FORMULA: H3C6H5O7+H2O=C3H4(OH)COOH)3.H2O. MOLECULAR WEIGHT: 209.50.
SYNONYMS: Acidum citri, Acidum limonum, Acidum limonorum, Acidum limonis.
"An organic acid, usually prepared from lemon juice"—(U. S. P.).
Source and History.—Citric acid of commerce is prepared from the juices of the lemon, the lime, and the bergamot. It was first obtained, in 1776, by Retzius, who distinguished it from acetic and tartaric acids. In 1784, Scheele first obtained it in crystalline form. In 1880, it was prepared from glycerin, and also from malic acid. Besides being derived from the sources named, it exists in a free or combined state as citrate of calcium or potassium in many fruit-juices, as of whortleberries, cranberries, gooseberries, strawberries; blackberries, raspberries, red elderberries, currants, cherries, tomatoes, tamarinds, cayenne, and in the fruits of bittersweet and of a solanaceous plant of South America and Mexico, known as the "tomato de la paz" (Cyphomandra botacea). It is also present in Jerusalem artichoke, dahlia tubers, and in the rhizomes of red puccoon, and Asarum europaeum. Both the tobacco plant and lettuce contain it. While abundant in the red elderberry the present supply is almost entirely derived from fruits of the orange family. England produces the acid on a large scale from Italian lemons and limes. It is also made from the sour oranges of Florida.
Preparation.—The juice, if fresh and from sound fruit, is first clarified by boiling, then strained and saturated to excess with calcium carbonate or milk of lime. If the juice be derived from decayed fruit, vinous fermentation is allowed to proceed instead of boiling it. The calcium citrate is then precipitated in hot water (being less soluble in hot than in cold water) and the supernatant liquid drawn off. The calcium salt is then washed with hot water to purify it. A slight excess of diluted sulphuric acid is now added to decompose the calcium citrate. By this process calcium sulphate is thrown out after which the citric acid solution is drawn off. The calcium sulphate is then agitated with either very cold or hot water, and the washings, together with the citric acid solution, are then heated by steam until of the consistence of syrup when it is crystallized in leaden vats. Crystallization is facilitated by the presence of sulphuric acid; by redissolving the acid in a little water and purifying with animal charcoal, the commercial crystals will be deposited from the filtered solution.
Description and Tests.—Citric acid is that acid whose sourness is typical of that of the lemon. It occurs in "colorless, translucent, right-rhombic prisms, odorless, having an agreeable, purely acid taste; efflorescent in warm air, and deliquescent when exposed to moist air. Soluble at 15° C. (59° F.), in 0.63 part of water, and in 1.61 parts of alcohol; in about 0.4 part of boiling water, and in 1.43 parts of boiling alcohol; also soluble in 18 parts of ether. When heated to about 75° C. (167° F.) the acid begins to lose its water of crystallization; at about 135° C. (275° F.) it becomes anhydrous, and melts between 135° and 152° C. (275° and 305.6' F.). When slowly ignited it is gradually decomposed without emitting the odor of burning sugar (difference from tartaric acid), and is finally consumed without leaving more than 0.05 percent of residue"—(U. S. P.). It reddens litmus and forms the line of salts known as citrates, of which the alkaline salts are freely soluble in water. Calcium citrate is soluble in cold, but not in hot water. The dehydrated acid when heated to 155° C. (311° F.) yields water and aconitic acid identical with that obtained from aconite, achillea, hellebore, and other plants.
The aqueous solution of citric acid spoils by keeping, forming acetic acid as one of its products. A solution of about the strength of lemon juice may be prepared by adding to one ounce of water about two-thirds of a drachm of citric acid. A portion of this added to water until sufficiently sour and then sweetened to taste, affords a fairly good substitute for lemonade. The addition of a small portion of lemon essence improves the taste. A solution of 1 part of the acid to 2 parts of distilled water will keep sufficiently long to be utilized at the "soda counter." Heated with caustic potash or with nitric acid, citric acid is converted into oxalic acid.
"On adding 1 Cc. of an aqueous solution (1 in 10) of the acid to 50 Cc. of calcium hydrate T.S. (or so much more of the latter that the mixture has an alkaline reaction), the liquid remains clear. Upon boiling this for about one minute, it becomes opaque through the precipitation of calcium citrate, which redissolves on cooling. If 1 Gm. of the powdered acid be dissolved in 5 Cc. of a. cold solution (1 in 3) of potassium acetate, the liquid should remain clear, even after the addition of an equal volume of alcohol (absence of tartaric or oxalic acid). On mixing 10 Cc. of a 10 per cent aqueous solution of the acid with a quantity of ammonia water insufficient to neutralize it completely, and adding to one-half of this liquid 1 Cc. of ammonium oxalate T.S., it should remain clear (absence of calcium). The other half, mixed with a few Cc. of hydrogen sulphide T.S., should not deposit a colored precipitate, nor acquire more than a faintly brownish-yellow tint (limit of metallic impurities). On treating 10 Cc. of a 1 per cent aqueous solution of the acid with 1 Cc. of barium chloride T.S. and a few drops of hydrochloric acid, the liquid should not show any turbidity within five minutes (limit of sulphuric acid). To neutralize 3.5 Gm. of citric acid should require 50 Cc. of potassium hydrate V.S. (each Cc. corresponding to 2 per cent of the pure acid), phenolphtalein being used as indicator"—(U. S. P.).
Action, Medical Uses, and Dosage.—Though fatal to rabbits, cats, and other animals, producing heart failure, violent tetanic convulsions, and finally death, it has not produced the death of man. Its continued use acts destructively on the teeth. Large doses are irritant to the gastro-intestinal tract, and may act as a poison in this way. It is principally carried off by the kidneys, rendering the urine acid if the amount taken be large. The coagulability of the blood is retarded by it. This acid is used as a refrigerant and antiscorbutic, rendering the blood more fluid by lessening its coagulability. In all febrile diseases a sweetened solution of it will be found a very beneficial drink, especially in those cases where the tongue is coated brown, dark or red; it may be flavored with a few drops of the essence of lemon. It is likewise beneficial in scurvy (though far less effective than fresh lemon juice), acidity of the stomach and some peculiar forms of sick-headache. It has also been advised in gout and inflammatory rheumatism, but its liability to occasion gastric acidity, and increase of uric acid in the urine, are serious objections to its employment in these diseases, aside from the failures that have followed its use on many occasions. Prof. Scudder recommends lemonade or lemon juice in those cases only where the tongue is red and long and the inflamed parts show excessive redness. A lemonade powder, which will keep for years if preserved dry, is made by mixing together 4 ounces of powdered white sugar with 3 drachms of powdered citric acid and 2 drops of oil of lemon. Half a teaspoonful of this mixture may be dissolved in a tumbler of water for a beverage. The continued use of citric acid disturbs the functions of the digestive organs. M. Girard, Chief of the Paris Municipal Laboratory, claims that citric acid is an efficient purifier of polluted water, 1 grain of the acid being destructive to the micro-organisms in a quart of water. For this reason he advises the use of lemonade during epidemics. Locally it is useful in hypertrophied tonsils, elongated uvula, and genital pruritis. The dose of citric acid is from 5 to 30 grains; lemonade at will.
Specific Indications.—Inflammatory rheumatism, showing excessive redness of inflamed parts and elongated, reddened tongue; scorbutus; febrile states, with red, elongated tongue. Lemon juice as an anti-rheumatic when the tongue is markedly red, papillae prominent, and usually thinly-coated white.
Related Product.—ACIDUM CITRICUM SACCHARATUM (N. F.), Saccharated citric acid. Formulary number, 5: "Citric acid (U. S. P.), in very fine powder, six hundred and twenty-five grammes (625 Gm.) [1 lb. av., 6 ozs., 20 grs.]; sugar in very fine powder, three hundred and seventy-five grammes (375 Gm.) [13 ozs. av., 100 grs.]. Triturate the powders together until intimately mixed, and preserve the product in well-stoppered bottles.
"Note.—This saccharate, when dissolved in water with an equal weight of saccharated sodium bicarbonate (F. 341), will form a neutral solution, and it is introduced into this formulary for the convenient preparation of effervescent powders (F. 319).
"This saccharate contains 62.5 per cent of crystallized citric acid"—(Nat. Form).
King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.