290 CONSIDERATION OF CARBON COMPOUNDS. 



matter, and often traces of other substances. Crude tartar is 

 the source of tartaric acid and its salts. 



Tartaric acid is obtained from potassium acid tartrate by 

 neutralizing with calcium carbonate, and decomposing the re- 

 maining neutral potassium tartrate by calcium chloride : 



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



Potassium Calcium Calcium Potassium Water Carbon 



acid tartrate. carbonate. tartrate. tartrate. dioxide. 



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



Potassium Calcium Calcium Potassium 



tartrate. chloride. tartrate. chloride. 



The whole of the tartaric acid is thus converted into calcium 

 tartrate, which is precipitated as an insoluble powder ; this is 

 collected, well washed, and decomposed by boiling with sul- 

 phuric acid, when calcium sulphate is formed as an almost in- 

 soluble residue, whilst tartaric acid is left in solution, from 

 which it is obtained by evaporation and crystallization : 



CaC 4 H 4 O 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. 



There are three acids which are isomeric with common tar- 

 taric acid, differing from it in physical, but not in chemical 

 properties. These acids are known as inactive tartaric acid, 

 levotartaric acid, and racemic acid, whilst the common tartaric 

 acid is termed dextrotartaric acid. Crude tartar sometimes 

 contains racemic acid. 



Analytical reactions. 



1. Neutral solutions of tartaric acid give with calcium chlo- 

 ride a white precipitate of calcium tartrate, which, after being 

 quickly collected on a filter and washed, is soluble in potassium 

 hydrate ; from this solution calcium tartrate is reprecipitated on 

 boiling. (Calcium citrate is insoluble in potassium hydrate.) 



2. A strong solution of a tartrate, acidulated with acetic acid, 

 gives a white precipitate of potassium acid tartrate-on the addi- 

 tion of potassium acetate. (Precipitate forms slowly.) 



