TARTARIC ACID, An acid com pound of carbon, hydrogen, and oxygen, of which four different modifications are known. Ordinary tartaric acid is usually seen in the form of color less, transparent crystals, which are not affected by the action of the air, have an agreeable acid taste, and are soluble in water and alcohol. The crystals when gently warmed become strongly electric, the opposite sides of the crystals ex hibiting the opposite forms of electricity. On heating tartaric acid to about 167-170° C. it fuses; and at slightly higher temperatures it be comes changed into metatartaric, tartralic, and tartrelic acids.
Tartaric acid occurs abundantly in the vege table kingdom both in the free and combined states. It is, however, from argol (q.v.). a prod uct of the fermentation of grape juice, that the tartaric acid of commerce is obtained. Argol, or crude tartar, is boiled with water and hydro chloric acid, and the solution is precipitated with lime. The insoluble tartrate of calcium tints obtained is purified by washing with water. then decomposed by treating with sulphuric acid. This transforms the tartrate into the sparingly soluble calcium sulphate; white tartaric acid goes into solution in the free state. The filtered :s liquid, when cooled and evaporated, yields crys talline tartaric acid.
Being a dibasic acid, tartaric acid can form both acid and neutral salts. The most important tartrates are the following: Neutral potassium tartrate, K,C,II,O,„ a soluble salt, which crystal lines with half a molecule of water. Acid potas sium tartrate, or bitartrate of potash, KlIC,H,0„ is prepared from argol by extraction with boiling water and filtering the solution thus obtained through charcoal. The salt crystallizes readily as the hot solution cools. The snowy white rhombic prisms which are thus deposited constitute cream of tartar, which is moderately soluble in cold water and but sparingly soluble in alcohol. It is an excellent saline purgative and diuretic and is therefore largely used in medicine. Calcium tartrate, 4H,0, is practically insoluble in cold water and but sparingly soluble in hot water. The insolubility of the acid tartrate of potassium and of the tar trate of calcium often helps to identify tartaric acid. Tartar emetic is described in a special article.
Tartaric acid has been known since the earliest times, in the form of tartar; Scheele was the first to obtain the acid in the free state and to determine its principal properties (1769), Like any other acid, tartaric acid promotes the secretion of saliva, and may therefore be used to allay thirst. When taken internally it is de composed in the blood with formation of alkaline carbonates, which cause an increase of the alka linity of urine.
A remarkable modification of tartaric acid is known as racemic or paratartaric acid, 211,0. It is a frequent associate of tartaric acid, but is especially abundant in the grapes of the Vosges district. While in most respects it ex hibits a close resemblance to tartaric acid, it crystallizes more readily from solution; contains two equivalents of water of crystallization; is less soluble in alcohol; and the racemate of lime is soluble in hydrochloric acid, and is precipitat ed unchanged on adding ammonia, Its most important difference, however, is that its solu tion does not rotate the plane of polarized light, while a solution of ordinary tartaric acid exerts a well-marked right-handed rotation. Pasteur proved that racemic acid is a mixture of ordinary tartaric acid (to which, from its optical prop erty, he applies the term deetro-raeemic acid) and of an acid which produces left-handed rota tion, to which he gives the name hero-racemic acid. (These acids are also known as dextro tartaric and hero-tartaric acids). He found that, by saturating raeemie acid with soda and ammonia, and allowing this solution to crystal lize slowly, two varieties of crystals are obtained, which may be distinguished by their form, in the same way as the image and the reflection of the image in a mirror differ, or as right-handed and left-handed. If the two kinds of crystals are separated, and then dissolved, each solution is found to act powerfully on polarized light, but in opposite directions. On separating these acids from their bases, and mixing equal parts of concentrated solutions of each, racemic acid is again formed. which exerts no action on a polar ized ray.
Besides the three modifications mentioned above, viz., ordinary o• dextro-tartaric acid, lfevo-tartaric acid, and racemic acid, a fourth modification of the same chemical composition and constitution, known as meso-tartaric, anti tartaric, or inactive tartaric acid, has been ob tained. It may be prepared by heating, in a sealed glass tube, 30 grams of ordinary tartaric acid with 3 or 4 cubic centimeters of water, at 165° C., for about 48 hours. In the anhydrous state it melts at about 140° C.; usually, how ever, it is obtained in the form of crystals having t he composition HP. It is very soluble in water. Its acid potassium salt, too, is very soluble in water, and its calcium salt crystallizes with 3 molecules of water: CaC,11,0, +3H,0. The different modifications of tartaric acid have also been prepared synthetically. Their relation to one another has been explained stereo-chemically. See STEREO-CHEMISTRY.