250 THE URINE. 



alkaline earths are likewise soluble only with great difficulty. In 

 the urine uric acid is said to be present as a quadriurate, viz., as 

 a hyperacid compound, in which one molecule of sodium (viz., 

 potassium or ammonium) is in combination with two molecules of 

 uric acid. Its solubility in the urine is largely dependent upon the 

 amount of water, the reaction, and the presence of mineral salts and 

 possibly of pigments. On standing, however, in the absence of 

 micro-organisms, the quadriurate is decomposed, with the liberation 

 of free uric acid and acid biurates, which latter are then again trans- 

 formed into quadriurates through the agency of the diacid phos- 

 phates, and through a repetition of this process all the uric acid 

 finally separates out, as is shown in the equations : 



(1) HNa.C 5 H 2 N 4 3 .C 5 H 4 NA + H 2 O = C 5 H 4 N 4 O 3 -f HNa.C 5 H ? N 4 O 3 



Sodium quadriurate. Uric acid. Acid sodium 



biurate. 



(2) 2HNa.C 5 H 2 N 4 3 + NaH 2 PO 4 = HNa.C 5 H 2 NA.C 5 H 4 N 4 O 3 + Na 2 HPO 4 



Acid biurate. Quadriurate. 



In the urine of birds and reptiles the uric acid is said to occur 

 exclusively in the form of quadriurates. Neutral urates are not 

 found in the urine. Of the compounds which uric acid forms with 

 the salts of the heavy metals, the silver and copper salts deserve 

 especial mention, as some of the methods which are employed in the 

 quantitative estimation of the substance are dependent upon their 

 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 104 and 105). 



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 7 drops of Fehling's 

 solution. On heating, white urate 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. 



