Carbonyl Sulphide, COS

Carbonyl sulphide, a gas discovered by Than, is formed synthetically when a mixture of carbon monoxide and sulphur vapour is passed through a hot tube. The reaction is reversible:

CO + S ⇔ COS,

so that the proportion of carbonyl sulphide formed depends upon the temperature and rate of passage of the gas.

Carbonyl sulphide results similarly from carbon disulphide and sulphur trioxide, thus:

CS₂ + 3SO₃ = COS + 4SO₂,

and also when sulphur vapour and air are passed over a red-hot mixture of clay and carbon, as well as when sulphur dioxide is passed over red-hot carbon:

4SO₂ + 9C = 6CO + 2COS + CS₂.

Some thiocarbonates yield carbonyl sulphide by decomposition; e.g. potassium ethylthioncarbonate is decomposed thus by hydrochloric acid:

KO-CS-OC₂H₅ + HCl = SCO + C₂H₅OH + KCl.

Sulphur may be substituted for chlorine in carbonyl chloride by means of cadmium sulphide at 270° C., thus:

COCl₂ + CdS = COS + CdCl₂;

but the most important method of preparing carbonyl sulphide consists in the decomposition of a thiocyanate with dilute acid, the liberated thiocyanic acid being hydrolysed thus:

SCNH + OH₂ = SCO + NH₃.

To a cooled mixture of 5 volumes of sulphuric acid and 4 volumes of water potassium thiocyanate is added, and the gas is evolved at about 20° C., together with hydrocyanic acid, formic acid, and carbon disulphide. The acid vapours are removed by passing the gas through strong alkali, and the carbon disulphide is absorbed in a mixture of triethylphosphine, pyridine, and nitrobenzene. After drying with sulphuric acid the gas may be further purified by liquefaction or absorption in toluene.

Carbonyl sulphide is a colourless gas possessing a resinous smell, but is said to be odourless when pure. Its density at 0° C. is 2.1046, that calculated from the molecular weight being 2.0749. That this compound stands about midway between carbon dioxide and carbon disulphide in physical properties as well as in chemical composition is shown in the following table:

	Critical Temp. °.C.	Boiling-point (1 atm.)	Vapour pressure at 0°. Atmospheres.	Heat of formation Calories.
Carbon dioxide.	31.4	-80	35.4	97
Carbonyl sulphide	105	-47.5	12.5	37
Carbon disulphide	275	46	0.17	-25.4

Liquid carbonyl sulphide is colourless and highly refractive; it dissolves sulphur and mixes with various organic liquids.

The gas is somewhat soluble in water, its solubilities at different temperatures being as follow:

Temperature ° C.	0°	5°	10°	15°	20°	25°	30°
1 vol. water dissolves	1.333	1.056	0.835	0.677	0.561	0.468	0.403

It is found in solution in certain mineral waters; e.g. those of Harkany and Parad in Hungary.

The following thermal values are given by Thomsen:

Heat of combustion = 131,010 calories.

Heats of formation:

C (amorph.) + S (rhomb.) + O = COS + 37,320 calories.

CO + S = COS + 8,030 calories.

Berthelot, however, gives a value for the heat of formation about half that obtained by Thomsen:

C (amorph.) + S (rhomb.) + O = COS + 19,600 calories.

Carbonyl sulphide may be ignited by means of a glowing splinter of wood, and burns with a blue, slightly luminous flame. With air it forms a mixture which is slightly explosive except when quite dry. The explosive limits lie between 11.9 and 28.5 per cent, of carbonyl sulphide. A white-hot platinum wire completely decomposes the gas into carbon monoxide and sulphur. Water slowly decomposes carbonyl sulphide, thus:

COS + H₂O = CO₂ + H₂S.

According to Buchbock the reaction proceeds in two stages, thiolcarbonic acid being an intermediate product:

COS + 2KOH ⇔ HO-CO-SH ⇔ CO₂ + H₂S.

Alkali solutions form salts of the same acid, which, however, soon decompose into carbonate and hydrosulphide:

COS + 2KOH = KO-CO-SK + H₂O;
KO-CO-SK + KOH = KO-CO-OK + KSH.

Alcoholic potash forms, in all probability, potassium ethylthioncarbonate, KO-CS-OC₂H₅, whence carbonyl sulphide is liberated again by acid. Heated mercury, copper, iron, and silver remove sulphur from carbonyl sulphide; cuprous chloride reacts thus:

COS + Cu₂Cl₂ + H₂O = CO₂ + 2HCl + Cu₂S,

and chlorine forms phosgene and sulphur dichloride: COS + 2Cl₂ = COCl₂ + SCl₂.