CITRIC ACID, Acidum citricum or HYDROXYTRICARBALLYLIC ACID, first obtained in the solid state by Karl Wilhelm Scheele, in 1784, from the juice of lemons. It is present also in oranges, citrons, currants, gooseberries and other fruits, bulbs and tubers. Citric acid, is made com mercially on a large scale from lime or lemon juice, and also by the fermentation of glucose under the influence of Citromycetes pfefferianus, C. glaber and other ferments. Lemon juice is fer mented for some time to free it from mucilage, then boiled, filtered and neutralized with powdered chalk and a little milk of lime ; the precipitate of calcium citrate so obtained is decomposed with dilute sulphuric acid, the solution filtered, evaporated to remove calcium sulphate and concentrated, preferably in vacuum pans. The acid is thus obtained in colourless rhombic prisms of the composition About 2ogal. of lemon juice should yield about To lb. of crystallized citric acid. The acid may also be prepared from the juice of unripe gooseberries.
The synthesis of citric acid was accomplished by L. E. Grimaux and P. Adam in 1881. (See any standard textbook of organic chemistry.) Citric acid has an agreeable sour taste. It is soluble in a of its weight of cold and in half its weight of boiling water, and dissolves in alcohol, but not in ether. At 150° C it melts, and on the continued application of heat boils, giving off its water of crystallization. At 175° C it is resolved into water and aconitic acid, a substance found in Equisetum fluviatile, monks hood and other plants. A higher temperature decomposes this body into carbon dioxide and itaconic acid, which, again, by the expulsion of a molecule of water, yields citraconic anhy dride, Citric acid digested at a temperature below 4o° C with concentrated sulphuric acid gives off carbon monoxide and forms acetonedicarboxylic acid.
The citrates are a numerous class of salts, the most soluble of which are those of the alkaline metals ; the citrates of the alkaline earth metals are insoluble. Citric acid, being tribasic, forms sev eral series of salts. On warming citric acid with an excess of lime water a precipitate of calcium citrate is obtained which is redis solved as the liquid cools.
Citric acid is used in calico printing, also in the preparation of effervescing draughts, as a refrigerant and sialogogue, and oc casionally as an antiscorbutic, instead of fresh lemon juice. In the form of lime juice it has long been known as an antidote for scurvy. Several of the citrates are much employed as medicines, the most important being the scale preparations of iron. Of these iron and ammonium citrate is much used as a haematinic, and as it has hardly any tendency to cause gastric irritation or constipa tion it can be taken when the ordinary forms of iron are inad missible. Iron and quinine citrate is used as a bitter stomachic and tonic. In the blood citrates are oxidized into carbonates; they therefore act as remote alkalis, increasing the alkalinity of the blood and thereby the general rate of chemical change within the body. (See ACETIC ACID.)