Thiocarbonic Acid, H₂CS₃

Carbon disulphide reacts with soluble sulphides and the salts produced are the sulphur analogues of the carbonates. It was discovered by Berzelius that when carbon disulphide is brought into contact with sodium sulphide solution, the former dissolves, producing a solution from which alcohol precipitates sodium thiocarbonate, Na₂CS₃, as a yellowish-brown oil. By the action of dilute hydrochloric acid on this salt thiocarbonic acid may be obtained as a reddish-yellow oil which possesses a very disagreeable odour and is decomposed by heat into carbon disulphide and hydrogen sulphide, no free sulphur being formed.

Pure thiocarbonic acid is better obtained by dropping pure solid ammonium thiocarbonate into a large excess of concentrated hydrochloric acid, when it separates as a bright red liquid which can be dried over phosphorus pentoxide. The liquid has a density of 1.47 at 17° C. and a surface tension at 12.5° C. of 48.3 dynes/cm.; it freezes to an orange-coloured solid at -30.5° C. The acid decomposes on keeping, giving carbon disulphide, in which it dissolves. It dissolves slightly in water, acids and many organic solvents, giving yellow solutions which gradually decompose with loss of colour. In acetone the decomposition is not accompanied by separation of sulphur even on long standing.

Calcium and sodium carbonates react with the acid forming the corresponding thiocarbonates, thus:

H₂CS₃ + Na₂CO₃ = Na₂CS₃ + H₂O + CO₂.

An investigation of the freezing curve for the system CS₂-H₂S has demonstrated the existence of two thiohydrates, CS₂.H₂S and CS₂.6H₂S, the former being isomeric with thiocarbonic acid.