CH. XII.] URIC ACID. 297 



Cool thoroughly under the tap. Filter off the uric acid. Neutralise 

 the filtrate with sodium carbonate and evaporate to dryness, finish- 

 ing the process on the water-bath, to prevent the conversion of the 

 urea to biuret (see Ex. 345). Extract the residue with strong 

 alcohol or acetone. The alcohol or acetone solution is carefully 

 evaporated to dryness, and the urea crystallises out. 



359. To demonstrate the presence of uric acid in urine. 



Treat 50 cc. of urine with two drops of ammonia and then stir 

 with powdered ammonium chloride till the solution is saturated. 

 Allow the excess of ammonium chloride to settle for 15 sees., and 

 pour off into another beaker. Note the gelatinous precipitate of 

 ammonium urate. Filter : scrape the precipitate off the paper and 

 transfer it to an evaporating dish. Add three or four drops of strong 

 nitric acid and place the dish on the water-bath till a pink, dry 

 residue is obtained. Treat this with a little dilute ammonia : the 

 purple colour produced indicates the presence of urates in urine 

 (see Exs. 350 and 352). 



360. Folin's method of demonstrating the presence of 

 uric acid in urine. To i to 2 cc. (20 drops) of urine in an 

 evaporating dish add one drop of a saturated solution of oxalic acid 

 and evaporate to complete dryness on a water-bath. Allow to cool, 

 add 10 cc. of strong alcohol and allow to stand for five minutes to 

 extract the polyphenols. Carefully pour off the alcohol. To the 

 residue add 10 cc. of water and a drop or two of saturated sodium 

 carbonate. Stir to secure complete solution of the uric acid and 

 transfer to a beaker. Add i cc. of Folin's uric acid reagent (Ex. 

 354) and 20 cc. of saturated sodium carbonate solution. The blue 

 colour that results indicates the presence of uric acid. 



361. Urine has been treated with about one-fiftieth its bulk 

 of strong hydrochloric acid, and allowed to stand from twelve to 

 twenty-four hours. Note the brown crystals of uric acid that have 

 formed on the sides of the vessel. Examine them microscopically : 

 they form very irregular crystals, usually arranged in sheaves. 

 Draw the crystals. 



NOTE. The chief pigment that associates itself with uric acid and urates 

 is known as uroerythrin (see p. 278). 



