380 PHYSIOLOGICAL CHEMISTRY 



and citrates, or the feeding of base-forming foods, by decreasing the 

 acidity of the urine increases its soivent power for uric acid and de- 

 creases the liability of formation of this type of calculus. 1 



EXPERIMENTS ON URIC Aero 



1. Isolation from the Urine. Place about 200 c.c. of filtered urine in a 

 beaker, render it acid with 2-10 c.c. of concentrated hydrochloric acid, stir 

 thoroughly, and stand the vessel in a cold place for 24 hours. Examine the pig- 

 mented crystals of uric acid under the microscope and compare them with those 

 shown in Fig. 136, page 461, and PL V, opposite. 



2. Solubility. Try the solubility of pure uric acid, furnished by the in- 

 structor, in water, dilute acid and alkali and in alcohol, ether and concentrated 

 sulphuric acid. 



3. Crystalline Form of Pure Uric Acid. Place about 100 c.c. of water in a 

 small beaker, render it distinctly alkaline with potassium hydroxide solution and 



FIG. 122. PURE URIC ACID. 





add a small amount of pure uric acid, stirring continuously. Cool the solution, 

 render it distinctly acid with hydrochloric acid and allow it to stand in a cool 

 place for crystallization. Examine the crystals under the microscope and com- 

 pare them with those reproduced in Fig. 122. 



4. Murexide Test. To a small amount of pure uric acid in a small evaporating 

 dish add 2-3 drops of concentrated nitric acid. Evaporate to dryness carefully 

 on a water-bath or over a very low flame. A red or yellow residue remains which 

 turns purplish red after cooling the dish and adding a drop of very dilute am- 

 monium hydroxide. The color is due to the formation of murexide. If potas- 

 sium hydroxide is used instead of ammonium hydroxide a purplish violet coloi 

 due to the production of the potassium salt is obtained. The color disappears 

 upon warming ; with certain related bodies (purine bases) the color persists undei 

 these conditions. 



1 Blatherwick: Arch. Int. Med., 14, 409, 1914. 



