Tartaric Acid

TARTARIC ACID, Acidum tartaricum, one of the four 'wine acids'. Wine acids (dioxytartaric C₄H₆O₆) correspond to the structural formula HOOC·CH(OH)·CH(OH)·COOH, in which the carbon atoms marked with an asterisk are asymmetric (see Asymmetric carbon). There are four wine acids in total: two optically active—right-handed and left-handed—and two inactive—mesotartaric (or antitartraric) and racemic. The study of the properties and conditions of mutual transformation of wine acids into each other played a very important role in the development of the chemistry of the structure of organic compounds and led to the establishment of the stereochemical hypothesis (Le Bel, van't Hoff), see Stereochemistry. - The right-handed (d) tartaric acid (Acidum tartaricum), usually called simple tartaric, or T. A., is one of the most common plant acids. First isolated by Scheele in 1768. It is found in significant quantities in grapes, tamarind, and mountain ash. It crystallizes in large monoclinic transparent prisms, melting at 170°; specific gravity 1.7598. It is well soluble in water and alcohol (100 parts of water at 15° dissolve 132 parts, at 100°—343 parts of T. A.), insoluble in ether. It rotates the plane of polarization to the right; specific rotation (Wd) = 12°. The source of technical production of T. A. is mainly waste from winemaking: grape pomace, wine yeast, and argol. The methods of preparation of T. A. come down to obtaining calcium tartrate, from which calcium is then precipitated as gypsum by H₂SO₄, and the filtrate is evaporated and subjected to crystallization.-T. A. is used as a mordant in dyeing and printing fabrics, for preparing various galenic preparations, in electroplating, in metal coloring, and for preparing photographic paper. Due to the low solubility of the acid potassium salt, T. A. is used in qualitative analysis as a reagent for potassium. Large quantities of T. A. are used to prepare salts (tartrates). The most important of these are: potassium antimonyl tartrate—tartar emetic (Tartarus stibiatus), K(SbO)C₄H₄O₆+ 4H₂O, used as a mordant and in medicine as an emetic; the neutral salt of potassium, K₂C₄H₄O₆, is used in medicine; the acid salt of potassium, cream of tartar (Cremor tartari), is used mainly for making baking powders ('dry yeast') and in dyeing; Rochelle salt, KNaC₄H₄O₆·4H₂O; tartaric acid alcohol, a pharmaceutical preparation and one of the components of Fehling's solution. The free tartaric acid is used to prepare effervescent powders with bicarbonate of soda, in doses of 0.2-1.0, as a refreshing drink. The left-handed (l) tartaric acid, in its chemical and physical properties, is completely identical to the right-handed one; optically active, it rotates the plane of polarization to the left by the same amount as the d-acid to the right ([α]_D²⁰ = 12°). It is prepared from racemic acid. It differs biochemically from the right-handed one in that when acted upon by the fungi Penicillium glaucum, Aspergillus niger, Aspergillus griseus, it is destroyed much more slowly than its right-handed enantiomer. Racemic acid (Acidum racemicum) is a racemic mixture of left-handed and right-handed tartaric acids. Optically, this acid is inactive; from aqueous solutions it precipitates in the form of crystals containing water, of composition (C₄H₆O₆)₂ + 2H₂O; at 110° it loses crystallization water; at 204° it decomposes. It is much less soluble in water than T. A. (in 100 parts of water at 15° 15 parts dissolve, at 100°—138 parts of racemic acid). When racemic acid is combined with optically active alkaloids and then subjected to fractional crystallization, it splits into the corresponding salts of right-handed and left-handed tartaric acids. It is prepared by heating T. acid or its derivatives. Mesotartaric (antitartraric) acid crystallizes in prisms of composition C₄H₆O₆ + H₂O. The anhydrous acid melts at 140°; at 15° it dissolves in 0.8 parts of water. It differs from optically active acids by the greater solubility of its acid salt. It does not split into optically active components (difference from racemic acid). It is optically inactive due to the complete mutual compensation of the rotation moments of both asymmetric carbon atoms. It is obtained from d-acid by boiling with a KOH solution. A characteristic reaction of T. A.: 1 part of resorcin in 100 parts of concentrated sulfuric acid when heated with T. A. to 130° gives a red-violet coloration.