1911 Encyclopædia Britannica/Citric Acid
CITRIC ACID, Acidum citricum, or Oxytricarballylic Acid, C3H4(OH) (CO·OH)3, a tetrahydroxytribasic 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 many other fruits, and in several bulbs and tubers. It is made 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 fermented for some time to free it from mucilage, then boiled and 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 C6H8O7 + H2O. Crystals of a different form are deposited from a strong boiling solution of the acid. About 20 gallons of lemon juice should yield about 10 lb of crystallized citric acid. The acid may also be prepared from the juice of unripe gooseberries. Calcium citrate must be manufactured with care to avoid an excess of chalk or lime, which would precipitate constituents of the juice that cause the fermentation of the citrate and the production of calcium acetate and butyrate.
The synthesis of citric acid was accomplished by L. E. Grimaux and P. Adam in 1881. Glycerin when treated with hydrochloric acid gives propenyl dichlorhydrin, which may be oxidized to s-dichloracetone. This compound combines with hydrocyanic acid to form a nitrile which hydrolyses to dichlor-hydroxy iso-butyric acid. Potassium cyanide reacts with this acid to form the corresponding dinitrile, which is converted by hydrochloric acid into citric acid. This series of operations proves the constitution of the acid. A. Haller and C. A. Held synthesized the acid from ethyl chlor-acetoacetate (from chlorine and acetoacetic ester) by heating with potassium cyanide and saponifying the resulting nitrile. The acetone dicarboxylic acid, CO(CH2CO2H)2, so obtained combines with hydrocyanic acid, and this product yields citric acid on hydrolysis.
Citric acid has an agreeable sour taste. It is soluble in ¾ths 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, C6H6O6, a substance found in Equisetum fluviatile, monks-hood and other plants. A higher temperature decomposes this body into carbon dioxide and itaconic acid, C5H6C4, which, again, by the expulsion of a molecule of water, yields citraconic anhydride, C5H4O3. Citric acid digested at a temperature below 40°C. with concentrated sulphuric acid gives off carbon monoxide and forms acetone dicarboxylic acid. With fused potash it forms potassium oxalate and acetate. It is a strong acid, and dissolved in water decomposes carbonates and attacks iron and zinc.
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 either acid monometallic, acid dimetallic or neutral trimetallic salts; thus, mono-, di- and tri-potassium and sodium citrates are known. On warming citric acid with an excess of lime-water a precipitate of calcium citrate is obtained which is redissolved as the liquid cools.
The impurities occasionally present in commercial citric acid are salts of potassium and sodium, traces of iron, lead and copper derived from the vessels used for its evaporation and crystallization, and free sulphuric, tartaric and even oxalic acid. Tartaric acid, which is sometimes present in large quantities as an adulterant in commercial citric acid, may be detected in the presence of the latter, by the production of a precipitate of acid potassium tartrate when potassium acetate is added to a cold solution. Another mode of separating the two acids is to convert them into calcium salts, which are then treated with a perfectly neutral solution of cupric chloride, soluble cupric citrate and calcium chloride being formed, while cupric tartrate remains undissolved. Citric acid is also distinguished from tartaric acid by the fact that an ammonia solution of silver tartrate produces a brilliant silver mirror when boiled, whereas silver citrate is reduced only after prolonged ebullition.
Citric acid is used in calico printing, also in the preparation of effervescing draughts, as a refrigerant and sialogogue, and occasionally 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 constipation it can be taken when the ordinary forms of iron are inadmissible. 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).