Perthiocyanic Acid, H₂C₂N₂S₃

Perthiocyanic Acid, H₂C₂N₂S₃, may be prepared by the action of mineral acids on thiocyanic acid. When potassium thiocyanate (100 parts) dissolved in water (60 parts), and slightly more sulphuric acid (100 parts, sp. gr. 1-44) than is required to liberate the thiocyanic acid, are mixed, on standing orange-coloured crystals of perthiocyanic acid separate out:

3HCNS = HCN + H₂C₂N₂S₃.

When heated above 220° C., perthiocyanic acid decomposes into carbon disulphide, ammonia, sulphur, and a residue of composition which varies according to the temperature. On hydrolysis with water the main reaction is:

H₂N₂C₂S₃ + 2H₂O = NH₄CNS + CO₂ + H₂S + S.

Heated with concentrated sulphuric acid, perthiocyanic acid gives sulphur dioxide, carbon dioxide, thiourea, thiocyanic acid and ammonium sulphate. Heated with chlorine the acid gives cyanogen chloride, sulphur chloride, hydrogen chloride and a reddish-brown insoluble residue. Ferric chloride gives a coloration similar to that produced with thiocyanic acid.