"A neutral principle obtained from several varieties of Aloes, chiefly Barbadoes Aloes (yielding Barbaloin); and Socotra or Zanzibar Aloes (yielding Socaloin)—differing more or less in chemical composition and physical properties, according to the source from which it is derived"—(U. S. P.).
Preparation.—There are several methods of preparing this substance. One part of aloes may be dissolved in 10 parts of boiling water, the fluid portion decanted, and slightly acidulated with hydrochloric acid (Groves). Or the aloes may be previously made acid with sulphuric, hydrochloric, or sulphurous acid, and allowed to stand a half-day (Tilden). Then rapidly evaporate to 2 parts and set aside, that crystals may form. To purify, recrystallize from alcohol very much diluted. These methods answer well for barbaloin. Socaloin is best obtained by digesting for a day 1 part of aloes in 3 parts of alcohol, and then boiling the mixture on a water-bath for 2 hours. Cool, filter, and allow crystals to form, which are to be washed with a small portion of alcohol and then dried (Plenge, Amer. Jour. Pharm., 1884).
Description and Tests.—Barbaloin, when pure, crystallizes in stellated groups of small, prismatic needles, whose purity is shown by the color, which should not deepen by exposure to the air in desiccation. Its solutions in the alkalies and their carbonates are orange-yellow, and the liquid absorbs oxygen upon contact with the atmosphere, which rapidly deepens its color. Boiled with alkalies or acids, it is speedily transformed into a brown resin. Corrosive sublimate, nitrate of silver, or neutral acetate of lead, do not cause its precipitation; concentrated subacetate of lead produces a precipitate of an intense yellow, soluble in excess of water, and becoming deeper colored on exposure. Cold fuming nitric acid dissolves it, without disengaging gas, forming a reddish-brown liquid; to which, if sulphuric acid be added in great excess, a yellow, pulverulent body is thrown down, which explodes when heated. By dry distillation, aloïn furnishes a slightly aromatic, volatile oil, and a quantity of resinous substance. It forms crystallized compounds with bromine, but not with chlorine, although it combines equally well with the latter. The official aloin may be either barbaloin or socaloin, or both. (For the chemistry of these substances, see under Aloe).
"Minute, acicular crystals, or a microcrystalline powder, varying in color from yellow to yellowish-brown, odorless, or possessing a slight odor of aloes, of a characteristic, bitter taste, and permanent in the air. Barbaloin is soluble at 15° C. (59° F.), in about 60 parts of water, 20 parts of alcohol, or 470 parts of ether. Socaloin is soluble in about 60 parts of water, 30 parts of absolute alcohol, 380 parts of ether, or 9 parts of acetic ether. When heated, aloin melts, and, on ignition, it is consumed without leaving a residue. An alcoholic solution of aloin is neutral to litmus paper. An aqueous solution of aloin is colored greenish-black by ferric chloride T.S., and slowly precipitated by basic lead acetate T.S. On adding a minute portion of barbaloin to a drop of cold nitric acid of specific gravity 1.200, on a white porcelain surface, a crimson color will be developed. Socaloin will produce scarcely any color when thus treated. In alkaline solutions, aloin is rapidly decomposed; in neutral, or acid solutions, only slowly"—(U. S. P.).
Action, Medical Uses, and Dosage.—The action of aloin is similar to that of aloes, except that this agent will produce the effects of aloes in about one-third as large a dose, and is said to be freer from irritant effects. Dose: 1/10 to ½ grain.