| Numerous boric acids are theoretically capable of being produced by the union of boron sesqui-oxide and water in different proportions, and the existence of various boric acids seems necessary in order to account for the types of borates known. At the present time, however, the existence in the solid state of only two boric acids, orthoboric acid, H3BO3 (i.e. B2O3.3H2O), and metaboric acid, HBO2 (i.e. B2O3.H2O), can be definitely affirmed. The latter is formed by heating the former to 100°-140°, and in aqueous solution it passes into the former, as is shown by molecular weight determinations. The existence of pyroboric acid, H2B4O7, has been assumed by various chemists, but cannot be regarded as proved. |