DIBASIC AND TRIBASIC ACIDS, 333 



in the carboxjl. As shown by the graphic formulas of the above 

 two acids, they contain also hydrogen in the hydroxyl form not in 

 combination with CO. This hydrogen, whilst not replaceable by 

 metals, may be replaced by alcohol radicals ; in other words, it be- 

 haves like the hydroxyl hydrogen in alcohols. In order to indicate 

 this difference in the function of the hydrogen, malic acid is said to be 

 dibasic, but triatomic ; tartaric acid is dibasic and tetratomic. A few 

 other acids behave in a similar manner, as, for instance, lactic ac!d. 



Malie add, H 2 C 4 H 4 O 5 , occurs in the juices of many fruits, as 

 apples, currants, etc. 



Tartaric acid, Acidum tartaricum, H 2 C 4 H 4 O 6 = 15O. Frequently 

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

 as the potassium or calcium salt ; grapes contain it chiefly as potas- 

 sium 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 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 O 6 + CaCl 2 == CaC 4 H 4 O 6 -f 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 6 + H 2 S0 4 = H 2 C 4 H 4 O 6 + CaSO 4 . 

 Calcium Sulphuric Tartaric Calcium 



tartrate. acid. acid. sulphate. 



Tartaric acid crystallizes in colorless, transparent 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.). 



