Preparation. - By dissolving benzoic acid in a slight excess of ammonia, evaporating and crystallising.

HC7H5O2 + NH4HO = NH4C7H5O2 + H2O.

Properties. - In colourless, laminar crystals; soluble in water and in alcohol.

Reactions. - It gives a bulky yellowish precipitate with persalts of iron (benzoate). Its aqueous solution when heated with caustic potash evolves ammonia, and, if it be not too dilute, when acidulated with hydrochloric acid it gives a deposit of benzoic acid.

Impurities. - Fixed salts.

Tests. - When heated it sublimes without any residue.

Physiological Action. - Benzoic acid is a stimulant and irritant to raw surfaces. It has an antiseptic action, destroying low organisms, and is used in the form of the tincture for ulcers, wounds, blisters, and chapped hands. It was owing to its antiseptic action that Friar's balsam was successfully used for the treatment of wounds in the Middle Ages, although at that time its mode of action was unknown (p. 104).

It acts as a stimulating expectorant, diminishing the secretion of the mucous membrane.

Benzoic acid when absorbed into the blood is excreted by the kidneys, and acts as a diuretic. It does not diminish the uric acid. In the kidneys it unites with glycocoll, and is excreted as hippuric acid, rendering the urine acid and somewhat irritating. This is proved by the following experiments: (1) If you give benzoic acid it is found in the urine as hippuric acid, but in the blood still remains as benzoic acid.

(2) If you give to a rabbit hippuric acid, it is excreted as such, but is found in the blood as benzoic acid.

(3) If you tie the renal arteries and give benzoic acid no conversion into hippuric acid takes place, but if you ligature the ureters the change takes place, and hippuric acid is found in the blood. This localises the seat of the change to the kidneys.

Uses. - Compound tincture of benzoin (5 per cent.) and glycerine (5 per cent.), in rose water is a useful application or a stimulant to the skin after the cure of acne. Friar's balsam is also useful in urticaria. As an inhalation (one drachm to one ounce in a pint of boiling water) it has a sedative effect in relieving the irritation and cough of sub-acute laryngitis and of tracheitis. It is also useful in bronchitis. Benzoic acid is used in chronic bronchitis and phthisis, both internally and as an inhalation, and extraordinary results have been ascribed to its uses; many, however, deny its beneficial effect. It is used in catarrh of the bladder to acidify the urine. Ammonium benzoate has a similar action to benzoic acid.