Prussic Acid

PRUSSIC ACID, also called hydrocyanic acid and hydrogen cyanide, is a compound of hydrogen, carbon and nitrogen (HCN), best known on account of its exceedingly poisonous nature. It is a very volatile colourless liquid boiling at 25.7° C and freezing at —13° C. As the parent substance of the cyanides it gives, on substitution of the hydrogen atom by metals, salts of prussic acid, e.g., potassium cyanide, KCN ; while substitution by organic radicals gives the organic cyanides, e.g., methyl cyanide, In its chemical character the radical, CN, resembles the halogen elements, e.g., chlorine, in being acidic (electronegative) and univalent, corresponding compounds having similar formulae, e.g., KC1 and KCN, and analogous chemical behaviour.

Prussic acid was discovered in 1782 by Scheele who obtained it from Prussian blue. In variable but generally very small propor tions it is widely distributed among plants in the form of corn pounds with sugars, the glucosides (q.v.) e.g., amygdalin (q.v.), contained in bitter almonds, from which the free acid can readily be obtained by hydrolysis.

Preparation.

The most convenient method of obtaining prussic acid in the laboratory is to add a concentrated solution of sodium cyanide gradually to sulphuric acid (about 70%), when hydrogen cyanide vapour is rapidly evolved : (more concentrated acid gives carbon monoxide, see below). On the large scale this process may be carried out in large covered iron vessels lined with lead and provided with mechanical stirrers. The vapour of hydrogen cyanide, after drying by calcium chloride, is condensed to an almost anhydrous liquid, from which the last traces of water may be removed by distilla tion with phosphoric oxide. An older method consists in distilling potassium ferrocyanide with dilute sulphuric acid: Hydrogen, carbon and nitrogen at very high temperatures, as in the electric arc, combine directly, with the absorption of much energy, to form hydrogen cyanide, which is thus an endothermic substance. Instead of the three elements, their compounds such as ammonia cyanogen acetylene C2H2, methane oxides of nitrogen or NO, may be used, often with advantage in the direction of greater yield and lower temperature of reaction. If alkaline substances are present, cyanides (q.v.) are formed.

These high-temperature reactions account for the presence of hydrogen cyanide in crude coal gas.

Physical and Chemical Properties.—In many of its physical properties hydrogen cyanide resembles water. Like the latter it is an ionizing solvent, many salts dissolving in it to form solutions which are good conductors of electricity. This property of liquids is associated with high dielectric constant, the magnitude of which is even greater for hydrogen cyanide than for water, indicating great electric polarity of the molecules (compare as a rough illustration a magnet with its north and south poles). Such polar molecules are more firmly held together than in liquids composed of non-polar molecules, and combination between them may occur to some extent, giving rise to association (q.v.). The physical properties of such "associated" liquids exhibit many characteristic abnormalities as compared with those of normal non-polar liquids; thus hydrogen cyanide has a latent heat of evaporation greater than the value expected if it were normal.

The vapour of prussic acid burns freely in air with a violet flame yielding nitrogen, carbon dioxide and water. As indicated by its structural formula H-C._=N or H-N=C, prussic acid is chemically unsaturated. It is capable of adding other atoms to its molecule as well as of substituting other atoms for its hydrogen. Thus it combines with hydrogen giving methylamine H3C-NH2, and yields addition compounds with hydrogen chloride. Substitu tion occurs with halogens, e.g., chlorine gives cyanogen chloride: HCN-I-C12=CNC1-1-HC1. The cyanides, being similarly un saturated, also undergo addition reactions. Thus molten potassium cyanide readily unites with free or combined oxygen giving potassium cyanate KCNO, or with sulphur giving potassium sul phocyanide (thiocyanate) KCNS, and accordingly at elevated temperatures potassium cyanide reduces many metallic oxides and sulphides to the free metal.

Prussic acid combines with water slowly when heated, more rapidly in the presence of mineral acid. The addition of water occurs in two stages thus : With concentrated sulphuric acid the last substance is converted by loss of water into ammonia (which combines with the sulphuric acid) and carbon monoxide gas CO thus: