HYDROXY-, KETO- AND DIBASIC ACIDS 117 



(6) The presence of tartaric acid or tartrates in a solution prevents 

 the precipitation of metallic hydroxides by caustic soda. 



(a) No precipitate is formed if caustic soda be added to ferric 

 chloride solution containing some tartrate solution, but a yellow- 

 brown solution results. 



(b) A dark-blue solution results if caustic soda be added- to copper 

 sulphate solution containing a tartrate. 



This property is used in the preparation of Fehling's solution. 



Citric Acid. 



Citric acid is another hydroxy acid, which occurs in the free state 

 in the juices of many plants. Small quantities are present in milk. 



Preparation. 



About 5-5 per cent, of citric acid is obtainable from good lemons; 

 about I per cent, from unripe gooseberries. It is usually extracted 

 from lemons, limes and bergamot. The hot liquid is neutralised with 

 calcium carbonate, and the calcium citrate so obtained is decomposed 

 by sulphuric acid in equivalent amount. The solution on evaporation 

 gives citric acid. 



Citric acid contains 6 atoms of carbon in its molecule, 5 being 

 in a normal straight chain. Its formula is 



CH 2 .COOH 



C(OH).COOH 



CH 2 . COOH 



which shows that it is a tribasic acid. It closely resembles tartaric 

 acid, but there are many points of difference. 

 Properties. 



Citric acid is obtainable either as a crystalline powder, or in trans- 

 parent colourless prisms having the formula C 6 H 8 O 7 . H 2 O. It has a 

 strong acid taste, is very easily soluble in water, and is also soluble in 

 dilute and absolute alcohol; it is almost insoluble in ether, chloro- 

 form, petroleum ether and benzene. 



Reactions. 



(1) On heating, citric acid loses water becoming anhydrous, melts 

 and decomposes, giving off acid fumes of aconitic acid 



(2) When heated with concentrated sulphuric acid, citric acid 

 chars slowly. 



(3) Silver citrate is precipitated on adding silver nitrate to a 

 neutral solution of citric acid or a citrate. The precipitate dissolves 

 in ammonia, but the solution on warming is not reduced and does 

 not form a silver mirror. 



