THE ORGANIC CONSTITUENTS OF THE URINE. 237 



formation. The salts of uric acid are readily decomposed by hydro- 

 chloric acid, and on standing the free substance crystallizes out from 

 the solution. The intimate relation which exists between uric acid 

 and the xanthin bases, as also its character as a diureid, has already 

 been considered (pages 80 and 82). 



Tests for Uric Acid. MUREXID TEST. If a few crystals of uric 

 acid are evaporated with a few drops of concentrated nitric acid on 

 a porcelain plate, a yellow or brick-red residue remains. On cool- 

 ing, a drop or two of ammonia are added, when a beautiful purple- 

 red color develops, owing to the formation of ammonium purpurate 

 (murexid). If now an excess of sodium hydrate solution is added, 

 the ammonium salt is transformed into the corresponding sodium 

 salt, and the purple red passes into a bluish violet. This disappears 

 on heating and does not return on cooling (compare with the similar 

 reaction of xanthin and guanin). 



COPPER TEST. A few crystals of uric acid are dissolved in 

 sodium hydrate solution and treated with a few drops of Fehling's 

 solution. On heating, white u rate of copper separates out. If more 

 copper solution is added, a partial reduction of the cupric oxide 

 occurs, owing to the formation of allantoin. 



DENNIGES' TEST. If uric acid is transformed into alloxan by 

 means of nitric acid, and the excess of acid is carefully evaporated, 

 a blue color results on treating the residue with a few drops of 

 concentrated sulphuric acid and commercial benzol containing 

 thiophen. 



SCHIFF'S TEST. If a piece of filter-paper is moistened with a 

 solution of nitrate of silver, and a drop of a solution of uric acid in 

 sodium carbonate is added, a brownish-black color develops, owing 

 to reduction of the oxide of silver. In the presence of only 0.002 

 milligramme of uric acid a yellow color is obtained. 



Isolation of Uric Acid. Uric acid is most conveniently prepared 

 from the excrements of snakes, in which, as has been stated, it 

 exists in the form of the quadriurate. To this end, the material is 

 boiled with a dilute solution of sodium hydrate so long as ammonia 

 is evolved, when -carbon dioxide is passed through the solution until 

 the alkaline reaction has largely disappeared. The acid biurate of 

 sodium which separates out is then washed with cold water and 

 dissolved in a dilute sodium hydrate solution. On adding an 

 excess of concentrated hydrochloric acid the uric acid crystallizes 

 out on standing. 



From human urine the substance can be obtained by adding con- 

 centrated hydrochloric acid in the proportion of 50 : 1000, and 

 keeping the mixture at a low temperature for about forty-eight 

 hours. The crystals which then separate out are treated with 

 water, dissolved in dilute sodium hydrate solution, decolorized with 

 animal charcoal, and reprecipitated with hydrochloric acid. This 

 method was formerly employed for estimating the amount of uric 

 acid in the urine, but has now been abandoned, as it does not furnish 



