ELEMENTARY CHEMICAL PHYSIOLOGY 259 



Hypoxanthine and Xanthine. Uric acid is not present as such in 

 the urine, but is combined with bases, like sodium, to form urates. 

 Two types of salts are formed : (a) the primary salts, the so-called 

 mono or acid urates which are formed when uric acid in an aqueous 

 solution acts as a monobasic acid, and (6) the secondary salts, the 

 so-called di or neutral urates, which can only exist in the presence 

 of excess of alkali. The group of salts which were formerly called 

 quadri-urates is probably a mixture of uric acid with a primary 

 salt. In urine then we have only to deal with primary salts. 



Preparation and Reactions of 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 

 probably 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. 186). These are crystals of uric acid admixed with pigment. 

 They can be purified by solution in 5 per cent. KOH and repre- 

 cipitation 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 is dissolved, with the aid of 

 heat, in 5 per cent, caustic potash cooled, filtered 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 on 

 a piece of glazed porcelain, add a few drops of dilute nitric acid, 

 evaporate slowly to dryness over a small flame. A yellow residue 

 is obtained. Add after cooling 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. This test cannot be applied directly to urine as urine 

 yields a red pigment on heating with nitric acid. 



Phosphotungstic Acid Test. Dissolve a few crystals of uric acid 

 in about 2 c.c. very dilute sodium hydrate, then add 1 c.c. of Folin's 

 uric acid reagent (see p. 283) and 10 c.c. of a saturated sodium car- 

 bonate solution. An intense blue colour develops. This test cannot 

 be applied direct to urine as certain other substances, such as the 

 phenols, present also give the same blue coloration. 



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 AgNO 3 . A black 

 stain of reduced silver results. This is called Schiff's reaction. 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 



