Thiocyanates

The salts of thiocyanic acid are generally obtained by the direct action of sulphur or a polysulphide on the cyanide, by the action of hydrogen cyanide or cyanogen on a polysulphide,

(NH₄)₂S₂ + HCN = NH₄CNS + NH₄HS,
Na₂S₂ + (CN)₂ = 2NaCNS,

or by the decomposition of a complex cyanide by fusion with an alkali carbonate and sulphur.

The thiocyanates are generally soluble in water, the exceptions being those of lead, silver, mercury and copper. Most of them dissolve also in alcohol and ether. Aqueous solutions of the alkali thiocyanates undergo atmospheric oxidation under the influence of sunlight; with solutions of medium concentration this change takes place rapidly, with separation of a yellow, amorphous precipitate consisting of pseudo-cyanogen sulphide, (CNS)₃. The concentration of thiocyanate most favourable to the separation of this sulphide is about 50 per cent, in summer and 10 per cent, in winter. In addition to this substance the products of the photochemical oxidation of potassium thiocyanate include hydrocyanic acid, sulphate, carbon dioxide, ammonia and ammonium salts:

12KCNS + 12O₂ + 6H₂O = 6K₂SO₄ + 3HCN + 3CO₂ + 3NH₃ + 2(CNS)₃.

An unstable intermediate peroxygenated compound is also always formed. This gives a blue coloration to fresh guaiacum tincture and is possibly analogous to Caro's acid.

Any pink colour that may develop in solutions of alkali thiocyanates on exposure to light is due to oxidation of traces of ferrous salts usually present by dissolved oxygen.

On heating, ammonium thiocyanate is converted into thiourea:

NH₄CNS ⇔ CS(NH₂)₂.

The existence of a compound of ammonium thiocyanate and thiourea, NH₄CNS.4CS(NH₂)₂, is indicated by the melting-point diagram. Ammonium thiocyanate forms a condensation product with formaldehyde, this being only sparingly soluble in water and ordinary solvents, and decomposed by strong acids or alkalis.

The thiocyanates, except those of the heavy metals, are decomposed in the cold by dilute mineral acids, and on heating, the free thiocyanic acid distils over. With moderately concentrated acid, carbonyl sulphide is produced, whilst concentrated sulphuric acid causes rapid decomposition, with evolution of pungent vapours containing carbonyl sulphide, formic acid, carbon dioxide and sulphur dioxide.

The reaction between ferric salts and alkali thiocyanate, which constitutes the well-known test for ferric iron, has been the subject of much investigation. It may be represented simply thus,

FeCl₃ + 3KCNS ⇔ Fe(CNS)₃ + 3KCl,

the resulting blood-red colour being said to be due to non-ionised ferric thiocyanate, its intensity depending on the product of the concentrations of ferric and thiocyanate ions. On dilution the solution becomes decolorised, possibly owing to hydrolysis of non-ionised thiocyanate into yellow colloidal ferric hydroxide and thiocyanic acid. Philip and Bramley, however, confirm the judgment of other observers that the loss of colour is associated with reduction of the iron, and they show that the following equation approximately represents the change in aqueous solution:

8Fe(CNS)₃ + 6H₂O = 8Fe(CNS)₂ + 7HCNS + CO₂ + H₂SO₄ + NH₃.

The discharging of the colour by oxalates, tartrates, etc., appears to be caused by the formation of complex ions with the ferric ions of the ionised ferric thiocyanate, which causes further dissociation of the red non-ionised salt and consequent loss of colour.

Ferric thiocyanate is readily soluble in aqueous ether, and the extract possesses a deep violet colour which can be completely discharged by the addition of ferric chloride. The explanation of this effect put forward by Clarens is that an excess of thiocyanate is necessary for the formation of ferric thiocyanate; when a ferric salt is added this excess of thiocyanate is removed and a salt of dithiocyanic acid, insoluble in ether, is formed.

Silver thiocyanate, formed as a white, flocculent precipitate by double decomposition, is insoluble in dilute mineral acids. It is upon the formation of this salt that Volhard's volumetric method for the determination of silver or of thiocyanate depends.

Both cupric and cuprous thiocyanates may be obtained by precipitation. The former, which forms as a black precipitate when excess of thiocyanate is added to a copper salt, is unstable, and if allowed to remain under water, loses thiocyanic acid and forms the cuprous salt. The latter is precipitated as a white powder by the addition of a soluble thiocyanate to a solution of copper sulphate in the presence of sulphurous acid.

Mercuric thiocyanate, which is formed as a white precipitate when mercuric nitrate and potassium thiocyanate solutions are mixed, is soluble in excess of either solution. When dried, this salt is inflammable, forming a voluminous ash known as "Pharaoh's serpents." By the interaction of a mercuric salt with ammonium thiocyanate and thio-carbamide in acetic acid solution in the presence of an oxidising agent, or by the action of hydrogen sulphide on mercuric thiocyanate, the phototropic compound HS.Hg.CNS is obtained.

A thiocyanate may be detected by its behaviour with sulphuric acid and by the blood-red colour produced with ferric chloride, which colour is discharged by mercuric chloride owing to the formation of the complex salt HgCNS.HgCl₂, and by Rochelle salt owing to the formation of undissociated ferric tartrate. The red compound is soluble in ether. The reaction is extremely sensitive, but it cannot be applied to the colorimetric estimation of thiocyanates.

Evolution of nitrogen from sodium azide-iodine mixture is brought about by traces of thiocyanate, and the latter may readily be detected by this means in the presence of most inorganic oxy-acids and the common organic acids.2 Sulphides and thiosulphates interfere and must previously be removed by means of mercuric chloride.

Thiocyanates may be estimated gravimetrically by precipitation as cuprous thiocyanate, or in the absence of halogen acids and hydrocyanic acid, as silver thiocyanate. Other methods depend on oxidation to sulphate and precipitation with barium chloride. The oxidation may be brought about by means of bromine water or by a reagent, containing chlorine, prepared by electrolysis of a solution containing sodium and magnesium chlorides. The thiocyanate ion is also quantitatively precipitated as copper-pyridine thiocyanate, [CuPy₂](CNS)₂, by the addition of a little pyridine and excess of cupric sulphate solution.

Volumetrically, thiocyanate is estimated by Volhard's method, which involves titration with standard silver nitrate solution containing nitric acid, ferric alum being used as indicator. Cuprous thiocyanate dissolved in ammonium hydroxide solution and acidified with dilute sulphuric acid may be titrated with permanganate. An iodometric method has also been described.

The alkali thiocyanates show a marked tendency to form double and complex salts with the thiocyanates of other metals. The double salts in general resemble in type the double halides. Such compounds as KAg(CNS)₂ and K₂Ag(CNS)₃ may be considered as derivatives of di- and tri-thiocyanic acids, but in the salt (NH₄)₅Ag(CNS)₆ the silver is not precipitated by the addition of chloride ion.

The alkaline earth metals yield compounds of the type R₂M(CNS)₄, where R is an alkali metal or silver; these are of interest since double halides of this type containing alkali and alkaline earth metals are not known.

Many compounds containing three metals, for example, of the types CS₂Ag₂M(CNS)₆, (M = Ca, Mg, Mn or Cd), and CsMNi(CNS)₅, (M = Ag₂ or Cu), have also been prepared.

Certain tervalent metals yield series of double thiocyanates which may be considered as salts of hypothetical acids of the type H₃[M(CNS)₆]; for example, iron, chromium and bismuth give such compounds.

Complex compounds of thiocyanates and arsenious oxide, for example KCNS.2As₂O₃ (microscopic hexagonal platelets), and an asbestos-like mass approximating in composition to 2NaOH.NaCNS.2As₂O₃.4H₂O, have been prepared, as also have alkali and ammonium salts of thio-cyanatocobaltous acid, K₂Co(CNS)₄ and (NH₄)₂Co(CNS)₄. By the action of ammonia on the latter salt, cobaltotetrammine thiocyanate, Co(CNS)₂(NH₃)₄, has been obtained as rose-red needles.