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Hydrocyanic Acid, HCN

Hydrocyanic acid, HCN, was discovered by Scheele in 1782 and first obtained anhydrous by Gay Lussac, who heated mercury cyanide with hydrochloric acid, and after passing the evolved gas over marble to remove HCl, and subsequently drying, condensed the hydrocyanic acid in a freezing mixture.

This acid occurs in plants, very seldom in the free state, but generally as a constituent of the glucoside amygdalin, which on hydrolysis yields hydrocyanic acid, benzaldehyde, or bitter almond oil, and glucose, as follows:

C20H27NO11 + 2H2O = HCN + C6H5CHO + 2C6H12O6.

This hydrolysis occurs naturally by the agency of the enzyme emulsin; it is brought about artificially by the action of dilute acids. Bitter almonds and laurel leaves are the chief sources of amygdalin; and by distilling these products with water a dilute solution of hydrocyanic acid is obtained.

Hydrogen cyanide has been synthesised from its elements by passing a mixture of hydrogen and air through a carbon tube heated in an electric arc. The reaction is endothermic, and, according to von Wartenberg, is represented by the following thermal equation:

2C (graphite) + H2 + N2 = 2HCN - 59,700 calories.

The amount of hydrogen cyanide formed increases with temperature, as theory indicates, and Wallis, by passing a mixture of nitrogen and hydrogen across the carbon poles of the electric arc, whose temperature is about 3500° C., converted 33.6 per cent, of the gaseous mixture into hydrogen cyanide.

By means of an electric flame an even more striking result has been obtained; for by employing a mixture of nitrogen and methane in the molecular proportion of 2-3:1, diluted with hydrogen so as to contain less than 10 per cent, of methane, Muthmann and Schaidhauf have converted the methane quantitatively into hydrogen cyanide. Lipinski also converted the whole of the methane in a mixture of 8 to 34 per cent. CH4, 75 to 53 per cent. N2, and 16 to 12 per cent. H2 into hydrogen cyanide by the passage of an alternating current at 2000 volts and 0.05 to 0.12 ampere between platinum terminals through 3.8 litres of the mixture for one to three hours.

A less complete synthesis of hydrogen cyanide would be effected by causing ammonia, in which nitrogen and hydrogen are already united, to react with carbon, according to the equation

NH3 + C = HCN + H2.

Bergmann has shown that when ammonia is passed over carbon heated to about 1300° C., 90 per cent, of it is converted into HCN. The reaction is endothermic, its heat being -39,500 calories. A modification of this reaction is that of Roeder and Grunwald, who pass a mixture of ammonia, and nitrous oxide over heated carbon, the reaction being:

2NH3 + N2O + 4C = 4HCN + H2O - 58,000 calories.

Owing to the heat of decomposition of nitrous oxide, which is endothermic, and the heat of formation of steam, it is not necessary to heat the carbon to so high a temperature as in the former case; indeed the yield of hydrogen cyanide is nearly quantitative when the temperature of the carbon is but 450° C.

Hydrogen cyanide is also produced by the explosion of a mixture of acetylene and nitrogen in a bomb:

C2H2 + N2 = 2HCN - 9400 calories,

and by the reaction between acetylene and ammonia at 300° C. in presence of a catalyst:

C2H2 + 2NH3 = 2HCN + 3H2 - 33,000 calories.

Methane and nitrogen also yield hydrogen cyanide under the influence of the silent electric discharge, or at 1000° C., the equation being:

2CH4 + N2 = 2HCN + 3H2 -103.5 calories.

Hydrogen cyanide is generally prepared, however, from potassium ferrocyanide or a simple cyanide. In either case it is synthetic in origin, for ferrocyanide was originally made by heating together a mixture of potassium carbonate, iron, and nitrogenous organic matter, whilst cyanide is obtained either from the hydrogen cyanide formed synthetically in gas manufacture or by passing ammonia over a heated mixture of alkali carbonate, and carbon (Beilby's process). When powdered potassium ferrocyanide is distilled with dilute sulphuric acid (1 part H2SO4 to 2 parts water) hydrocyanic acid is evolved according to the reaction:

2K4Fe(CN)6 + 3H2SO4 = 6HCN + K2Fe[Fe(CN)6] + 3K2SO4.

The vapour may be dried by passing it through calcium chloride tubes kept at 30° C. by immersion in warm water, and then condensed in a freezing mixture; or the vapour may be at once passed into water if only a solution of the acid is required.

Practically anhydrous hydrocyanic acid may be obtained by dropping sulphuric acid diluted with an equal volume of water on to 98 per cent, potassium cyanide.

Anhydrous hydrocyanic acid is also formed when mercuric cyanide, heated to 30° C., is decomposed by hydrogen sulphide.

Physical Properties of Hydrocyanic Acid

Anhydrous hydrogen cyanide is a colourless, mobile liquid having a smell of bitter almonds, and is exceedingly poisonous. Owing to the danger of working with this substance its physical properties have not been completely elucidated. Its density at 18° C. is 0.6969, and at 7° C. is 0.7058, whence the density at 0° C. has been calculated to be 0.7115; its boiling-point at atmospheric pressure is 26.5° C. (Gay Lussac); its vapour density at high temperature is 0.947 (air = 1) and 13.69 (H = 1); at -15° C. it solidifies to a mass of white feathery crystals.

The heat of vaporisation of hydrogen cyanide is 210.7 calories per gram, or 5700 calories per gram molecule; this value is high on account of polymerisation. Rapid evaporation causes a drop of the liquid on a glass rod to solidify.

The following thermal values have been determined by Berthelot and Thomsen:

Heat of formation HCN vapour: -30,500 calories
Heat of formation HCN liquid: -24,800 calories
Heat of formation HCN in solution: -24,400 calories
Heat of formation HCN from diamond: -30,200 calories
Heat of formation HCN from graphite: -29,850 calories

The heat of combustion at constant pressure is 159,300 calories (Berthelot) or 158,600 calories (Thomsen); the heat of aqueous solution is 400 calories (Berthelot).

Liquid hydrogen cyanide dissolves many organic and inorganic substances. Interesting results relating to the electric conductivity and chemical reactivity of such solutions have been obtained by Kahlenberg and Schlundt.

Hydrogen cyanide mixes in all proportions with water, alcohol, and ether. Solution in these cases is accompanied both by fall of temperature and diminution of volume. No definite hydrate of hydrogen cyanide is known.

Chemical Properties of Hydrocyanic Acid

Hydrogen cyanide vapour burns with a violet flame of slight luminosity; both the anhydrous liquid and its concentrated aqueous solution appear combustible. When passed over heated copper oxide the vapour burns to carbon dioxide, water, and nitrogen.

A polymer of hydrogen cyanide is slowly produced when an aqueous solution of the latter is kept in presence of alkali carbonate or cyanide. Under these conditions the liquid turns brown, and a black mass separates after some days, from which ether extracts a substance which crystallises in colourless leaflets. This substance possesses the composition (HCN)3, turns brown at 140° C. with partial decomposition, and melts at 180° C. It is decomposed by being heated with baryta water into carbonic acid, ammonia, and amino-acetic acid; it is therefore probably the dinitrile of aminomalonic acid, NH2CH(CN)2.

That hydrogen cyanide is itself the nitrile of formic acid is shown by the fact that it is resolved into this acid and ammonia when boiled with concentrated mineral acids and alkalis:

HCN + 2H2O = HCOOH + NH3.

Hydrogen cyanide is reduced to methylamine, CH3NH2, by nascent hydrogen or by hydrogen gas at 140° C. in presence of platinum black.

In sunlight chlorine converts hydrogen cyanide into cyanogen chloride; cyanogen bromide and iodide are also produced, though less readily, from hydrogen cyanide and the corresponding halogen according to the equation:

CNH + X2 = CNX + HX.

Hydrogen cyanide reacts with an organic carbonyl group, producing a cyanhydrin, thus:

>CO + HCN = >C(CN)(OH)

whence by hydrolysis h hydroxyacid results:

>C(CN)(OH) + 2H2O = >C(COOH)(OH) + NH3

An aqueous solution of hydrogen cyanide is a very weak acid: hydrocyanic or prussic acid.

A solution of the acid does not redden litmus; its soluble salts are largely hydrolysed in aqueous solution, and are decomposed by atmospheric carbon dioxide, so that they smell of hydrocyanic acid.

The electric conductivity of aqueous hydrocyanic acid has been measured by Walker and Cormack at 18° C., with the following results:


The dissociation constant, which may be taken to be 1.3×10-9, shows the strength of hydrocyanic acid to be only one-fortieth that of hydrogen sulphide and one two-hundredth that of carbonic acid. From measurements of the hydrolysis of potassium cyanide at 25° C. van Laar has found the value K=3.1×10-8, whilst by measuring the potential between a silver electrode and a solution of potassium silver cyanide at 18°C. Morgan found K=2.6×10-8. The percentage degree of electrolytic dissociation of hydrocyanic acid in decinormal solution is 0.011.

The catalytic influence of the cyanide ion in promoting the change of benzaldehyde into benzoin has been studied by Stern, and the mechanism of the addition of the elements of HCN to carbon compounds by Lap worth.


The cyanides of the alkali and alkaline earth metals are soluble in water and are c.onsiderably ionised in solution. Other cyanides except mercuric cyanide are insoluble.; Because of the weakness of hydrocyanic acid, mercuric cyanide shows, to a greater degree than other mercuric salts, chemical inertness in solution owing to feeble ionisation. Silver cyanide resembles silver chloride in physical properties; it is a white, curdy precipitate, which, however, differs from the chloride by its solubility in concentrated nitric acid. The solubility of silver cyanide in potassium cyanide solution with the formation of KAg(CN)2, containing the anion Ag(CN)2', illustrates an important general property of the cyanides, that of forming complex cyanides. These are of different degrees of stability: from the least stable, as for instance K2Ni(CN)4, dilute mineral acids reprecipitate the simple insoluble cyanide; from the most stable, as for instance K4Fe(CN)6, the free, solid acid, e.g. H4Fe(CN)6, is separated by mineral acid. The following list includes representatives of the complex cyanides, which, like the ammines, are formed by the members of the eighth and contiguous groups. These compounds will be dealt with in detail under the respective metals.

Copper, silver, gold, and cadmium form the following complex cyanides:

K2Cu2(CN)4, K6Cu2(CN)8 KAg(CN)2

KAu(CN)2, HAu(CN)4 K2Cd(CN)4;

whilst chromium, manganese, and the metals of the eighth group form complex cyanides of several different types, as the following table shows. Where the free acid is known its formula appears.


Cyanogen Halides

Hydrogen cyanide, cyanogen halide, and a metallic cyanide may be formulated respectively as:

H-CN, X-CN, M-CN or H-N=C, X-N=C, M-N=C.

The normal formulae, first given, have generally been preferred, because they show the carbon atom to be quadrivalent, whilst the nitrogen atom is tervalent; but there is a reason for preferring the iso formulae, in which hydrogen, halogen, or metal is attached to nitrogen rather than to carbon.

The reactivity of halogen and hydrogen atoms attached to nitrogen is superior to the reactivity of the same atoms attached to carbon; and a study of the cyanogen halides shows them to possess all the characteristics of compounds having halogen attached to nitrogen. This is borne out by such a reaction as the following:

C=N-Br + 2HI = C=N-H + HBr + I2,

which, being easy of achievement, is regarded as proving that bromine is attached to nitrogen.

Now the cyanogen halides are readily formed from hydrocyanic acid and its salts, which therefore in all probability have a similar composition, being iso compounds, thus:

C=N-H and C=N-M.

Additive Compounds of Hydrogen Cyanide

When hydrogen chloride is passed into anhydrous hydrogen cyanide at - 10° C., and the solution is then heated to 35° C., the compound HCN.HCl is formed, and crystallises on cooling. If hydrogen cyanide has the constitution :C:N-H, the compound with HCl is . This compound is very hygroscopic and is decomposed by water into formic acid and ammonium chloride, but may be sublimed. Another compound, 2HCN.3HCl, exists which appears to be the hydrochloride of dichlormethylformamidine, (NH=CH-NH-CHCl2).HCl.

Physiological Action of Hydrocyanic Acid

Hydrocyanic acid is one of the most powerful poisons known, and it is very rapid in its action. An amount equal to 1/100000th part of the weight of its blood suffices to kill a dog, and a few drops brought into a dog's eye kills the animal in thirty seconds; 0.05 gram has proved a fatal dose for a man, though larger quantities have been taken without fatal effects. The symptoms of poisoning by prussic acid are headache, nausea, difficulty of breathing, palpitations, tetanic spasms affecting the muscles of the jaws and limbs, paralysis, and insensibility.

The cause of the poisonous action of hydrocyanic acid and the soluble cyanides is not known, but an analogy has been traced between this action and the inhibiting effect of the same substance on catalysts such as ferments and colloidal metals.

Ammonia or chlorine water appears to serve as an antidote, though in neither case is the chemical action understood.

Detection and Estimation

In cases of poisoning hydrocyanic acid is separated from the matter containing it by distillation with tartaric acid. For the detection of small quantities of hydrocyanic acid in aqueous solution several methods are available.

  1. The cyanide may be converted into Prussian blue by boiling its solution with alkali with the addition of ferrous and ferric salt. On acidifying ferric ferrocyanide separates as a deep blue precipitate, or, if only a trace of cyanide was present, shows a blue or green colour. By this test 1 part of cyanide in 50,000 can be detected. Extremely dilute solutions of hydrogen cyanide containing as little as 0.00002 gram per c.c. may be made alkaline, evaporated to small bulk, and then tested for in this way.
  2. Conversion into nitroprusside, which gives a purple colour with alkali sulphide, serves to detect 1 part of cyanide in 300,000.
  3. By evaporation to dryness with yellow ammonium sulphide cyanide is converted into thiocyanate, which gives a blood-red colour with ferric chloride. This test will detect 1 part of cyanide in even 4,000,000 parts of solution.
  4. Cyanide solution gives a deep red colour with picric acid.
Cyanide is estimated volumetrically by adding to its alkaline solution

standard silver nitrate solution until a permanent precipitate appears. One molecule of silver nitrate then corresponds to two molecules of cyanide owing to the formation of KAg(CN)2, according to the reaction:

AgNO3 + 2KCN = KAg(CN)2 + KNO3.

The estimation may be carried out gravimetrically by weighing the corresponding silver cyanide, or by reducing it and weighing the metallic silver.

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