TARTARIC AND CITRIC ACIDS 47 



oxide or carbonate is left in the case of metallic 

 tartrates. 



(ii.) On heating in a test-tube with concentrated 

 H2SO4, immediate charring takes place. The tartrate 

 is oxidised with production of CO and COg, whilst the 

 sulphuric acid undergoes reduction to SOg, all of which 

 three gases may be recognised by the usual tests. 



(iii.) Metallic tartrates reduce ammoniacal silver 

 oxide to the metallic state, and under suitable conditions 

 the metal is deposited in the form of a silver mirror. 



To obtain the best results this test should be carried 

 out as follows. To a neutral solution of the tartrate, 

 silver nitrate is added until a decided white crystalline 

 precipitate of silver tartrate is produced. Very dilute 

 ammonia solution is then added until the precipitate is 

 nearly dissolved, and the mixture is then heated by 

 immersing the test-tube in a beaker of boiling water. 

 Care should be taken not to move the test-tube during 

 the heating. 



(iv.) Calcium chloride solution added in small 

 quantities to a solution of tartaric acid gives no 

 precipitate, owing to the formation of soluble acid 

 calcium tartrate. On adding a further quantity of the 

 calcium salt, however, the insoluble normal tartrate is 

 precipitated. This precipitate is soluble in tartaric 

 acid and also in acetic acid. 



(v.) A small crystal of solid tartaric acid, or a 

 tartrate, and an equal quantity of pyrogallic acid are 

 warmed with i or 2 c.c. of concentrated HgSO^ in a 

 test-tube. A fine violet-blue coloration is produced 

 (distinction from citrate). 



28. Preparation of Citric Acid from Lemons. 



Citric acid occurs in lemons to an extent of about 8 per cent., 

 and can be readily prepared from the juice. 



