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 CnH2nO2. 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  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.
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
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;