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      Thiocyanic Acid
      Sulphocyanic Acid
      Isoperthiocyanic Acid
      Cyanogen Sulphide
      Thiocyanic Anhydride
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Thiocyanic Acid, HSCN






The potassium salt of this acid was first prepared in 1808 by Porret by boiling potassium sulphide solution with prussian blue; it was examined quantitatively by Berzelius in 1820.

The salts of the acid are formed by the direct union of cyanides with sulphur, either by fusion or by evaporating their aqueous solutions with polysulphide. They may also be prepared from complex cyanides; potassium thiocyanate, for instance, is obtained by heating together potassium ferrocyanide, potassium carbonate, and sulphur. Ammonium thiocyanate is formed by warming together hydrocyanic acid and yellow ammonium sulphide solutions:

(NH4)2S2 + HCN = NH4SCN + NH4HS,

and is thus prepared from the cyanide formed in gas manufacture. It may also be obtained by the interaction of carbon disulphide and an alcoholic solution of ammonia. When 350-400 grams of carbon disulphide are mixed with 600 grams of 95 per cent, alcohol and 800 grams of ammonia solution of 0.912 density there is formed on standing a mixture of ammonium thiocarbonate and thiocarbamate; and these on heating yield thiocyanate, thus:

H4NS-CS-SNH4 = NCS-NH4 + 2H2S

H2N-CS-SNH4 = NCS-NH4 + 2H2S

Potassium thiocyanate may also be prepared by passing cyanogen gas over heated potassium disulphide:

(CN)2 + K2S2 = 2KSCN.

Thiocyanic acid may be obtained in solution by decomposing barium thiocyanate with an equivalent quantity of dilute sulphuric acid, or 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 phosphoric oxide in an atmosphere of hydrogen. The acid distils over under a pressure of 40-60 mm. The acid may also be obtained from its insoluble salts, such as those of lead and mercury, by decomposing them with hydrogen sulphide. At ordinary temperature thiocyanic acid, whose vapour is stable, 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., which melt at 5° C. and readily decompose, with evolution of heat, into hydrocyanic acid and isoperthiocyanic acid, C2N2S3H2 (q.v.). The acid is soluble in ether, and is stable in dilute aqueous solution. Cryoscopic measurements in benzene, etc., indicate a mixture of single and double molecules.

Thiocyanic acid may be represented by the formula NC-SH or SC=NH. Probably the inorganic salts have the constitution represented by the first formula, as well as the esters derived from them. These esters are converted into sulphonic acids and hydrocyanic acid by oxidation:

NCSR + H2O + 2O = RSO3H + NCH,

and are reduced by nascent hydrogen to mercaptans:

NCSR + 2H = RSH + NCH.

They can, however, undergo isomeric change into the esters of isothio- cyanic acid, which are the mustard oils:

NC-SRSC=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 by Dixon and Taylor.

The heat of formation of thiocyanic acid in aqueous solution from its elements is -19,900 calories, and from HCN Aq. is +5800 calories. An aqueous solution of thiocyanic acid is largely ionised and approaches hydrochloric acid in strength.

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

vλvγK
23260.883464.74
43370.913284.81
83450.934954.75
163520.953924.94
5123691.00000-
Mean 4.81


Silver Thiocyanate is a curdy white precipitate resembling silver chloride, and insoluble in nitric acid; cuprous thiocyanate, formed by adding thiocyanate solution to copper sulphate in presence of sulphurous acid, is also a white precipitate.

Ferric Thiocyanate is the blood-red substance formed in solution in the well-known test for iron. This reaction has been the subject of extended investigation by Bongiovanni, who has shown that it may be quite simply represented thus:

FeCl3 + 3KCNS ⇔ 3KCl + Fe(CNS)3,

the colour being due to non-ionised Fe(CNS)3, and its intensity depending on the product of the concentrations of ferric and thiocyanate ions. The decolorisation on dilution is due to hydrolysis of non-ionised Fe(CNS)3 into yellow, colloidal ferric hydroxide and thiocyanic acid. Philip and Bramley, however, confirm the judgment of other observers that loss of colour is associated with reduction of iron, and 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., is caused by the formation of complex ions with the ferric ions of the ionised ferric thiocyanate, which causes further ionisation of the red non-ionised salt, and consequent loss of colour.

Molybdenum and vanadium thiocyanates are also red and behave similarly to the ferric salt.

Mercuric Thiocyanate, formed as a white precipitate when mercuric chloride and potassium thiocyanate solutions are mixed together, is soluble in excess of either solution. When dried this salt is inflammable, and in burning forms a voluminous residue known as "Pharaoh's serpents." Ammonium thiocyanate is converted when heated in the fused state into thiourea:

NH4CNSCS(NH2)2.

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

3CNSH = C2N2S3H2 + HCN.

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

CNSH + H2OHS - CO - NH2COS + NH3.

Besides these products there are others, including hydrogen sulphide and sulphur, which react with 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 = NH4-CNS + CS2 + S,

By the action of zinc and hydrochloric acid thiocyanic acid is reduced to trithioformaldehyde, ammonia, methylamine, hydrogen sulphide, and hydrocyanic acid.

Bromine oxidises thiocyanate quantitatively in aqueous solution, thus:

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


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