PHYSIOLOGICAL CHEMISTRY 425 



PREPARATION AND REACTIONS or URIC ACID. 



EXPERIMENT I. To 100 c.c. urine add 5 c.c. HC1 (cone.), and 

 allow the mixture to stand overnight. It will then be found that a 

 dark-brown sediment, like cayenne pepper, has settled down, and pro- 

 bably also that a brown scum has formed on the surface. Filter and 

 examine the sediment under the microscope. It consists of large dark- 

 brown clumps of crystals, whetstone or barrel-shaped (Fig. 245). These 

 are crystals of uric acid admixed with pigment. They can be purified 

 by solution in 5 per cent. KOH and reprecipitation by HC1. Preserve 

 the crystals for further use. 



EXPERIMENT II. Pure crystals can be obtained from the solid urine 

 of a snake or bird. This urine, which consists of sodium urate, is 

 dissolved in caustic potash and acidified with HC1. Pure uric acid 

 separates out. 



From these two experiments we learn that uric acid exists in urine 

 as a salt. If this salt be decomposed by a mineral acid the liberated 

 uric acid, being very insoluble, is precipitated. 



The following are the most important reactions of uric acid. 



EXPERIMENT III. The Murexide Test. Place some uric acid or 

 bird's urine in a capsule, add a few drops of dilute nitric acid, evaporate 

 slowly to dryness on a water-bath. A yellow residue is obtained. Add 

 a drop of ammonia, a crimson colour results, which is changed to purple 

 by adding caustic soda. If overheated, the residue will turn crimson 

 without the addition of ammonia. 



EXPERIMENT IV. Uric acid has the power of reducing metallic 

 oxides in alkaline solution. This may be demonstrated by the following 

 tests. Some uric acid is dissolved in weak sodium carbonate solution, 

 which is then poured on to a piece of filter paper moistened with a 

 solution of AgN0 3 . A black stain of reduced silver results. This is 

 called Schiffs reaction. In the presence of neutral salts, and more 

 especially of magnesium mixture (MgCl 2 , NH 4 C1, NH 3 ), the uric acid 

 and other purine bodies unite with the silver to form a double salt. 

 This salt separates out as a gelatinous precipitate, and is employed for 

 quantitatively estimating the purine bodies (Salkowski's method). Uric 

 acid can also exercise its reducing powers on cupric salts in alkaline 

 solution. By applying Trommer's test, or one of its modifications, to 

 an alkaline solution of uric acid, it will be noticed that reduction 

 ensues. The reduction precipitate is, however, of a dull brown colour 

 instead of being yellowish red, as it usually is. This is because a 

 certain amount of the cuprous oxide unites with some of the uric acid 

 to form a brown compound. 



