ORGANIC ACIDS 45 



Acid potassium tartrate, COOK. CHOH . CHOH. COOH, in 

 grapes. 



Citric acid, COOH . CHgCCOHXCOOH). CHg. COOH, in 

 lemons, sour gooseberries. 



The organic acids present in unripe fruit are converted into 

 other non-acid bodies during the process of ripening. 



26. Preparation and Qualitative Examination of 

 Crystallised Oxalic Acid {Z00Y\)^2Y{^0. 



Oxalic acid can be conveniently prepared in the 

 laboratory by the oxidation of cane sugar with nitric 

 acid. 



About twenty-five grams of cane sugar are dissolved 

 in 130 c.c. water and 100 c.c. concentrated HNO3 added. 

 The mixture is gently warmed and the heating dis- 

 continued when oxidation commences, as evidenced 

 by the rapid production of brown fumes of oxides of 

 nitrogen. 



When the reaction slackens, the liquid is placed on 

 the water-bath, or gently boiled, until its volume is 

 reduced by about one-half. It is then placed aside to 

 cool and crystallise, and may with advantage be seeded 

 with a ready-formed crystal of oxalic acid. 



The oxalic acid is recrystallised from small quantities 

 of water until free from nitric acid. 



The following tests for oxalic acid should be 

 performed with the crystals : — 



(i.) On heating the crystallised acid in a dry tube, it 

 melts in its own water of crystallisation, and on further 

 heating sublimes. 



(ii.) On heating oxalic acid or an oxalate with 

 concentrated HgSO^, CO and COg are produced by 

 dehydration of the oxalic acid — 



COOH 



I =C02+C04-H20 



COOH 



