DIBASIC A1STD TRIBASIC ORGANIC ACIDS. 293 



Of officinal scale compounds containing tartaric acid may be 

 mentioned the iartrate of iron and amm.oniam, and the tartrate of 

 iron and potassium. The first compound is obtained by dissolving 

 freshly precipitated ferric hydrate in a solution of ammonium 

 acid tartrate, the second by dissolving ferric hydrate in potas- 

 sium acid tartrate. The clear solutions, after having been suf- 

 ficiently evaporated, are dried as mentioned above on glass 

 plates. 



Citric acid, Acidum citricum, H 3 C C H 5 0-.H 2 = 210. Citric acid 

 is a tribasic acid containing three atoms of hydrogen replaceable 

 by metals; its constitution may be expressed by the graphic 

 formula : 



C 3 H 4 



^NC0 2 H 

 X C0 2 H 



Citric acid is found in the juices of many fruits (strawberry, 

 raspberry, currant, cherry, etc.), and in other parts of plants. 

 It is obtained from the juice of lemons by saturating it with 

 calcium carbonate and decomposing the calcium citrate thus 

 formed by sulphuric acid. It forms colorless crystals, easily 

 soluble in water. 



Analytical reactions. 



1. Xeutral solutions of citrates yield with calcium chloride on 

 boiling (not in the cold) a white precipitate of calcium citrate, 

 which is insoluble in potassium hydrate, but soluble in cupric 

 chloride. 



2. Neutral solutions of citrates are precipitated white by 

 silver nitrate. The precipitate does not blacken on boiling, as 

 in the case of tartrates. 



3. A neutral or alkaline solution of a citrate to which a few 

 drops of a solution of potassium permanganate have been 

 added, renders the red solution green or reddish-green, whilst 

 tartrates decolorize it. 



Citrates. Potassium citrate, K 3 C 6 H 5 O..II 2 O, and Lithium citrate, 

 Li 3 C 6 II 5 7 , are both colorless substances, easily soluble in water 

 and obtained by dissolving the carbonates in citric acid. 



