Tartaric Acid

TARTARIC ACID, is now one of the most important of the organic acids and probably one of the first to be recognised, for its sparingly soluble acid potassium salt (see TARTAR) was known to the Greeks and Romans in the form of a deposit from fermented grape juice. K. W. Scheele first isolated tartaric acid in 1769 by boiling tartar with chalk and decomposing the product with sulphuric acid. Ordinary tartaric acid crystallises from water in large colourless hemihedral monoclinic prisms containing no water of crystallisation. It melts at 168-17o° C and at higher temperatures decomposes into a variety of products; its formula is (See PYRUVIC ACID.) Manufacture.—The starting material, argol or wine lees, is boiled with dilute hydrochloric acid and the solution treated with milk of lime and calcium carbonate, when calcium tartrate is precipitated. This deposit is then decomposed with dilute sul phuric acid, when sparingly-soluble calcium sulphate remains largely undissolved whereas the tartaric acid passes into solution, which is concentrated to the crystallising point in vacuum pans. The liquid is then cooled in tubs by stirring in order to obtain the crude tartaric acid in granular form. This product is redis solved in water and the solution decolorised by heating with animal charcoal previously freed by acid from calcium phosphate. The clear solution is concentrated and allowed to crystallise, when purified commercial tartaric acid is obtained.

Applications.

Tartaric acid is used as a mordant in wool dye ing. It is employed by the calico printer in conjunction with bleach ing powder for the liberation of chlorine, as a resist for alumina and other basic mordants and in the discharge colours for Turkey red. By successive nitration and autoxidation, tartaric acid is con verted into dinitrotartaric acid and dihydroxytartaric acid, the latter being an intermediate in the manufacture of the wool dye, tartrazine. Tartaric acid also serves in certain photographic processes for printing and developing. It is employed for the production of baking powders, Seidlitz powders, sherbet and simi lar effervescent drinks. Medicinally it is used to make saline draughts and cooling drinks for febrile and diabetic patients. If

unneutralised it must be taken in largely diluted solution other wise severe gastroenteritis may result. Lime water, magnesia and alkalis are antidotes.

Chemical Properties.

Tartaric acid is oxidised by hydrogen peroxide in presence of ferrous salts to dihydroxymaleic acid (H. J. H. Fenton, 1894) and reduced by hydriodic acid and phos phorus to malic and succinic acids. Tartaric acid prevents the precipitation by alkalis of many metallic hydroxides (e.g., copper and iron). Calcium chloride gives a white calcium tartrate in neutral solution, the precipitate being soluble in cold aqueous potash but reprecipitated on boiling. Added to concentrated sul phuric acid containing 1% of resorcinol, tartaric acid develops a violet red coloration. With warm ammoniacal silver nitrate it furnishes a silver mirror.

Stereoisomeric Tartaric Acids.—Four varieties of tartaric acid are recognised : The ordinary dextrorotatory tartaric acid found either free or as potassium and calcium tartrates in the juices of tamarinds, mulberries, pineapples, unripe beetroot and especially in grapes; (2) laevorotatory tartaric acid which does not occur in nature but is obtained by the resolution of the salts of racemic acid. Its chemical properties are identical with those of the dextro-acid and in all its physical properties it resembles this acid except that it turns the plane of polarisation of light to the left and its crystalline salts show hemihedral faces like those of the dextro-acid but oppositely situated, as object to mirror image; (3) racemic acid, (C4H606)2,2H20, which is obtained from the tartar mother-liquor, melts at 206° C and is produced syn thetically by the oxidation of fumaric acid or by mixing molecular proportions of the d- and /-acids; (4) mesotartaric acid, melting at 143° C, is obtained when cinchonine tartrate is heated for some time at 170° C, and also by the oxidation of malefic acid; it is inactive like racemic acid, but unlike the latter is not resolvable into optically active varieties. The chemical study of the optical properties of these four tartaric acids by L. Pasteur forms the foundation of our modern conceptions of stereoisomerism. (See