CH. XII.] URIC ACID. 295 



Uric acid can assume a great variety of crystalline forms, resembling 

 dumb-bells, whetstones, butcher-trays, stars, and sheaves. 



350. To another portion of the solution add two drops of 

 ammonia and saturate with ammonium chloride. A white amor- 

 phous precipitate of ammonium urate is formed. 



NOTE. This is the basis of Hopkins' original method for the estimation of 

 urates in urine. It is an important reaction for separating urates from physio- 

 logical fluids, such as urine (see Ex. 359), since no other organic substance, 

 likely to be met with in physiological analysis, is precipitated by saturation 

 with ammonium chloride. The murexide reaction can be applied to the 

 precipitate obtained. 



351. Treat a little uric acid with a little strong sulphuric acid : 

 it dissolves. Pour the solution into water: the uric acid may 

 separate out. 



352. Murexide test. Treat a little uric acid in a porcelain 

 dish with two or three drops of strong nitric acid. Heat on the 

 water bath till every trace of nitric acid and water has been re- 

 moved. A reddish deposit remains. Treat this with a dilute 

 solution of ammonia (five drops of ammonia to about a test-tube 

 full of water). The residue turns reddish- violet in colour. Add 

 a little caustic soda. The colour turns to a blue- violet. 



NOTES. i. This important test needs a certain amount of care. The 

 heating must be performed on the water-bath, and should be continued as 

 long as is necessary to ensure the complete removal of every trace of nitric acid. 



2. Xan thine and guanine give a yellow substance (nitro-xanthine) when 

 treated with nitric acid. On evaporation the colour goes to a violet shade, 

 which turns yellow with dilute ammonia. Adenine and hypoxanthine give no 

 colour reaction. 



3. The chemistry of the reaction is as follows : From uric acid arises 

 by oxidation dialuric acid and alloxan. They condense together to form 

 alloxantin. By the action of ammonia on alloxantin, purpuric acid is formed. 

 Murexide is ammonium purpurate, 



HN-CO HN-CO HN-CO OC-NH 



I I H |i | I OH | | 



OC C-^ + OC CO = OC C^i C CO 



! I X OH || II" HO^- | I 



HN-CO HN-CO HN-CO OC-NH 



Dialuric acid Alloxan. Alloxantin. 



__ NH OC-NH 



I I "J> 

 Alloxantin + NH 8 



HN-CO OC-NH 



Purpuric acid. 



