Ammonium thiocarbonate, (NH₄)₂CS₃

Ammonium thiocarbonate, (NH₄)₂CS₃, may be prepared by digesting ammonium pentasulphide with carbon disulphide under a reflux condenser; a white deposit of ammonium monosulphide first forms on the condensing surface and is gradually converted into the thiocarbonate, becoming pale orange-coloured. On exposure to moist air it turns red.

The thiocarbonates of the alkali and alkaline earth metals may also be obtained by the action of carbon disulphide on the corresponding hydroxides under certain conditions. On shaking carbon disulphide with aqueous sodium hydroxide, a deep red solution is obtained, the reaction being facilitated by warming to 80° C., and especially by the presence of emulsifying agents. The solution contains sodium trithio-carbonate sufficient in amount to satisfy the equation:

3CS₂ + 6NaOH = 2Na₂CS₃ + Na₂CO₃ + 3H₂O.

No dithiocarbonate is formed, nor in freshly prepared solutions are polysulphides or thiosulphates present. With oxidising agents such as hydrogen peroxide or chlorine, the red solution yields sulphates, whilst salts of the heavy metals give coloured precipitates which are unstable when moist, decomposing to carbon disulphide and the metal sulphide.

It was generally supposed that the removal of carbon disulphide from crude coal gas when purified by lime was due to its absorption by calcium sulphide with the formation of calcium thiocarbonate, but Yeoman suggests that it is the perthiocarbonate, CaCS₄, which is formed. This suggestion accounts for the necessity of the presence of some oxygen in order that absorption may take place.

The thiocarbonates of the alkali and alkaline earth metals are soluble in water. The alkali thiocarbonates are yellow in colour. In concentrated solution these salts are fairly stable, but in dilute solution they are gradually decomposed, forming the carbonate and hydrogen sulphide.

Under certain conditions thiocarbonates behave like simple sulphides; thus, benzoyl chloride and a thiocarbonate in equimolecular proportions yield benzoyl disulphide, which substance is also formed by the interaction of benzoyl chloride and potassium hydrogen sulphide followed by oxidation. The complex salts of the thiocarbonates are more stable than the simple salts.

Thiocarbonates, especially the alkali thiocarbonates, are used in agriculture to prevent certain diseases, notably phylloxera of the vine. It is carbon disulphide which is the active material of these fungicides.

It has been shown that when a solution of potassium thiocarbonate is boiled in an atmosphere of nitrogen, the following change takes place:-

K₂CS₃ = K₂S + CS₂,

the sulphide subsequently undergoing hydrolysis, thus:

K₂S + 2H₂O = 2KOH + H₂S

In the presence of air or oxygen, however, the reaction is as follows:- 2K₂CS₃ + 2H₂O + 2O₂ = K₂S₂O₃ + K₂CO₃ + CS₂ + 2H₂S,

and in an atmosphere of carbon dioxide:

K₂CS₃ + CO₂ + H₂O = K₂CO₃ + CS₂ + H₂S.

It is believed that the reaction

K₂CS₃ + 3H₂O = K₂CO₃ + 3H₂S

cannot take place.

When carbon disulphide is heated with sodium alone in a sealed tube at 150° C., no reaction other than slight superficial tarnishing of the metal occurs; with a sodium-potassium alloy yellowish-brown explosive crusts are formed. When the liquid is shaken with sodium amalgam (0.8 per cent. Na) and the mixture then treated with 90 per cent, alcohol, a solution containing a complex mixture of compounds is obtained, from which by treatment with methyl chloride the following compounds may be isolated: methyl sulphide, two isomeric methyl tetrathio-oxalates, methyl methylxanthate, tetramethylthiolethylene and methyl trithiocarbonate (b.pt. 224° C. at 760 mm.). Similar products (except the xanthate) may be obtained by allowing carbon disulphide to react with sodium in solution in liquid ammonia and treating the resulting brown mass with methyl sulphate.

Salts and esters of the intermediate thio-acids are known, being prepared from carbon disulphide or carbon oxysulphide.