POLYBASIC AND HYDROXY-ACIDS. 



513 



Tartaric acid crystallizes in colorless, translucent prisms ; it has a 

 strongly acid, but not disagreeable taste ; it is readily soluble in water 

 and alcohol, and fuses at 135 C. (275 F.). 



FIG 71. 



Isomerism of tartaric acid. Four tartaric acids are known. They are : 

 dextrotartaric or common tartaric acid; favot ar tar ic acid ; mesotartaric or inact- 

 ive tartaric acid; and racemic add. These four acids have the same composi- 

 tion and show the same chemical reactions, proving that they are built up from 

 the same radicals ; but in some respects they possess different physical proper- 

 ties. Thus, mesotartaric and racemic acids are optically inactive ; the others, 

 as indicated by their names, are active, one turning polarized light to the right, 

 the other to the left. 



Pasteur first observed that the spontaneous evaporation of a solution of 

 ammonium sodium racemate yields two 

 kinds of stereo-isomeric crystals. These 

 crystals (Fig. 71) are rectangular prisms P, 

 M, T, having the lateral edges replaced by 

 the faces b' , and the intersection of these 

 faces with the face T replaced by a face h. 

 The crystals are hemihedral, having four of 

 these h faces placed alternately. In the two 

 kinds of crystals these hemihedral faces oc- 

 cupy opposite positions, so that if one kind 

 of crystal be placed before a mirror its re- 

 flection will represent the arrangement of 

 the hemihedral faces of the other kind of crystal. The crystals are called 

 right-handed and left-handed respectively. 



From these two kinds of crystals two tartaric acids can be separated ; one is 

 dextrotartaric acid, the other laevotartaric acid. When the two acids are brought 

 together in solution they unite forming racemic acid. These observations, sup- 

 ported by chemical data, have led to assume in tartaric acids the existence of 

 two asymmetric carbon atoms, about which the hydrogen atoms and the radi- 

 cals are arranged differently. Three of these forms may be represented by the 

 formulas : 



Isomeric salts of tartaric acid. 



C0 2 H 



H C OH 

 OH C II 



C0 2 H 



Dextrotartaric acid. 



CO 2 H 

 OH C H 



H 



-OH 



II- 



-C-OH 



C0 2 H 

 Laevotartaric acid. 



H O-OH 



! 

 CO,H 



Mesotartaric acid 



Racemic acid results from the combination of dextrotartaric and laevotartaric 

 acids. 



In a tenth-normal solution of tartaric acid at 25 C., 8.2 per cent, of the acid 

 is dissociated into H g and H.C^O/ ions. 

 33 



