: Hg(CH3COO)2 + H2S = HgS + 2CH3COOH A solution of 35 g. of Hg(CH3COO)3 and 25 g. of NH4SCN in 100
ml. of hot glacial acetic acid is prepared. A moderately fast
stream of H3S is then introduced until precipitation is complete.
The acetic acid is then slowly evaporated (caution, HCN!), and
the black precipitate transforms to the red form. The glacial
acetic acid must be present until conversion is complete; overheating must be avoided. During the last stage the paste must be
constantly stirred. If this is neglected, the product is dull red
or brown. When the acid has been completely removed and the
product cooled, 200 ml. of HSO is added and the mixture is filtered
through a Buchner funnel. The product is washed and dried between two layers of thick filter paper. Yield 25 g.
