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 N 4 O s or C s H 4 N 4 0,-.H 2 0). This substance yields, with ammonia, 
ammonium purpurate, which is the purple product of the test. 
(b) 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 phosphomolybdic 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 Ztschr.f. anal. Chem., Wiesbaden, 1885. Bd. xxiv. S. 637. 
4 Hopkins, Joum. Path, and Bacteriol., Edin. and London, 1893, vol. i. p. 450. 
5 Cf. v Jaksch, " Klinische Diagnostik," 1896, 4th edition, S. 428, 431 ; Bitter, 
Ztschr. f. $>hysiol. Chem., Strasburg, 1895, Bd. xxi. S. 288 ; Luff, Goulstonian Lectures, 
1897, Leet. i." 
