This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
An organic acid very widely diffused: it occurs in fruits partly in the free state, and partly combined with potash or lime.
From "cream of tartar" - acid tartrate of potash- which is derived from grape-juice. The process of preparation involves three distinct reactions: (it is a favorite test-question at examinations).
1. The salt having been boiled with sufficient water, prepared chalk is gradually added, and an insoluble tartrate of lime is formed and precipitates: but tartaric acid is dibasic, and the other equivalent of basic potash remains in the solution as a neutral tartrate (K2T).
2(KHC4H1O6) + CaCO3 = CaC4H1O6 + K2C4H1O6 + CO2 + H1O.
2. To precipitate this element of tartaric acid also as tartrate of lime, solution of chloride of calcium is added, giving rise to formation of chloride of potassium, and precipitating the tartrate of lime.
K2C4H1O6 + CaCl2 = CaC4H1O6 + 2KCl.
3. The tartrate of lime, having been washed, is decomposed by sulphuric acid, which precipitates an insoluble sulphate, tartaric acid being left in solution.
CaC4H1O6 + H1SO4 = H1C4H1O6 + CaSO4.
Tartaric acid occurs in fine white powder, of strongly acid taste, or in large colorless oblique rhombic prisms, which become luminous in the dark on friction. While dry, these are permanent in air, but an aqueous solution becomes mouldy on keeping, with formation of acetic acid (a change which may be prevented by the addition of some rectified spirit). A usual test for tartaric acid in solution (not too dilute) is the formation of a crystalline white precipitate of tartrate of potash on the addition of acetate of potash. Solutions neutralized by an alkali also give with chloride of calcium a white precipitate of tartrate of lime soluble in cold liquor potassae, but falling again when heated. Tartaric acid may be added to bicarbonate of potash to saturation without any precipitate, but if the bicarbonate be added to the acid, bitartrate is at once formed and precipitates (Squire).
Tartaric acid, in moderate doses, is readily absorbed, but we do not exactly know what changes it undergoes in the system. That it combines with earthy bases is probable, for Woh-ler found it in the urine (by which secretion it is eliminated) in the form of tartrate of calcium (Medical Times, ii., 1845). Dragendorff, Buchheim, and Pietrowski found only a small amount in the urine, and conclude that the greater part is oxidized in the body.
On the skin, concentrated solutions of tartaric acid produce temporary irritation and burning.