512 CONSIDERATION OF CARBON COMPOUNDS. 



Dibasic and tribasic hydroxy-acids. 



Mono-hydroxy-succinic acid, or malic acid = 



/OH CH.OH.COJH 



C 4 H 6 5 or C 2 H 3 ^C0 2 H or j 



X CO 2 H CH 2 .CO 2 H 



Di-hydroxy-succinic acid, or tartaric acid = 



/OH 



//OH CH.OH.CO 2 H 



C 4 H 6 O 6 or C 2 H 2 or | 



\>C0 2 H CH.OH.C0 2 H 



\CO 2 H 



Malic acid, H 2 C 4 H 4 O 5 , occurs in the juices of many fruits, as 

 apples, currants, cherries, etc. It may be extracted from these fruits 

 or prepared synthetically. 



Tartaric acid, Acidum tartaricum, H 2 C 4 H 4 O 6 - 148.92. Fre- 

 quently found in vegetables, and especially in fruits, sometimes free, 

 generally as the potassium or calcium salt ; grapes contain it chiefly 

 as potassium acid tartrate, which is obtained in an impure state as a 

 by-product in the manufacture of wine. During the fermentation of 

 grape-juice, its sugar is converted into alcohol ; potassium acid tar- 

 trate is less soluble in alcoholic fluids than in water, and therefore is 

 deposited gradually, forming the crude tartar, or argol, of commerce, 

 a substance containing chiefly potassium acid tartrate, but also cal- 

 cium tartrate, some coloring matter, and traces of other substances. 

 Crude tartar is the source of tartaric acid and its salts. 



Tartaric acid is obtained from potassium acid tartrate by neutral- 

 izing with calcium carbonate, and decomposing the remaining neutral 

 potassium tartrate by calcium chloride : 



2(KHC 4 H 4 O 6 ) + CaCO 3 == CaC 4 H 4 O 6 + K 2 C 4 H 4 O 6 + H 2 O + CO 2 . 

 Potassium acid Calcium Calcium Potassium Water. Carbon 



tartrate. carbonate. tartrate. tartrate. dioxide. 



K 2 C 4 H 4 6 + CaCl 2 = CaC 4 H 4 6 + 2KC1. 

 Potassium Calcium Calcium Potassium 



tartrate. chloride. tartrate. chloride. 



The whole of the tartaric acid is thus converted into calcium tar- 

 trate, which is precipitated as an insoluble powder ; this is collected, 

 well washed, and decomposed by boiling with sulphuric acid, when 

 calcium sulphate is formed as an almost insoluble residue, while tar- 

 taric acid is left in solution, from which it is obtained by evaporation 

 and crystallization : 



CaC 4 H 4 O 6 + H 2 SO 4 = H,C 4 H 4 O 6 -f CaSO 4 . 



Calcium Sulphuric Tartaric Calcium 



tartrate. acid. acid. sulphate. 



