FORMULA: HC3H5O3. MOLECULAR WEIGHT: 89.79.
SYNONYMS: Isolactic acid, Oxypropionic acid, Ethylidene-lactic acid, Ethidene-lactic acid.
"An organic acid, usually obtained by subjecting milk-sugar or grape-sugar to lactic fermentation; composed of 75 per cent, by weight, of absolute lactic acid [HC3H5O3=89.79], and 25 per cent of water"—(U. S. P.).
Source and History.—Lactic acid is formed from milk-sugar by the process of "lactic fermentation" induced by the presence of casein. It is the acid principle of sour milk and is produced when vegetable and other tissues become soured, as in sauerkraut, or when beet juice, or rice-water are allowed to ferment, and it may also be obtained from refuse liquor produced in the manufacture of starch and the tanning of leather. It is found in natural form in the juice of the bittersweet (Solanum Dulcamara) (Wittstein), and occurs normally in some of the fluids of the body, notably the gastric juice; also in certain pathological conditions. That it exists in the gastric juice, however, is contradicted by Maly and others. This acid was first obtained from sour milk by Scheele in 1780. Berzelius discovered it in a modified form in meat juices in 1807 and named it sarcolactic acid, which, however, has been shown to be a combination of Paratactic and ethylenelactic acids. These acids are isomeric with the lactic acid of fermentation, but behave differently toward polarized light. The greater portion of lactic acid comes to us from Germany, though a considerable amount is now prepared in the United States. Alcohol, mannit, a gum, and several other acids may be obtained in making lactic acid.
Preparation.—This acid is usually prepared by allowing milk-sugar to digest for a few weeks, in the sunlight, at a temperature of about 30° C. (86° F.), in a mixture of milk, rotten cheese, and chalk. The casein of the cheese induces lactic fermentation, the resulting acid being neutralized by the calcium compound, forming calcium lactate. The calcium salt is then crystallized, decomposed with oxalic acid by which calcium oxalate is formed, and lactic acid is liberated. At no time should the temperature fall below 20° C. (68° F.), or rise above 40° C. (104° F.), for at the former the formation of the acid is retarded and at the latter a certain amount of butyric acid is produced. (We wouldn't want that, would we? Butyric acid is responsible for the smell of rotten eggs.—Henriette)
A more rapid process than that customarily followed, and one which gives a greater yield, is that of Kiliani (1882). Cane sugar is converted into invert sugar by dissolving in two hundred and fifty grammes (250 Gm.) [8 ozs. av., 358 grs.] of water, five hundred grammes (500 Gm.) [1 lb. av., 1 oz., 279 grs.] of cane sugar, and boiling it with ten cubic centimeters (10 Cc.) [162♏] of sulphuric acid. To this is gradually added four hundred and fifty cubic centimeters (450 Cc.) [15 fl℥, 104♏] of a solution of equal parts of sodium hydroxide and water. This is heated to 60° C. (140° F.), or 70° C. (158° F.), until it no longer reacts, or gives but a very faint greenish color, with Fehling's test solution. Sulphuric acid is then added to neutralization, sulphate of sodium crystallizes, and 93 per cent alcohol is added until the remainder ceases to throw down a precipitate. Then over a sand-bath one-half of the alcoholic solution is boiled and neutralized with carbonate of zinc, and, while boiling hot, filtered, added to the other half, and allowed to cool. Lactate of zinc at once begins to crystallize, requiring about 36 hours for completion. The crystals are expressed and recrystallized. This salt is then treated with oxalic acid, and lactic acid results. When well-soured sauerkraut is boiled with zinc carbonate, lactate of zinc is formed.
ACIDUM LACTICUM DILUTUM (Br.) Diluted lactic acid: "Lactic acid 3 fluid ounces (Imp.); distilled water, q.s. to make 1 pint (Imp.) Mix"—(Brit. Phar.).
Description and Tests.—Pure lactic acid is colorless and odorless. The official preparation is "a colorless, syrupy liquid, odorless, of a purely acid taste, and absorbing moisture on exposure to damp, air. Specific gravity, about 1.213 at 15° C. (59° F.). Freely miscible with water, alcohol, or ether; insoluble in chloroform, benzin, or carbon disulphide. Lactic acid is not vaporized by a heat below 160° C. (320° F.); at a higher temperature it emits inflammable vapors, and is finally dissipated. 5 Gm., after combustion, should not leave more than 0.05 Gm. of fixed residue. Lactic acid has a strongly acid reaction"—(U. S. P.).
Albumen is coagulated by it. Treated with nitric acid it yields oxalic acid. It gives rise to the lactates, and when treated with chromic acid, acetic and formic acids result. Heated to 150° C. (302° F.), it is converted into the anhydride, lactide (C3H4O2=concrete lactic acid), a crystalline solid. When painted upon substances it dries to form a smooth varnish which gradually absorbs moisture from the air.
"On adding some potassium permanganate to a mixture of equal volumes of lactic and sulphuric acids, and gently heating, the odor of aldehyde will become perceptible. 10 Cc. of a 1 per cent aqueous solution of the acid should not be rendered opalescent by the addition of 1 Cc. of silver nitrate T.S. (limit of chloride). 10 Cc. of a 10 per cent aqueous solution should remain unaffected by the addition of 1 Cc. of barium chloride T.S. (absence of sulphate), or by 1 Cc. of copper sulphate T.S. (absence of sarcolactic acid), or after supersaturation with ammonia by 1 Cc. of ammonium sulphide T.S. (absence of iron, lead, etc.). On adding a few drops of lactic acid to 10 Cc. of hot alkaline cupric tartaric V.S., no red cuprous oxide should be separated (absence of sugars). If a small portion of the acid be heated with an excess of zinc carbonate, the mixture dried at 100° C. (212° F.), and then extracted with absolute alcohol, upon evaporation of the latter no sweet residue should remain (absence of glycerin). On mixing equal volumes of lactic and colorless, concentrated sulphuric acids in a small, clean, glass-stoppered vial, the mixture should not acquire a tint deeper than a pale straw-color (absence of more than traces of organic impurities). To neutralize 4.5 Gm. of lactic acid should require 37.5 Cc. of potassium hydrate V.S. (each cubic centimeter corresponding to 2 per cent of absolute acid), phenolphtalein being used as indicator"—(U. S. P).
Action, Medical Uses, and Dosage.—Lactic acid is a normal constituent of the gastric juice, and to its excess in the system has been attributed the cause of rheumatism; but whether it is the cause or a result has not yet been definitely determined. Very large doses are reputed hypnotic (Mendel).
Lactic acid quickly dissolves oxalate of calcium and phosphate of calcium, especially that which is contained in the bones, and hence has been recommended in oxalic and phosphatic urinary deposits. It has not, until quite recently, been much employed in medicine in its uncombined state, but has been used in the preparation of lactate of iron and lactate of quinine. According to Pereira this acid was introduced into medicine by Magendie, who suggested its employment in dyspepsia and in phosphatic urine. It has more recently been advised in gout. The dose is from half a drachm to 2 drachms, in sweetened water, or in the form of lozenges. It is better to take the acid during or immediately following meals. Added to pepsin, as prepared for therapeutical use, this acid renders it still more valuable as a solvent of the food received into the stomach. According to Bricheteau and Adrian, the false membranes of diphtheria, croup, pseudo-membranous bronchitis, etc., are soluble in a solution of lactic acid, forming a translucent liquid with almost imperceptible fragments of a gelatiniform substance floating upon its surface and looking like froth, while acetic, citric, formic, and chromic acids have no such action. They recommend in croup, diphtheria, etc., a gargle composed of lactic acid 5 parts, water 100 parts, and orange syrup 30 parts; in conjunction with the use of the same, minus the syrup, in the form of spray thrown upon the affected parts.
Lactic acid has been used topically in ulcerated states of the nasal fossae and in suppurative otitis, though in the latter disease the results are not commensurate with the irritation of the external canal which it occasions. In caries of the aural bony structures it has been advised, but it is open to the same objection mentioned above. It fails to destroy dense granulations of the tympanum. Webster (Dynam. Therap.) declares it second only to acetic acid in obstinate tinea versicolor, and states that it does not attack the true skin. "Liver spots" and ephilides are said by him to be removed by the application of the concentrated acid, the procedure to be repeated until the epidermis and underlying pigment are removed in such a manner as to avoid cicatrization. The lesions are then dressed with mild zinc ointment or a similar dressing. Epithelial excrescences, such as warts, horny growths, and tylosis of the palms and soles, are removed by the continued application of the acid with a camel's-hair pencil.
The most important use of this remedy, however, is in gastro-intestinal affections. Where an acid is indicated and the digestive powers are feeble this acid should be preferred to hydrochloric and other acids. It may therefore be employed in feeble digestion and even in advanced dyspepsia. It is a well-known fact that delicate stomachs that can scarcely retain any other food, kindly receive buttermilk and coagulated or "clabbered" milk, and this often forms an excellent means of administration. The irritable stomach with evident lack of, or but scanty secretion of gastric juice, clearly points to its use. Its action in infantile diarrhoea with a painful and irritable stomach and the passage of green evacuations is most positive. It should be given in water that has been boiled and may be sweetened if desired. The dose should be regulated so that the little patient receives from ½ to 1 drop at a dose. The dose of lactic acid may range from a half drop to a half ounce in 24 hours. The small doses, from one-half drop to 5 drops, are those which act specifically.
Specific Indications and Uses.—Gastric irritation with thirst, deep-red tongue, and diarrhoea with green stools, itching skin, and cutaneous eruptions, especially when arising from gastric disturbances.