Carbolic Acid, PHENOL, or PHENIC ACID, , is one of the most important substances derived from coal-tar. Although called an acid, and forming salts, it is neutral to test-paper, and has more in common with the alcohols than with the acids (see ACIDS and ALCOHOLS). It is obtained by distilling coal-tar, reserving the portion passing over between and (— C.). After rectification, this constitutes the crude carbolic acid used so largely for disinfecting purposes. By careful purification it can be obtained pure, when it forms minute white or colourless plates or needles, possessing a burning taste, and an odour resembling that of creosote. By exposure to the air it becomes pinkish, but is not otherwise injured. It rapidly absorbs water from the air, forming an oily liquid, which does not readily mix with water until about 15 volumes have been added, when it forms a solution. Carbolic acid is apt to be confounded with cresylic acid and creosote, which possess an odour somewhat similar. This remark applies only to the liquid acid, as neither of these substances can be crystallised. Liquid carbolic acid has no action on polarised light, in this respect differing from creosote. Carbolic acid is readily soluble in alcohol, ether, chloroform, glycerin, olive-oil, and volatile oils. Although called an acid, it forms but very weak combinations with the alkalies, some of the so-called carbolates used for disinfecting purposes being probably only earthy substances mixed with carbolic acid.
In solution carbolic acid coagulates albumen, arrests fermentation, destroys parasites, whether animal or vegetable, and prevents moulding or putrefaction. Owing to these valuable properties it has come largely into use as an antiseptic and disinfectant. Applied to the cavity of a decayed tooth it quickly relieves certain forms of toothache, acting as a local anaesthetic. Owing to its powerful action on the mucous membranes, it must not be allowed to touch the interior of the mouth. Taken internally, concentrated carbolic acid acts as an irritant poison, accompanied by a numbing of the stomach, which prevents the action of emetics. Medicinally, carbolic acid may be given in doses of one to three grains dissolved in glycerine or water. See ANTISEPTIC SURGERY.