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• Physical Properties

  Mercury(II) sulfide has several modifications, the only two stable allotropic forms are:
  1. the red hexagonal form known as cinnabar (alpha form), and 2. the black cubic modification (beta form).
  Cinnabar is a red crystalline or powdery substance; hexagonal crystal system; refractive index 2.854; density 8.10 g/cm3; sublimes at 583.5°C; color changes to brown at 250°C and converts to black sulfide at 386°C; reverts to red color on cooling; insoluble in water, alcohol and nitric acid; soluble in aqua regia and solutions of alkali metal sulfides; decomposed by hot concentrated sulfuric acid.
  Black sulfide is a black amorphous powder or crystalline substance (beta form); cubic structure; metastable at ordinary temperatures; converts to red sulfide by sublimation at ordinary pressure; density 7.73 g/cm3; melts at 583.5°C; insoluble in water, alcohol and nitric acid; soluble in aqua regia, alkalies, and solutions of alkali metal sulfides. ;

• Preparation

  Red sulfide occurs natively and is mined from mineral cinnabar. Also it can be prepared by heating mercury with a solution of potassium pentasulfide, producing a scarlet cinnabar:
  Hg + K2S5 → HgS + K2S4
  The red sulfide also may be made from black sulfide by heating in a concentrated solution of alkaline polysulfide. The shade of pigment varies with temperature, reaction time, and concentration of the black sulfide.
  Alternatively, vermilion may be made by grinding sodium sulfide with sulfur and slowly adding mercury. The shades are not bright when prepared at 0°C.
  The black mercury(II) sulfide is prepared usually by precipitation from an aqueous solution of mercury(II) salt with hydrogen sulfide. Thus, when H2S is passed into a solution of HgCl2, a pale yellow precipitate of composition HgCl2•2HgS forms. This converts to amorphous black HgS on further treatment with H2S.
  The black sulfide may also be made by other methods such as adding sodium thiosulfate, Na2S2O3 in excess to a dilute solution of sodium mercurichloride, Na2HgCl4 and treating mercury with molten or powdered sulfur. ;

• Reactions

  When heated in a current of air, mercury(II) sulfide is converted into metallic mercury and sulfur dioxide:
  HgS + O2 → Hg + SO2
  Similar reduction to metallic mercury occurs when the sulfide is heated with several metals or metal oxides:
  HgS + Fe → Hg + FeS
  4HgS + 4CaO → 4Hg + 3CaS + CaSO4
  Mercury(II) sulfide dissolves in concentrated solutions of alkali or alkalineearth metal sulfides forming thiosalts, such as Na2[HgS2]•xH2O. Such thiosalts are stable in solution only when alkaline hydroxides are present in excess. These salts also are obtained as bright and deliquescent needles when HgS is heated with sulfur and alkaline hydroxides. ;

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