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Cream of Tartar

potassium, tartrate and acid

CREAM OF TARTAR, a white, crystal line compound of tartaric acid (q.v.) and potassium. Tartaric acid is dibasic, its molecule containing two atoms of hydrogen that are replaceable by metals. Cream of tartar is the substance that is formed when only one of these typical hydrogen atoms is replaced by potassium; and it is therefore known to chem ists as hydrogen potassium tartrate, or acid potassium tartrate, to distinguish it from the normal potassium tartrate, in which both of the typical hydrogen atoms of the acid are replaced by potassium. Cream of tartar is obtained from argol (q.v.), which forms about vats or casks in which wine is undergoing fermentation. The argot is dissolved in hot water and the solution decolorized by albumin or animal charcoal, the cream of tartar being then extracted by evap oration and crystallization, and purified by recrystallization. Cream of tartar is soluble in water, but it does not dissolve as freely as the other familiar compounds of potassium. It con

stitutes the chief commercial source of tartaric acid and its compounds, and is used in medicine to some extent. The best baking powders consist of cream of tartar mixed with sodium bicarbonate in the proportion of the relative molecular weights of the two substances. These salts do not act upon each other when dry, but when they are moistened they combine to produce the normal tartrate of potassium and sodium, with the formation of water and the liberation of carbon dioxide gas. The formula of cream of tartar is KH5C40., and that of sodium bicarbonate is HNaCO2. The reaction that occurs is represented by the equation HNaCO. KH5C40e= H20 A- CO2-1- KNaH.C.O., the last formula on the right being that of the normal tartrate of sodium and potassium. The carbon dioxide gas that is liberated in the re action is retained by the dough or batter with which the baking powder is mixed and serves to make it porous, or alight.*