Acidum Oleicum (U. S. P.)—Oleic Acid.
Related entry: Acidum Stearicum (U. S. P.)—Stearic Acid
FORMULA: HC18H33O2. MOLECULAR WEIGHT: 281.38.
SYNONYM: Elaic acid.
"An organic acid, prepared in a sufficiently pure condition by cooling commercial oleic acid to about 5° C. (41° F.), then separating and preserving the liquid portion"—(U. S. P.).
Source and Preparation.—This acid was first obtained by Chevreul in 1821. It is a constituent of most fats, and of the non-drying fixed oils in the form of glycerin trioleate (triolein). It is obtained in large quantities as a by-product in the manufacture of candles. In this condition it is very impure, of a dark, reddish-yellow or brownish-red color (red oil), and at ordinary temperatures somewhat turbid. An early method of obtaining this acid was as follows: To procure oleic acid, treat oil of bitter almonds with caustic potash, and to the soap formed add hydrochloric acid; this separates the oleic and other acids. To the decomposed mixture add about half its weight of oxide of lead, and digest for 2 or 3 hours, at a temperature of 100° C. (212° F.), by which means oleates of the fatty acids are formed. Ether is now added, which dissolves only the oleate of lead; the ethereal solution is mixed with an equal volume of water, to which hydrochloric acid is added as long as is required for decomposition, and the mixture is then well shaken. The ether rises to the surface, holding the oleic acid in solution; decant it and distill it off; there remains a compound of pure oleic acid with oxidized acid. By subjecting this compound to a temperature of about -7.2° C. (19° F.), the pure crystals of oleic acid form, while the oxidized acid remains in solution. Wolff recommends the substitution of petroleum benzin in the place of ether. The method of Chas. T. George (A. J. P., 1881) is to dissolve dry, white Castile soap (30 parts) in hot water (100 parts) and then to decompose the solution with sulphuric acid (6 parts). After washing the resulting oily layer with warm water, powdered litharge (2 parts) is added to it, and the lead oleate formed is dissolved, while warm, in petroleum benzin (10 parts). This solution is decomposed with weak hydrochloric acid (1 part with 12 of water). Evaporate the benzin, and oleic acid (15 parts) remains.
Description and Tests.—Oleic acid is a tasteless, inodorous, colorless, oily fluid at temperatures above 13.8° C. (57° F.), but when once melted it does not solidify until cooled to 4.4° C. (40° F.), and when solid it does not melt until heated to 13.8° C. (57° F.) It is not soluble in water, but readily soluble in alcohol or ether, floats on water, and becomes brown by the absorption of atmospheric oxygen, with which it has a tendency to combine. It forms salts or soaps with bases. The oleic acid, as it occurs in non-drying fat oils, like olive oil, is chemically different from that occurring in drying oils, the type of which is linseed oil. Oleic acid, when in contact with nitrous acid, is converted into an isomeric solid compound called elaïdic acid.
Oleic acid is not decomposed when distilled in vacuo, but if heated in the air it decomposes, forming hydrocarbons, sebacic acid, and volatile fatty acids. When treated with fused caustic potash, decomposition takes place, with acetic and palmitic acids as a result. With sodium and potassium hydroxides it forms soaps known respectively as oleate of sodium and oleate of potassium, classed among hard and soft soaps. This acid is used in the preparation of the oleates, a class of external applications now used in considerable quantities. The United States Pharmacopoeia describes, as follows, the oleic acid that should be employed in pharmacy: "A yellowish or brownish-yellow, oily liquid, having a peculiar lard-like odor and taste; becoming darker and absorbing oxygen on exposure to air. Specific gravity about 0.900 at 15° C. (59° F.). Insoluble in water; soluble in alcohol, chloroform, benzol, benzin, oil of turpentine, and fixed and volatile oils. When cooled to about 4° C. (39.2° F.), oleic acid becomes semi-solid, and, on further cooling, congeals to a whitish, solid mass. When heated to a temperature of about 95° C. (203° F.), the acid begins to be decomposed, giving off acrid vapors. At a higher temperature it is completely dissipated. An alcoholic solution of oleic acid has a feebly acid reaction upon litmus paper. Equal volumes of oleic acid and alcohol, mixed at the ordinary temperature, should give a clear solution without separating any oily drops upon the surface (absence of fixed oils). If 1 Gm. of oleic acid be heated with 20 Cc. of alcohol, 2 drops of phenolphtalein T.S. added, and then a strong solution (1 in 4) of sodium hydrate, drop by drop, until the liquid has acquired a permanent red tint and the acid is saponified; next acetic acid added until the red color of the liquid is just discharged, and the liquid filtered—10 Cc. of the filtrate mixed with 10 Cc. of ether should not be rendered more than slightly turbid by the addition of 1 Cc. of lead acetate T.S. (absence of notable quantities of palmitic and stearic acids)"—(U. S. P.).
Linoleic acid is used to adulterate oleic acid. It may be detected with potassium permanganate, which converts oleic acid into azaleinic acid, and linoleic acid into sativic acid. The latter does not dissolve in ether while the former does. As little as 1 per cent of this impurity may be detected by this salt (A. P. A. Proceed., 1890). Linoleic and ricinoleic acids constitute the fatty acids occuring in the drying oils.
Use.—Used only in preparing the oleates.
Related Products.—SULPHO-OLEIC ACID. Sulpholeic acid (C17H22CO2HSO3H). This acid is formed when sulphuric acid reacts upon castor, olive, almond, or other fixed ohs, provided the temperature be not higher than 50° C. (122° F.). To obtain the acid in an anhydrous state ether is employed to remove the oil not acted upon, and the crude product is treated with alkaline carbonates or hydroxides, treated again with sulphuric acid, and finally evaporated in pure condition from benzin, or from ether. The substances known as oleite, polysolve and solvine, are neutral salts of this acid from its combination with alkaline bases. Water dissolves these salts (sulpholeates) and they in turn may be employed to render many otherwise insoluble substances miscible with water. Sulphoricinoleic acid is the name applied to that prepared with castor oil. The sulpholeates mix with many organic substances.
SULPHORICINIC ACID. Prepared by acting upon castor oil at a heat not higher than 50° C. (122° F.) with from 30 to 40 per cent sulphuric acid. Upon adding water, the sulphoricinic acid together with some free oil, separates as an oily stratum, an aqueous acid solution underlying. It is an active topical irritant. It, as well as its soluble salts are decided deodorizers and antiseptics. They have been used in ozena and like conditions. Salol sulphoricinate, creosote sulphoricinate, and naphthol sulphoricinate, have been employed as local medicines. Phenolum natrio sulphoricinicum is a yellowish liquid containing sodium sulphoricinate (80 per cent) and phenol (20 per cent). It is miscible in all proportions with water. A 20 per cent solution has been recommended as a topical application in diphtheria. Skin and mucous-surface affections, tubercular and otherwise, have been treated with it.
OLEITE (Sodium sulphoricinoleate) is a scarcely odorous, transparent, gelatinous fluid, soluble in water, alcohol, volatile oils, and chloroform. Its taste is acrid. It has extraordinary solvent powers upon many substances employed in medicine. It is non-poisonous internally or externally, and being soothing, is recommended as an ointment base. It is said to act kindly in cuts, scalds, burns, etc.