716 PHYSIOLOGICAL CHEMISTRY. 



The exogenous uric acid is increased on a diet rich in nucleopro- 

 teins (sweetbreads) ; the endogenous uric acid, being derived mainly 

 from the muscles and the leucocytes, is increased after exercise, in 

 leukaemia, etc. 



Experiment 97. (Preparation of uric acid.) Add 100 c.c. of hydrochloric 

 acid to 1 liter of urine and set aside for a day. Collect the highly colored 

 crystals of uric acid, wash with water, transfer them to a beaker with a little 

 water, heat, and add enough sodium hydroxide to dissolve the crystals. Decol- 

 orize the solution of sodium urate with boneblack, filter while hot, acidify with 

 hydrochloric acid, and allow to crystallize. Examine the crystals microscop- 

 ically and chemically. 



Tests for uric acid. 



1. Murexide test. Place a few fragments of uric acid in a porcelain 

 dish, add a drop of nitric acid, and carefully evaporate over a flame. 

 To the dry residue add a drop of ammonia- water, which produces a 

 beautiful purplish-red color. (Plate VIII., 4.) 



To distinguish from xanthine and guanine, add a drop of caustic 

 soda, when the red changes to a deep blue color. Moisten with water 

 and evaporate to dryness, when the color disappears. With xanthine 

 or guanine the color persists. 



For the following tests use solution of sodium urate prepared by dissolving 

 uric acid in warm water with the aid of sodium carbonate. 



2. Schif's reaction. Place a drop of the solution on a piece of 

 filter-paper previously moistened with silver nitrate. A dark stain 

 is formed, due to the reduction of the silver salt. 



3. Boil with Fehling's solution. A gray precipitate is formed when 

 uric acid, a reddish precipitate when the copper solution, is in excess. 

 (The reaction shows the necessity of exercising judgment in drawing 

 conclusions when testing for sugar in urine with reducing agents.) 



4. Add magnesia-mixture and then silver nitrate. Uric acid is 

 precipitated as a gelatinous magnesia-silver salt. (This reaction may 

 be used to precipitate uric acid from urine, especially in those cases 

 in which hydrochloric acid fails to precipitate the acid.) 



Quantitative estimation of uric acid. Of the many methods de- 

 scribed for this purpose, the one which is based on the separation of 

 uric acid and its subsequent titration with potassium permanganate 

 is best adapted for the needs of the physician. It is carried out 

 thus : 



Uric acid is precipitated by ammonium sulphate as ammonium 

 urate, which is filtered off and isolated. On the addition of sul- 



