CHOHCOOH merle modifications; these are known as (1) Dextrotartaric acid, or Ordinary tartaric acid, (2) Lvvotartaric acid, (3) Racemic acid and (4) Mesotartaric acid.
1. Dextrotartaric Acid and some of its salts are widely distributed in the vegetable kingdom, its acid potassium salt being present in large quantities in grape juice. When grape juice is fermented, the acid salt, being only sparingly soluble in the alcohol produced, forms a dirty-white crust or sediment in the fermenta tion casks; this deposit is known as °argol° or °lees." Cream of tartar and ordinary tartaric acid are usually obtained from this source. For the preparation of cream of tartar, argol is simply freed from impurities by treatment with animal charcoal and by subsequent crystal lization. For the manufacture of ordinary tartaric acid, argol is boiled with dilute hydro chloric acid and then precipitated as calcium tartrate with calcium hydroxide. The yield is increased by adding calcium chloride to the mother-liquor. From the calcium salt thus pre cipitated tartaric acid is liberated with dilute sulphuric acid.
A number of synthetic methods are also in common use for the manufacture of tartaric acid. According to British Patent 12,467, the compound may be obtained by the elec trolytic reduction of glyoxylic acid or its derivatives; the glyoxylic acid used in this process being formed from oxalic acid by the action of amalgams. The method of Baekeland (United States Patent 1,190,845) consists in building up the acid from the oxides of carbon by converting them into formates; from these, oxalates are obtained at high tem peratures and pressures; the oxalates are re duced to glyoxylates, and the latter electrolyzed into tartrates. Tartaric acid has also been ob tained by the oxidation of carbohydrates in the presence of catalyzers (British 108,494).
Dextrotartaric acid crystallizes in trans parent prisms, free from water of crystalliza tion. It melts at 170° C.; above this tempera ture it yields pyrotartaric acid, acetone, acetaldehyde and other decomposition products. It is very soluble in water and alcohol, but is insoluble in ether. In aqueous solution it turns the plane of polarization to the right, reduces ammoniacal silver compounds, is readily de composed into glyoxal and other products in the presence of uranium salts, and yields large quantities of carbonic acid and acetaldehyde in the presence of manganese dioxide at or above C. Under the influence of ultra-violet rays tartaric acid evolves carbon dioxide. The crystals of the acid show triboluminescence. In the form of its acid potassium salt the com pound has been used by the ancients. The pure acid was first described by Scheele in 1769.
2. Lrevotartaric Acid differs from the dex tro compound in that its aqueous solutions turn the plane of polarization to the left. It was first prepared by Pasteur from sodium am monium racemate. When this salt is allowed to crystallize it deposits two sets of crystals from dextro- and lxvo-tartaric acids may be obtained. Pasteur's observations showed in cidentally that the molecule of racemic acid is a complex of dextro- and lxvo-tartaric acids. Pasteur also found that when the cinchonine salt of racemic acid is allowed to crystallize, the first crystals consist of the cinchonine derivative of lmvotartaric acid. The physical and chemi cal properties of this compound are practically those of dextrotartaric acid.
3. Racemic Acid occurs with ordinary tar taric acid in the grapes of certain districts. It may be prepared (1) by mixing strong equimolecular solutions of dextro- and lzvo-tar taric acids, (2) by the saponification of glyoxal cyanhydrin, (3) by heating dextrotartaric acid for a number of hours with caustic alkalis, and by other methods. It separates in triclinic crystals with one molecule of water. It melts at 205-206° C. It is only moderately soluble in cold water or alcohol. Solutions of racemic acid do not show optical activity, the compound being inactive by external compensation.
4. Mesotartaric Acid is formed together with racemic acid when a solution of ordinary tartaric acid is heated with caustic alkalis. It has also been prepared synthetically by heating dibromsuccinic acid with water or silver oxide. It crystallizes in long prisms with one molecule of water. The dehydrated acid melts at 140° C. It is optically inactive by internal compensation.
Of the four tartaric acids described in this article, ordinary or dextrotartaric acid is a most important compound. It is extensively employed in dyeing, bleaching, calico-printing, in the preparation of effervescent drinks, in the manufacture of baking powders, seidlitz powders, in photography, and in medicine. Sodium potassium tartrate, or Rochelle salt, forms Fehling's solution with copper sulphate and is used in urine analysis; potassium acid tartrate, or cream of tartar, is a mild cathartic; potassium antimonyl tartrate, or tartar emetic, is occasionally used as a counter-irritant in the form of an ointment, and finds extensive appli cation as a mordant; potassium magnesium tar trate is used as a purgative; iron and potas sium tartrate as a mild chalybeate; morphine tartrate has been recommended for hypodermic injection; dimethyl piperazine tartrate as a solvent for uric acid in chronic and acute gout; and paraphenetidin acid tartrate as an anti pyretic.