THE ORGANIC CONSTITUENTS OF THE URINE. 251 



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 been abandoned, as it does not furnish 

 reliable results and in its place the method of Folin may be recom- 

 mended. 



FOLIN'S METHOD. This is based upon the precipitation of uric 

 acid by means of ammonium sulphate as ammonium urate, the de- 

 composition of the latter by means of sulphuric acid and its titration 

 with potassium permanganate. To precipitate the uric acid, and also 

 to remove the small amount of mucoid substance which is found in 

 every urine, the following reagent is employed : 500 grammes of 

 ammonium sulphate, 5 grammes of uranium acetate, and 60 c.c. of a 

 10 per cent, solution of acetic acid are dissolved in 650 c.c. of water. 

 The resulting solution measures about 1000 c.c. Seventy-five c.c. of 

 the reagent are added to 300 c.c. of urine in a flask holding 500 c.c. 

 After standing for five minutes the mixture is filtered through two 

 folded filters, and thus freed from the mucoid body, which is carried 

 down with the uranium phosphate in acid solution. The filtrate is 

 divided into two portions of 125 c.c. each, which are placed in 

 beakers and treated with 5 c.c. of concentrated ammonia. After 

 stirring a little the solutions are set aside until the next day. The 

 supernatant fluid is then carefully poured off through a filter 

 (Schleicher and Schull No. 597) ; the precipitated ammonium 

 urate is collected with the aid of a small amount of a 10 per 

 cent, solution of ammonium sulphate and washed with the same 

 reagent. Traces of chlorides do not interfere with the subsequent 

 titration, and the process of filtration and washing can be completed 

 in from twenty to thirty minutes. The ammonium urate is then 



