Carbamic Acid, HOCONH₂

Carbamic Acid, HO-CO-NH₂, the half-amide of carbonic acid, is unknown, but several of its salts exist, the most important of which is ammonium carbamate, NH₄O-CO-NH₂. This salt is formed by the union of carbon dioxide with ammonia:

CO₂ + 2NH₃ = NH₄-CO₂-NH₂,

which is best effected by passing the mixed, dry gases through absolute alcohol. Ammonium carbamate also occurs, together with ammonium bicarbonate, in commercial carbonate of ammonia, prepared by sublimation when a moistened mixture of calcium carbonate and ammonium sulphate is heated; it gradually sublimes at atmospheric temperature from this compound, and is frequently deposited in transparent prisms upon the neck of the bottle containing the commercial salt. The carbamate may also be obtained from the commercial salt by acting upon it for thirty or forty hours with saturated solution of ammonia at 20°-25° C.; but dilute ammonia solution, like water, converts carbamate in solution into normal carbonate. When vaporised, ammonium carbamate is completely dissociated into ammonia and carbon dioxide. This is shown by the vapour density, which is 0.892 (air = 1) between 37° C. and 100° C. Recombination on cooling is slow, owing to the necessary intramolecular change. Sodium and potassium carbamates and a basic calcium salt, NH₂-CO-OCa(OH), are known. Ethyl carbamate, NH₂-CO-OC₂H₅, is urethane.