592 THE CHEMISTRY OF THE URINE. 



acid. After cooling, the crystals of uric acid which fall may be washed 

 by decantation. 



Tests and reactions (a) The murexide test. If a small quantity of 

 uric acid be placed upon a watch glass, a little strong nitric acid, or a 

 few drops of bromine water added, and the whole taken to dryness upon 

 the water-bath, an orange-red residue is obtained which, if touched with 

 a drop of ammonia, yields a fine purple colour. If a minute quantity of 

 sodic -hydrate solution be subsequently added, the purple colour changes 

 to blue ; while, on warming the alkaline solution, all colour is discharged. 

 The water-bath should always be used for evaporation in applying this test, 

 and if the watch glass be allowed to remain on the bath for a considerable 

 time, after evaporation is complete, a red colour will develop without 

 further treatment, and the residue will dissolve to a purple solution in 

 distilled water. This is the most delicate method of applying the test. 



The residue left by the action of the nitric acid or bromine water consists 

 of various oxidation products of uric acid, amongst which is alloxantin 

 (C 5 H C N 4 8 or C 8 H 4 N 4 6 .H 2 0). This substance yields, with ammonia, 

 ammonium purpurate, which is the purple product of the test. 



(&) If uric acid be dissolved in a little caustic soda, a few drops of 

 Fehling's solution added, and the solution boiled, a yellowish precipitate 

 of cuprous oxide is obtained (cf. p. 608). 



(c) An alkaline solution of uric acid gives, on the addition of a few 

 drops of a solution of phospliomolyldic acid, a dark blue precipitate with 

 a metallic lustre, which under the microscope is seen to consist of small 

 six-sided prisms. 1 



Estimation. The methods now used for the estimation of uric acid depend 

 either upon the insolubility of its silver compound in ammoniacal solutions, or 

 upon the depression in solubility which ammonium urate undergoes in the 

 presence of other ammonium salts. Of the silver processes the Salkowski- 2 

 Ludwig 3 method is the most accurate. In this the phosphates of the urine are 

 first precipitated by the addition of an ammoniacal solution of magnesium 

 chloride, containing ammonium chloride (magnesia mixture). Without filter- 

 ing off the phosphates, a solution of ammoniacal silver nitrate is next added, 

 which gives a further precipitate of silver-magnesium urate. After standing, 

 the mixed precipitates are filtered off, washed, and treated with a solution of 

 potassium-hydrogen sulphide, which decomposes the silver compound, forming 

 silver sulphide and potassium urate. The black precipitate of the former is 

 filtered off, and the uric acid liberated in the filtrate by the addition of hydro- 

 chloric acid. It is finally separated by filtration and weighed. 



The writer 4 has modified the previous methods employed for the separation 

 of uric acid as ammonium urate in such a way that the precipitation is absol- 

 utely complete, and the results are as accurate as those of the foregoing method, 

 while much more easy to obtain. 5 The urine (100 c.c.) is saturated with 

 chloride of ammonium, and allowed to stand for two hours, when the resulting 

 ammonium urate precipitate is filtered off, washed from the filter with hot 

 water, and the uric acid liberated by warming with hydrochloric acid. 

 After standing it is filtered off, washed, and weighed. 



1 Offer, Centralbl.f. Physiol., Leipzig u. Wien, 1894, Bd. viii. S. 801. 



2 Arch. f. d. ges. Physiol., Bonn, 1872, Bd. v. S. 210. 



3 Zlschr.f. anal. Chem., Wiesbaden, 1885. Bd. xxiv. S. 637. 



4 Hopkins, Journ. Path, and Bacterial., Edin. and London, 1893, vol. i. p. 450. 



5 Cf. v Jaksch, " Klinische Diagnostik," 1896, 4th edition, S. 428, 431; Ritter, 

 Ztsc.hr. f. physiol. Chem., Strasburg, 1895, Bd. xxi. S. 288 ; Luff, Goulstonian Lectures, 

 1897, Lect. i. 



