CHROMIC ACID. (FR., Acide chromique ; Gain., Chromsdure.) Formula This acid does not occur in nature, but it may be prepared from its salts in several ways, in the form of beautiful, deep red, needle-shaped crystals. The following are the most important methods : 1. By the decomposition of bichromate of potash by strong sulphuric acid. One volume of a saturated solution of the bichromate is poured gently into one and a half volume of oil of vitriol, the mixture being carefully stirred during the operation ; on cooling, the chromic acid separates out in long needles of a beautiful red colour. These are dried and purified from sulphuric acid by re-crystallization.
2. By treating chromate of baryta with a large excess of strong nitric acid. This method is employed by Mr. Charles Watt. The nitrate of baryta formed by the reaction is insoluble in nitric acid, and may he easily separated from the chromic acid by decantation, or by filtration through asbestos. The filtrate is evaporated to dryness, the nitric acid being volatilized ; the residue is pure chromic acid. If the quantity of nitric acid required be large, it may be condensed and employed a second time.
3. By the decomposition of chromate of lead by sulphuric acid. The mixture is allowed to
stand for twenty-four hours and is then diluted. The sulphate of lead thus formed is filtered off and the filtrate evaporated at a gentle heat. If the chromic acid be required to be perfectly pure, the crystals are dissolved in pure water and re-crystallized.
Chromic acid dissolves in a small quantity of water, forming a dark-brown liquid having an acrid, astringent taste. It is a powerful oxidizing agent, readily giving up part of its oxygen and passing into the sesquioxide. It reacts upon alcohol with such energy that the latter becomes inflamed.
Free chromic acid is seldom used in the arts as an oxidizing agent. A mixture, however, of a solution of bichromate of potash with sulphuric acid and some neutral vegetable matter yields the free acid. Obtained in this manner, it acts as a powerful bleaching agent, and is often used as a substitute for chlorine in calico-printing, the corrosive properties of the latter rendering it unfit for many purposes.
Pure chromic acid is used in mounting microscopic objects, to harden preparations of soft tissues. When combined with oxide of tin, it forms one of the pink colours used in porcelain painting.