1911 Encyclopædia Britannica/Formic Acid

FORMIC ACID, H₂CO₂ or H·COOH, the first member of the series of aliphatic monobasic acids of the general formula C_nH_2nO₂. It is distinguished from the other members of the series by certain characteristic properties; for example, it shows an aldehydic character in reducing silver salts to metallic silver, and it does not form an acid chloride or an acid anhydride. Its nitrile (prussic acid) has an acid character, a property not possessed by the nitriles of the other members of the series; and, by the abstraction of the elements of water from the acid, carbon monoxide is produced, a reaction which finds no parallel in the higher members of the series. Finally, formic acid is, as shown by the determination of its affinity constant, a much stronger acid than the other acids of the series. It occurs naturally in red ants (Lat. formica), in stinging nettles, in some mineral waters, in animal secretions and in muscle. It may be prepared artificially by the oxidation of methyl alcohol and of formaldehyde; by the rapid heating of oxalic acid (J. Gay-Lussac, Ann. chim. phys., 1831 [2] 46, p. 218), but best by heating oxalic acid with glycerin, at a temperature of 100–110° C. (M. Berthelot, Ann., 1856, 98, p. 139). In this reaction a glycerol ester is formed as an intermediate product, and undergoes decomposition by the water which is also produced at the same time.

C₃H₅(OH)₃ + H₂C₂O₄ = C₃H₅(OH)₂·OCHO+CO₂ + H₂O
C₃H₅(OH)₂O·CHO + H₂O = C₃H₅(OH)₃ + H₂CO₂.

Many other synthetical processes for the production of the acid or its salts are known. Hydrolysis of hydrocyanic acid by means of hydrochloric acid yields formic acid. Chloroform boiled with alcoholic potash forms potassium formate (J. Dumas, Berzelius Jahresberichte, vol. 15, p. 371), a somewhat similar decomposition being shown by chloral and aqueous potash (J. v. Liebig, Ann., 1832, 1, p. 198). Formates are also produced by the action of moist carbon monoxide on soda lime at 190–220° C. (V. Merz and J. Tibiçira, Ber., 1880, 13, p. 23; A. Geuther, Ann., 1880, 202, p. 317), or by the action of moist carbon dioxide on potassium (H. Kolbe and R. Schmitt, Ann., 1861, 119, p. 251). H. Moissan (Comptes rend., 1902, 134, p. 261) prepared potassium formate by passing a current of carbon monoxide or carbon dioxide over heated potassium hydride,

KH + CO₂ = KHCO₂ and KH + 2CO = KHCO₂ + C.

A concentrated acid may be obtained from the diluted acid either by neutralization with soda, the sodium salt thus obtained being then dried and heated with the equivalent quantity of anhydrous oxalic acid (Lorin, Bull. soc. chim., 37, p. 104), or the lead or copper salt may be decomposed by dry sulphuretted hydrogen at 130° C. L. Maquenne (Bull. soc. chim., 1888, 50, p. 662) distils the commercial acid, in vacuo, with concentrated sulphuric acid below 75° C.

Formic acid is a colourless, sharp-smelling liquid, which crystallizes at 0° C., melts at 8·6° C. and boils at 100·8° C. Its specific gravity is 1·22 (20°/4°). It is miscible in all proportions with water, alcohol and ether. When heated with zinc dust, the acid decomposes into carbon monoxide and hydrogen. The sodium and potassium salts, when heated to 400° C., give oxalates and carbonates of the alkali metals, but the magnesium, calcium and barium salts yield carbonates only. The free acid, when heated with concentrated sulphuric acid, is decomposed into water and pure carbon monoxide; when heated with nitric acid, it is oxidized first to oxalic acid and finally to carbon dioxide. The salts of the acid are known as formates, and are mostly soluble in water, those of silver and lead being the least soluble. They crystallize well and are readily decomposed. Concentrated sulphuric acid converts them into sulphates, with simultaneous liberation of carbon monoxide. The calcium salt, when heated with the calcium salts of higher homologues, gives aldehydes. The silver and mercury salts, when heated, yield the metal, with liberation of carbon dioxide and formation of free formic acid; and the ammonium salt, when distilled, gives some formamide, HCONH₂. The esters of the acid may be obtained by distilling a mixture of the sodium or potassium salts and the corresponding alcohol with hydrochloric or sulphuric acids.

Formamide, HCONH₂, is obtained by heating ethyl formate with ammonia; by heating ammonium formate with urea to 140° C.,

2HCO·ONH₄ + CO(NH₂)₂ = 2HCONH₂ + (NH₄)₂CO₃;

by heating ammonium formate in a sealed tube for some hours at 230° C., or by the action of sodium amalgam on a solution of potassium cyanate (H. Basarow, Ber., 1871, 4, p. 409). It is a liquid which boils in vacuo at 150°, but at 192–195° C. under ordinary atmospheric pressure, with partial decomposition into carbon monoxide and ammonia. It dissolves mercuric oxide, with the formation of mercuric formamide, (HCONH)₂Hg.