2. In the MOIST WAY:—
a. (Ph. D. 1826; Scheele’s Process.) Equal parts of benzoin and hydrate of lime, in fine powder, are intimately mixed together and boiled for about an hour, with 40 parts of water; the liquor, after filtration, is evaporated to 1⁄5th, and the lime saturated with hydrochloric acid; the benzoic acid crystallised out as the liquor cools, and is then either washed with very cold water, and dried by a gentle heat, or it is dried and sublimed in the manner already explained. The product of the sublimation is extremely white and pure.
Obs. An economical and productive process; but, to ensure success, a perfect mixture of the dry ingredients must be first made; as otherwise the benzoin runs into a solid mass in the boiling water, and the operation fails. Prod. “1 lb. of (gum) benjamin yields 1 oz. 6 dr. 2 scr. of flowers.” (Gray.)[150]
[150] A quantity which, in our own experiments, we were never able to obtain.
b. (Process of Stoltze.) The benzoin is dissolved in 3 times its weight of alcohol, the solution introduced into a retort, and a solution of carbonate of soda in weak spirit-and-water, is gradually added, until all the free acid present is neutralised; water, equal to about twice the weight of the benzoin employed, is next poured in, and the alcohol removed by distillation. The floating resin is now skimmed off the residual liquid and washed with a little water, and the washings added to the contents of the retort, which will deposit crystals of benzoate of soda on cooling, and more by subsequent evaporation. From this salt the benzoic acid is obtained by saturating the alkali with an acid (as the hydrochloric), and by subsequent sublimation of the crude precipitated crystals.
3. Other Methods:—
a. Ordinary hippuric acid is very gently boiled, for about 15 minutes, in nitric acid[151] (sp. gr. 1·42); water is then added, and the solution allowed to cool and crystallise. The crystals are collected on a filter, washed with a little very cold water, dried by pressure in bibulous paper, and lastly, purified by sublimation, as before.
[151] Hydrochloric acid as well as sulphuric acid also convert hippuric acid into benzoic acid; as does likewise a sufficient degree of heat. See Hippuric Acid.
b. From the urine of horses, cows, and other graminivorous animals, in a similar way to that by which hippuric acid is obtained, only allowing the urine to acquire a slight degree of putridity before evaporation, which last should be effected by a heat slightly under that of ebullition. The crude acid thus obtained is purified as previously directed.
Obs. Large quantities of benzoic acid are said to be obtained in this way on the Continent; but, owing to the process being clumsily conducted, it is generally of inferior quality, and hence unsaleable. It may, however, by skilful purification, be rendered quite equal to that obtained from gum benzoin.[152]