The sulphocarbonates, or thiocarbonates (from the Greek theion, sulphur) form a class of salts analogous to the carbonates, both in their formulas and in the method of their preparation. Carbon disulphide, a volatile liquid, boiling at 46°, possessing a disagreeable smell, is produced when sulphur vapour is led over charcoal heated to redness in a fireclay tube ; in fact, the carbon is burned in sulphur gas. When shaken with a concentrated aqueous solution of the sulphide of sodium or potassium, it dissolves, forming the compound Na2CS3, or K2CS3. These thiocarbonates, like the carbonates, are white, crystalline salts; on adding acid, thiocarbonic acid separates as an oil; it slowly decomposes, especially if warmed, into hydrogen sulphide and carbon disulphide. Many of its salts are insoluble, and may be prepared by precipitation.
The formula of carbon dioxide is CO2, that of carbon disulphide CS2; and it is evident that an intermediate substance should exist of the formula COS. This substance is carbon oxysulphide. It is a gas, prepared by heating with water thiocyanic acid, HSCN, the ammonium salt of which is produced when ammonia is passed through a mixture of carbon disulphide and alcohol: CS2 + 2NH3. Ale = H2S + (NHJSCN.Alc. On evaporation of the alcohol the ammonium thiocyanate crystallises out. This salt, distilled with sulphuric acid, yields in passing the acid HSCN, which, on account of the high temperature, reacts with water, forming ammonia (which yields ammonium sulphate with the sulphuric acid) and carbon oxysulphide, COS : HSCN + H2O = NH3 + COS.
Like nitrous oxide, carbon disulphide is an endothermic compound, and can consequently be decomposed by shock ; when a fulminate is exploded in it, it is resolved into carbon and sulphur. On the other hand, carbon dioxide and oxysulphide are exothermic compounds, heat being evolved during their formation.