Thiocyanic Acid, HSCN

Thiocyanic Acid, Sulphocyanic Acid or Rhodanic Acid, HSCN, was isolated by Winterl (1790), Buchholz (1798) and Rink (1804). Its potassium salt was first prepared by Porret in 1808 by boiling potassium sulphide solution with Prussian blue. The composition of the acid was first determined by Berzelius in 1820. The question of the formation of the acid in animals has been the subject of investigation by Dezani and others, but their results have led to the conclusion that the acid is not produced in the animal organism, but is purely exogenetic.

An aqueous solution of thiocyanic acid may be prepared by the decomposition of either silver or mercuric thiocyanate with hydrogen sulphide, or by decomposing barium thiocyanate with an equivalent quantity of sulphuric acid. It may also be obtained by distilling potassium thiocyanate with a dilute acid such as sulphuric, phosphoric, oxalic or tartaric acid.

The acid may be obtained in the anhydrous state by distilling its potassium salt with dilute sulphuric or phosphoric acid, passing the vapour through a long calcium chloride tube and then condensing it in a freezing mixture. A better way is to drop concentrated sulphuric acid on to a mixture of potassium thiocyanate and phosphorus pent- oxide in an atmosphere of hydrogen. The acid distils over under a pressure of 40 to 60 mm. It is stated that still better results are obtained by this latter method if potassium hydrogen sulphate is used in place of sulphuric acid.

At ordinary temperatures thiocyanic acid is a clear, yellowish, volatile, oily liquid, of unknown boiling-point, which when sufficiently cooled forms colourless crystals, stable in dry hydrogen at -15° C., melting at 5° C., and readily decomposing with evolution of heat into hydrocyanic acid and isoperthiocyanic acid, C₂N₂S₃H₂. The vapour of thiocyanic acid is stable. The acid is readily soluble in water, alcohol, ether and benzene. It has a corrosive action on the skin. Hydrogen peroxide oxidises it according to the equation:

HCNS + 3H₂O₂ = HCN + H₂SO₄ + 2H₂O.

This reaction is accelerated by the presence of nickel and cobalt salts.

Cryoscopic measurements on the acid in benzene, etc., indicate a mixture of single and double molecules. Single molecules of thiocyanic acid may be represented by the formula or S:C=NH. Probably the inorganic salts and the esters derived from them have the constitution represented by the first formula. The esters are converted into sulphonic acids and hydrocyanic acid by oxidation,

NCSR + H₂O + 2O = RSO₃H + HCN,

and are reduced by nascent hydrogen to mercaptans:

NCSR + 2H = RSH + HCN.

They can, however, undergo isomeric change into the esters of iso-thiocyanic acid, these esters constituting the mustard oils:

→ SC=NR.

This change is accompanied by the evolution of heat, which for the methyl ester amounts to 6800 calories.

Measurements of the molecular refraction of thiocyanates have been made.

The heat of formation of thiocyanic acid in aqueous solution from its elements is -19,900 calories, and from HCN aq. + 5800 calories.

An aqueous solution of thiocyanie acid is largely ionised and approaches hydrochloric acid in strength.

In the following table are given the conductivity (λ_v) and degree of dissociation (γ) at various dilutions (v litres) at 25° C., whence the constant K is calculated.

v	λv	γ	K.
2	326	0.88346	4.74
4	337	0.91328	4.81
8	345	0.93495	4.75
16	352	0.95392	4.94
512	369	1.00000	. . .
			Mean = 4.81

When an alkali thiocyanate is warmed with moderately concentrated sulphuric or hydrochloric acid, a yellow solid separates and carbonyl sulphide gas is evolved, which burns with a pale sulphurous flame. The yellow substance contains isoperthiocyanic acid, formed, together with hydrocyanic acid, according to the reaction:

3HCNS = C₂N₂S₃H₂ + HCN.

The carbonyl sulphide is produced, together with ammonia, by hydrolysis of the thiocyanic acid, which hydrolysis, according to Klason, is preceded by the formation of thiolcarbamic acid, thus:

CNSH + H₂O → HS.CO.NH₂ → COS + NH₃.

Besides these products there are others, including hydrogen sulphide and sulphur, which react with the thiocyanic acid to produce the disulphide of thiolthioncarbamic acid, thus:

2CNSH + H₂S + S = H₂N.CS.S.S.CS.NH₂.

This compound, however, decomposes on warming, yielding ammonium thiocyanate, carbon disulphide and sulphur, thus:

(NH₂.CS.S.)₂ = NH₄CNS + CS₂ + S.

Under the action of zinc and hydrochloric acid, thiocyanic acid is reduced to trithioformaldehyde, ammonia, methylamine, hydrogen sulphide and hydrocyanic acid. Bromine oxidises thiocyanates quantitatively in aqueous solution, thus:

KSCN + 4Br₂ + 4H₂O = KBr + CNBr + H₂SO₄ + 6HBr.