SALTS OF TARTARIC Acqn.—The commercially important salts formed by tartaric acid are three in number, known respectively as Cream of tartar, Rochelle or Seignette's salts, and Tartar emetic.
(i.) Cream of tartar.—This derives its name from the Venetian method of preparation. The im purities are first removed by repeated solutions and crystallizations of the granulated tartar. During the last solution, a certain proportion of wood ashes and white of eggs is added to the boiling liquor. This occasions effervescence, which brings the remaining impurities to the surface so that they can be removed by skimming the liquid. When allowed to cool, suddenly the surface will be covered with a thin coating of very fine crystals.
The French plan varies somewhat from the foregoing. The granulated tartar is reduced to powclor, saturated with water and placed in vats holding water at a temperature of 100°. After 2 or 3 days it will be found that the insoluble matters have subsided, and the clear liquid is then run into earthenware vessels and allowed to crystallize. Of these crystals 100 parts are dissolved in water containing 4 to 6 parts of clay and the same quantity of animal charcoal in suspension. This liquid is evaporated till the surface is covered with a film, and is then drained off into coolers. After standing for a week or more, a crop of colourless crystals will be found collected round the sidcs of the vessels, while the colouring matters, clay and charcoal, will be accumulated at the bottom. The crystals are dried and bleached in the sun. ' (ii.) Rochelle salts.—M. Guido Schnitzer takes equivalent weights of nitrate of soda and crude potashes aud heats them, with just sufficient water to produce a solution, in an iron vessel, mean while thoroughly agitating the compound. After completion of the mutual decomposition that ensues, enough milk of lime is added to the boiling liquid to conveit the carbonate into caustic soda. The solution coutaining the nitrates of potash and soda is then evaporated, to allow the
nitrate of potash to crystallize out, which it is said to do perfectly. The crystals are then drained from the soda solution and washed with water slightly acidulated with hydrochloric acid, to remove the last traces of soda. The soda solution is then boiled in a copper vessel, with a sufficiency of rough tartar to make a neutral solution. Some tartrate of lime will be deposited, and may be washed and used subsequently in the process of making tartaric acid. When filtered from this deposit, the solution is evaporated for the crystals.
(iii.) Tartar emetic.—This salt consists of white, transparent, inodorous, rhombic octahedral crystals, having a slightly sweet aud metallic taste, which dissolves in 14 to 15 parts of water at 15°, and in two parts at 100°. It is represented by the formula It is obtained by boiling 3 parts of antimony teroxide with 4 parts of cream of tartar and filtering the hot solution and allowing it to crystallize.
Of the other salts formed by tartaric acid, the tartrate and ammonio-tartrate of iron are used in medicine, the chromium tartrate in calico-printing, the tartrate of potash and tin in wool-dyeing, and the tartrate of potash and copper as a water-colour (Brunswick green).
Tartaric acid is used chiefly as a discharge in calico-printing, and also largely for mordanting woollen goods in conjunction with tin salts and alum. It is also frequently made to take the place of citric acid, which it closely resembles in many respects, in the manufacture of acidulated drinks. A. L.