CITRIC ACID, or Oxytricarballylic Acid, C 6 H 8 O, or C 3 H 4 (OH)(CO.OH) 3 , a tetrahydric tribasic acid, first prepared in the solid state by Scheele, in 1784, from the juice of lemons, in which it exists in large quantity. 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, chiefly in the months of November and December. The 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 hme; the precipitate of calcium citrate so obtained is decomposed with dilute sulphuric acid, and the resulting solution of citric acid is separated by filtration, evaporated to remove calcium sulphate, and concentrated. The concentration is best effected in vacuum pans. The acid is thus procured in colourless rhombic prisms of the composition C 6 H 8 O 7 + H 2 O∙ 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 ftι of crystallized citric acid. The acid may also be prepared from the juice of unripe gooseberries. Calcium citrate for exportation in the place of lemon juice 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....
Citric acid has an agreeable sour taste. It is soluble in ∣ths 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, C 6 H 6 O 6 , a substance found in Equisetum fluviatile, monkshood, and other plants. A higher temperature decomposes this body into carbonic anhydride and itaconic acid, C 5 H 6 O 4 , which, again, by the expulsion of a molecule of water, yields citraconic anhydride, C 5 H 4 O 3 . Citric acid digested at a temperature below 40° C. with concentrated sulphuric acid gives off carbonic oxide. With fused potash it forms potassium oxalate and acetate. It is a strong acid, and dissolved in water decomposes the carbonates and attacks iron and zinc. Citric acid, in common with other tribasic acids, evolves about three times the amount of heat disengaged by acetic acid when quantities of these two bodies in the proportion of their molecular weights are saturated with soda,—one molecule of the tribasic being equivalent to three of the monobasic acid.
The citrates are a numerous class of salts, the most soluble of which are those with alkaline bases; the alkaline earthy citrates are insoluble. Citric acid, being tribasic, forms either acid monometallic, acid di metallic, or neutral trimetallic salts; thus, mono-, di-, and tri-potassic and sodic citrates are known. In a few salts a fourth atom of hydrogen is replaced by a metal, and citric acid is therefore considered by some chemists to be tetrabasic. Citric acid gives with excess of lime-water a slight precipitate of calcium citrate; a further precipitate is produced by boiling, but it is redissolved as the liquid cools. Solution of citric acid may be approximately titrated by means of baryta-water and litmus.
The impurities occasionally present in 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 of the sample of acid to be tested. 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 is furthermore distinguished from tartaric acid by the fact that an ammonia solution of silver tartrate produces a brilliant silver mirror when boiled, whereas citric of silver is reduced only after prolonged ebullition.
Citric acidis used in calico-printing, also in the preparation of effervescing draughts, and occasionally as a refrigerant and antiscorbutic, instead of fresh lemon juice, to which, however, it is therapeutically inferior. In the form of lime juice it has long been known as an antidote for scurvy, and several of the citrates are much employed as medicines.
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