Chromic acid (H2CrO4) is produced by two or three methods. In one, 2 parts (by measure) of a cold saturated solution of bichromate of potassia are mixed with 3 parts of sulphuric acid: on cooling, the chromic acid is deposited in crystals, The mother liquor being then decanted. Perhaps the method of producing chromic acid more generally followed commercially is to decompose chromium sulphate with lime and to heat to redness the resultant paste of lime, gypsum, and chromium oxide. The chromate of lime formed is treated with sodium sulphate to yield soluble sodium chromate and gypsum. The addition of sulphuric acid liberates the chromic acid. A less wasteful process than this is the electrolytic one now being worked in Germany by Lucius & Bruning. In a solution of chromium sulphate in sulphuric acid are immersed both lead anode and lead cathode, chromic acid being liberated on the former and hydrogen on the latter. A current at 3'5 volts with a current density of 300amperes per square metre is required, the cells being at the temperature of 50° C. (122o F.).