326 PHYSIOLOGICAL CHEMISTRY 



Examine microscopically. The crystals are colored dark brown 

 by urine pigment, and occur in a variety of forms, lozenge or 

 "gunboat" shapes, thick rods, etc. These crystals are not pure 

 but serve for a study of the reactions of uric acid. 

 Reactions of uric acid. 



15. Solubility. Test solubility of uric acid in ammonia, so- 

 dium hydroxide, and concentrated sulphuric acid. 



16. Murexid test. To a few crystals of uric acid in an evap- 

 orating dish add 2-3 drops cone, nitric acid and evaporate to 

 dryness. Cool the dish and add a drop of dilute ammonia. Ob- 

 serve the red or purple spot. This test is given also by xanthine. 

 To distinguish between the two substances again evaporate to 

 dryness. The color disappears if the substance is uric acid. If 

 it is xanthine, the red color persists. 



17. Boil a few crystals of uric acid with a small amount of 

 Fehling's solution. A slight reduction will occur, but it may be- 

 come apparent only by allowing the cuprous oxide to settle to 

 the bottom of the tube. This should occur inside of ten or fif- 

 teen minutes after the boiling (1/2 minute) is stopped. This 

 result should be kept in mind in testing supposed diabetic urines. 

 The amount of uric acid in normal urine is so small, however, 

 that it will not reduce Fehling's solution perceptibly. 



PURINE BASES. 



18. Preparation (Salkowski Method). To a liter of urine 

 add 200 c.c. of magnesia mixture. Filter off the precipitate, add 

 an excess of strong ammonia, and then 50-60 c.c. of 3% silver 

 nitrate. Allow the mixture to stand for an hour and filter, 

 washing the precipitate to remove excess of silver. Suspend the 

 precipitate in 400 c.c. boiling water, acidify with a few drops of 

 cone. HC1. Pass hydrogen sulphide into the hot solution as long 

 as a precipitate forms. Boil the mixture a few minutes (Hood) to 

 remove excess H,S and filter off the precipitated silver sulphide. 

 Evaporate the clear filtrate to dryness on the water bath. The 

 residue contains uric acid and other purine bases. Extract the 

 residue with boiling 3% sulphuric acid. This dissolves the xan- 

 thine bases and leaves the uric acid as a residue. Allow to stand 



