Tartaric Acid, an organic tetratomic acid, which is now regarded as belonging to a group derived from corresponding tetratomic alcohols by the substitution of oxygen for hydrogen molecules. Only one of the acids, the erythric (C4H8O5), has been actually formed, from ery-thrite (C4H10O4), but the composition and behavior of tartaric acid favors the theory that it has a similar derivation. The formula of tartaric acid is C4H606, and includes four bi-basic acids having different crystalline forms, and different properties in regard to polarized light, viz.: dextrotartaric acid, which turns the plane of polarization to the right; laevotartaric acid, which turns it to the left with equal force; paratartaric or racemic acid, which has no rotatory power, but is separable into two equal parts of right-handed and left-handed acids; and an inactive acid not thus separable.
Dextrotartaric acid is the ordinary tartaric acid found in grapes, tamarinds, pineapples, and several other fruits, usually in combination with potassium, and frequently with a small portion of calcium. The acid of commerce is prepared from tartar or argol, and was first separated from it by Scheele in 1770. The present mode of manufacture is as follows. The crude tartar is dissolved in hot water in which is stirred a little pipe clay and bone black to remove coloring matter. The filtered or decanted liquid deposits on cooling crystals of cream of tartar, from which the acid may be prepared by dissolving them in boiling water, or the original solution may be employed. Powdered chalk is added as long as there is effervescence or the liquid reddens litmus. The product consists of an insoluble tartrate of calcium and a soluble normal tartrate of potassium, which latter, after separation of the calcium salt, is mixed with an excess of chloride of potassium, which throws down the remaining tartaric acid also as tartrate of calcium. Both precipitates are washed and digested with sulphuric acid diluted with eight or ten parts of water, by which means sulphate of lime is precipitated while the tartaric acid is left free in the solution.
The filtrate is carefully evaporated to the consistency of a sirup, and placed in a warm situation to crystallize. Liebig found that tartaric acid is produced by the action of nitric acid upon milk sugar. It may also be obtained from succinic acid by submitting the latter to the action of bromine and treating one of the products, dibromosuccinic acid, with oxide of silver and water. Tartaric acid crystallizes in transparent, oblique rhombic prisms of sp. gr. 1.75, which are inodorous, permanent in the air, and easily soluble in hot and cold water and in alcohol, but insoluble in ether. The aqueous solution soon spoils, becoming covered with a fungoid growth. Tartaric acid is used in calico printing to liberate chlorine from bleaching powder, and in medicine, principally for the preparation of effervescing powders. (See Effervescence.) - Other Varieties of Tartaric Acid. The grapes cultivated in certain districts of the upper Rhine and in the Vosges contain, besides ordinary tartaric acid, an isomeric acid, called paratartaric or racemic acid, which resembles it in many particulars, but differs much in others; for instance, it is rather less soluble, and has not the power of rotating the plane of a polarized ray of light.
Pasteur has made some interesting researches upon the subject, and finds that if racemic acid is united with single bases, a salt is formed whose crystals are all identical; but if it is united with two bases, after the manner of Rochelle salt, and the solution allowed to crystallize slowly, two varieties of crystals are formed, bounded by the same number of faces, inclined to one another at exactly the same angles. They however have certain hemihedral faces which are developed on opposite sides of the two crystals, so that one crystal is like the reflected image of the other, and may be denominated morphologically right-handed and left-handed crystals. If these crystals are selected and separately recrystal-lized, each variety will produce its own particular form of crystal, and one will have right-handed and the other left-handed rotatory powers on polarized light. The acids obtained from these two varieties of crystals have also corresponding right-handed and left-handed rotatory powers, one being in fact ordinary or dextrotartaric, the other laevotartaric acid. As these two acids have equal rotatory powers in opposite directions, if their solutions are mingled in equal proportions the mixture will have no effect upon polarized light.
When concentrated solutions of the two acids are mingled, crystals of racemic acid are deposited with sensible evolution of heat. Both acids also exhibit pyro-electricity, but in opposite directions. Pasteur also found that racemic acid may be artificially produced by the action of heat upon certain compounds of tartaric acid which are capable of resisting a high temperature; for instance, when tartrate of cinchonine is exposed to a temperature of about 338° F. and afterward repeatedly boiled in water and treated with chloride of calcium, racemate of calcium is formed. Left-handed tartaric acid may in like manner be converted into racemic acid. The formation of racemic acid in these reactions is accompanied by the production of a fourth modification, which Pasteur calls inactive acid, having, like racemic, no action on polarized light, but which, unlike racemic, cannot be resolved into right-handed and left-handed acids.