Acidum Benzoicum, B.P. Benzoic Acid.
C7H6O2 = 122.048.
Benzoic acid, C6H5COOH, may be obtained from benzoin, or prepared by oxidising toluene; or it can be prepared from hippuric acid and other organic compounds. It is also official in the U.S.P. It occurs in white or yellowish-white crystalline plates or needles, odourless when pure, but when obtained from benzoin possessing a pleasant aromatic odour. The pure acid melts at from 120° to 122° and boils at 249°, but that obtained from benzoin melts at about 120°, and boils at about 239°. It should leave only a slight residue on ignition, and should be free from chlorine compounds and from hippuric and cinnamic acids. The presence of chlorine compounds may be indicative of artificial acid, and in employing the B.P. test care must be taken that the calcium carbonate used as a reagent is free from chlorine, which is never absent from the commercial carbonate.
Soluble in water (1 in 390), boiling water (1 in 12), alcohol (1 in 3), ether (1 in 2.5), chloroform (1 in 7), or the fixed and volatile oils.
Action and Uses.—Benzoic acid has an action very similar to that of salicylic acid; it is even more powerfully antiseptic, for the presence of 0.1 per cent. inhibits the growth of most bacteria. Taken internally, it is rapidly absorbed and affects metabolism in the same way as the salicylates. Benzoic acid combines with glycocoll in its course through the kidneys, and is excreted in the urine as hippuric acid. Unlike salicylic acid, it has little influence on rheumatic diseases. It is used in chronic cystitis and other genito-urinary diseases to diminish the alkalinity and putridity of the urine. It has also a more limited use as an expectorant, to assist the coughing tip of tenacious mucus. After the use of this drug, the urine contains less aromatic sulphate and indican, and this is regarded as evidence of diminished putrefaction in the intestines. Benzoic acid has a preservative action on articles of food and drink resembling that of salicylic acid. The acid is frequently administered in cachets, but it may also be given in pills massed with glucose and a little tragacanth; for its local action on the throat as a stimulant and antiseptic, Trochisci Acidi Benzoici are suitable. When prescribed in mixture form it should be suspended with mucilage of tragacanth, or with syrup. The ammonium and sodium salts are, however, to be preferred for use in mixtures, as they have not the irritating action of the acid upon the mucous membranes. When used for inhalation the natural acid is preferable; for this purpose, however, the compound tincture of benzoin is commonly employed, and is of great service when inhaled with steam in acute laryngitis with hoarseness. Benzoic acid is incompatible with ferric salts, lead acetate, and mercuric chloride.
Dose.—3 to 10 decigrams (5 to 15 grains).
- Trochisci Acidi Benzoici, B.P.—BENZOIC ACID LOZENGES.
- Benzoic acid, ½ grain, in each lozenge. These lozenges are used as a local antiseptic, and as a stimulant in pharyngeal affections.