dihydroxysuccinic acids, dibasic organic acids, HOOCCH(OH)—CH(OH)COOH, with two asymmetric carbon atoms in the molecule. All the theoretically possible tartaric acids are known: D-tartaric acid (I), solutions of which rotate the plane of polarized light to the right; its optical antipode, L-tartaric acid (II); D,L-tartaric acid (a
racemic mixture of D- and L-tartaric acids); and mesotartaric acid (III), which is optically inactive because of intramolecular compensation of the rotation of the plane of polarization owing to the presence of an additional axis of symmetry (ab).
The most important tartaric acid is D-tartaric acid (ordinary tartaric acid), which is present either in the free state or in the form of salts in many fruits, particularly grapes. When grape juice is fermented, the poorly soluble acidic potassium salt of tartaric acid, so-called cream of tartar, is precipitated. D-tartaric acid is obtained by the action of inorganic acids on cream of tartar.
D-tartaric acid and its salts (tartrates) are used extensively in the food industry (for example, in the preparation of soft drinks and in baking), in medicine (tartar emetic), in dyeing, in organic synthesis, and in analytical chemistry. In an alkaline medium, D-tartaric acid dissolves Cu(OH)2, yielding a transparent bright blue solution—Fehling solution—that is used as a reagent in testing the reducing capacity of substances.
V. N. FROSIN