TARTARIC ACID (dihydroxy-succinic acid), QHjOs, or HO 2 C-CH(OH)-CH(OH)-C0 2 H. Four acids of this composition are known, namely dextro- and laevo-tartaric acids, racemic acid and mesotartaric acid, the two last being optically inactive (see Stereoisomerism). Their constitution follows from their formation from dibromosuccinic acid and from their synthesis from glyoxal cyanhydrin, these two methods producing the inactive racemic form which may then be split into the active components. Dextro-tartaric acid occurs in the free state or as the potassium or calcium salt in grape juice and in various unripe fruits. During the alcoholic fermentation of grape juice it is deposited in the form of an impure acid potassium tartrate which is known as arg&l, and when purified as cream of tartar. For the preparation of the acid the crude argol is boiled with hydrochloric acid and afterwards precipitated as calcium tartrate by boiling with milk of lime, the calcium salt being afterwards decomposed by sulphuric acid. It may also be obtained (together with racemic acid) by oxidizing milk sugar, saccharic acid, &c, with nitric acid, and by the reduction of oxalic ester with sodium amalgam (H. Debus, Ann., 1873, 166, p. 100). It crystallizes from water in large prisms which melt at 168-170 C, and on further heating gives an anhydride and finally chars, emitting a characteristic odour and forming pyroracemic and pyrotartaric acids. It behaves as a reducing agent. Chromic acid and potassium permanganate oxidize it to formic and carbonic acids, whilst hydrogen peroxide in the presence of ferrous salts gives dihydroxymaleic acid (H. J. H. Fenton, Jour. Chem. Soc, 1894, p. 899; 1895, pp. 48, 774; 1896, p. 546). Hydriodic acid and phosphorus reduce it to malic acid and finally to succinic acid. Calcium chloride gives a white precipitate of calcium tartrate in neutral solutions, the precipitate being soluble in cold solutions of caustic potash but re-precipitated on boiling. It prevents the precipitation of many metallic hydroxides by caustic alkalis. It carbonizes when heated with strong sulphuric acid, giving, among other products, carbon monoxide and carbon dioxide. A small crystal of oxalic acid added to concentrated sulphuric acid containing about 1 per cent, of resorcin gives a characteristic violet red coloration.
Laevo-tartaric acid is identical in its chemical and in most of its physical properties with the dextro-acid, differing chiefly in its action on polarized light, the plane of polarization being rotated to the left. By mixing equal quantities of the two forms in aqueous solution heat is evolved and racemic acid, (C4H t O«)j-2HjO, is obtained. This variety is also formed by the hydrolysis of glyoxal cyanhydrin (F. Pollak, Monats., 1894, 15, p. 469); by heating a solution of desoxalic acid; by the oxidation of fumaric acid with potassium permanganate; by the action of silver oxide on dibromosuccinic acid, and by the oxidation of mannite, dulcite, inulin, &c, with nitric acid. In the anhydrous state it melts at 205-206 C. Mesotartaric acid is formed when cinchonine tartrate is heated for some time at 170 C. (L. Pasteur, Ann., 1853, 88, p. 212); by heating tartaric or racemic acid for some time with water to 165° C; by the oxidation of laevulose; and by the oxidation of phenol or maleic acid with an alkaline solution of potassium permanganate (O. Doebner, Ber., 1891, 24, p. 1755; A. Kekule and R. Anschutz, ibid., 1881, 14, p. 714). It crystallizes in prisms, and in the anhydrous state meits at 140 C. On prolonged boiling with aqueous hydrochloric acid it yields racemic acid. The sodium ammonium salt is not capable of decomposition into its optical antipodes, as is sodium ammonium racemate.
Tartaric acid as used in medicine is derived from potassium acid tartrate. Its impurities are lead, oxalic acid, lime and potassium tartrate. It is incompatible with potassium, calcium, mercury and vegetable astringents. Tartaric acid is rarely used alone, but is contained in prlula quininae sulphatis and in Seidlitz powder (see Sodium), and is a constituent of many proprietary granular effervescent preparations. If taken in overdose or in a concentrated form tartaric acid produces severe gastro-enteritis. In these cases lime-water, alkalis and magnesia should be used as antidotes, and opium may be required.