POLYBASIC AND HYDEOXY- ACIDS. 517 



bonate and decomposing by sulphuric acid the calcium citrate thus 

 formed. (100 parts of lemons yield about 5 parts of the acid.) It 

 forms colorless crystals, easily soluble in water. 



Analytical reactions. 

 (Potassium citrate, K 3 C 6 H 5 O 7 , may be used.) 



1. Neutral solutions of citrates yield with calcium chloride on 

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

 is insoluble in potassium hydroxide, 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. Silver citrate is soluble in a solution of an alkali ci- 

 trate ; hence, sufficient silver nitrate solution must be added to obtain 

 a permanent precipitate. 



3. A solution of citrate made alkaline with a little sodium hydrox- 

 ide solution, to which a few drops of potassium permanganate solu- 

 tion are added, turns green slowly, whereas, atartrate under the same 

 conditions decolorizes permanganate quickly, with precipitation of 

 brown manganese dioxide. 



4. When ignited, it is decomposed without emitting an odor resem- 

 bling burning sugar. (Difference from tartaric acid.) 



5. Tartaric acid in citric acid may be detected by adding about 1 

 c.c. of an aqueous solution of ammonium molybdate to about 1 

 gramme of the citric acid, then 2 or 3 drops of sulphuric acid, 

 and warming on the water-bath. The presence of 0.1 per cent, or 

 more of tartaric acid gives a blue color to the solution. 



Citrates. Potassium citrate, K 3 C 6 H 5 O 7 .H 2 O, and Lithium citrate, 

 Li 3 C 6 H 5 O 7 .4H 2 O, are official. Both are white deliquescent salts, easily 

 soluble in water, and obtained by dissolving the carbonates in citric 

 acid. Sodium citrate, 2Na 3 C 6 H 5 O 7 .llH 2 O, is also official. 



The effervescent potassium citrate, lithium citrate, and magnesium sulphate are 

 granulated mixtures, all containing citric acid, tartaric acid, and sodium bicar- 

 bonate, mixed respectively with potassium citrate, lithium citrate, and mag- 

 nesium sulphate. 



The official solution of magnesium citrate is made by dissolving magnesium 

 carbonate in an excess of citric acid solution to which some syrup is added, 

 and dropping into this mixture, which should be contained in a strong bottle, 

 potassium bicarbonate. The bottle is immediately closed with a cork in order 

 to retain the liberated carbon dioxide. 



Bismuth citrate, BiC 6 H 5 O 7 , is obtained by boiling a solution of citric acid 

 with bismuth nitrate, when the latter is gradually converted into citrate, while 



