Tartaric Acid, , exists free or in combination with bases in the fruits and juices of many plants. In the rowan, grape, and pine-apple it gives a subacid flavour. The commercial source is tartar, which is boiled with chalk, and the insoluble tartrate of lime is removed by straining. On treatment with sulphuric acid and purification, tartaric acid is obtained in the form of large transparent, prismatic crystals, which are readily soluble in water. They possess a pleasant acid taste, and the solution reddens litmus. When rubbed in the dark the crystals become luminous. When tartaric acid is heated it melts, forming the isomeric metatartaric acid, and by further heating it is broken up into a number of bodies, formic acid, acetone, and carbonic acid being among the number.
Besides the tartaric acid of commerce there are others, isomeric with the above, known to chemists. These differ mainly in the power of deviating the plane of Polarised Light (q.v.) to the right or the left. The ordinary acid deviates it to the right, and another deviates it to the left, so that when mixed in equal proportions and dissolved the solution has no action on polarised light at all. Such a compound exists in nature, and is known as racemic acid. It gave rise to many important researches and discoveries, owing to its similarity to tartaric acid, except in its being inactive to polarised light. To Pasteur is due the above explanation of its peculiar behaviour. Tartaric acid is largely used in dyeing and calico-printing, and in the manufacture of aerated waters and confections. Being a dibasic acid (see ACIDS), it forms a large number of salts, many of which are important. Bitartrate of potash, or cream of tartar, , is obtained by purifying the crude argol by crystallisation. It is used, along with baking soda, by housewives, as a baking powder. Medicinally it is a useful purgative, and is a favourite domestic remedy for clearing the blood in spring-time. Tartarated iron or tartrate of iron and potash is a mild tonic, which, when dissolved in sherry, constitutes iron wine. Tartrate of potash and antimony, or tartar emetic, , has been long used in medicine, in doses of to of a grain; it is a powerful sudorific, but in larger doses, 1 to 3 grains, it acts as an emetic. It has gained considerable notoriety through being employed in several famous poisoning cases which have come before the public. It should never be in unskilful hands, as grain has proved fatal to a child, and 2 grains in one case at least were fatal to an adult.