Chemical elements
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      Hydrogen Sulphide
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      Thionyl Fluoride
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      Fluorosulphonic Acid
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      Chlorosulphonic Acid
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      Sulphites
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      Pyrosulphates
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      Persulphuric Anhydride
      Persulphuric Acid or Perdisulphuric Acid
      Perdisulphates
      Permonosulphuric Acid
      Amidopermonosulphuric Acid
      Thiosulphuric Acid
      Thiosulphates
      Polythionic Acids
      Dithionic Acid
      Trithionic Acid
      Trithionates
      Tetrathionic Acid
      Tetrathionates
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      Pentathionates
      Wackenroders Solution
      Hexathionic Acid
      Polythionic Acids
      Sulphur Sesquioxide
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      Hydrosulphites
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      Nitrogen Persulphide
      Nitrogen Pentasulphide
      Sulphammonium
      Hexasulphamide
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      Trithiazyl Chloride
      Thiotrithiazyl Chloride
      Dithiotetrathiazyl Chloride
      Nitrogen Bromosulphide
      Thiotrithiazyl Bromide
      Thiotrithiazyl Iodide
      Thiotrithiazyl Nitrate
      Thiotrithiazyl Hydrogen Sulphate
      Thiotrithiazyl Thiocyanate
      Thionylamide
      Sulphamide
      Imidodisulphamide
      Sulphimide
      Sulphonic Acids
      Amidosulphonic Acid
      Imidosulphonic Acid
      Nitrilosulphonic Acid
      Hydroxylamine-monosulphonic Acid
      Nitrososulphonic Acid
      Hydroxylamine-disulphonic Acid
      Hydroxylamine-isodisulphonic Acid
      Hydroxylamine-trisulphonic Acid
      Dihydroxylamidosulphonic Acid
      Sulphazinic Acid
      Sulphazotinic Acid
      Dehydrosulphazotinic Acid
      Nitrosulphonic Acid
      Nitrosulphonyl Chloride
      Nitrosulphonic Anhydride
      Nitrosulphuric Acid
      Nitrosodisulphonic Acid
      Sulphonitronic Acid
      Sulphates of Hydroxylamine
      Hydroxylamine Dithionate
      Hydrazine Dithionate
      Hydrazine Amidosulphonate
      Carbon Subsulphide
      Carbon Monosulphide
      Carbon Disulphide
      Thioformaldehyde
      Thiocarbonic Acid
      Ammonium thiocarbonate
      Thiolcarbonic Acid
      Xanthic Acid
      Perthiocarbonic Acid
      Sodium perthiocarbonate
      Carbonyl Sulphide
      Thiocarbonyl Chloride
      Thiocarbonyl Tetrachloride or
      Carbon Hexachlorosulphide
      Trichloromethyl Disulphide
      Thiocarbonyl Sulphochloride
      Carbon Bromosulphide
      Amino-derivatives of Thiocarbonic Acid
      Dithiocarbamic Acid
      Thiocarbamide
      Azidodithiocarbonic Acid
      Thiocyanogen
      Cyanogen Monosulphide
      Cyanogen Trisulphide
      Sulphur Thiocyanate
      Disulphur Dithiocyanate
      Thiocyanic Acid
      Thiocyanates
      Dithiocyanic Acid
      Trithiocyanuric Acid
      Perthiocyanic Acid
      Perthiocyanogen
      Sulphates

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 (λv) and degree of dissociation (γ) at various dilutions (v litres) at 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


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 + H2OHS.CO.NH2COS + 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.


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