1888.] 



Volumetric Determination of Uric Acid. 



285 



the second 25 c.c. of water were added. When estimated the two 

 fluids should show a difference equal to the quantity of salt added." 

 Results very closely corresponding to this were obtained. 



These results do not agree with results obtained by Salkowski 

 (Pfluger's 'Archiv,' vol. 5, 1872, p. 210) and Maly (Pfliiger's 

 'Archiv,' vol. 6, p. 201). Salkowski proposed a volumetric method 

 for the determination of uric acid, very similar to that proposed by 

 Dr. Haycraft ; in this he added excess of silver nitrate and estimated 

 the excess of silver present. He gave up this method, however, as 

 on examining the silver precipitate obtained from urine, after com- 

 plete precipitation of the phosphates by magnesia mixture, he found 

 that it contained magnesium as well as silver, and that the proportion 

 of magnesium to silver varied considerably in precipitates from 

 different urines, though constant for the same urine. Haycraft con- 

 siders that the presence and variation in amount of magnesium in 

 these precipitates may be due to varying quantities of magnesium 

 ammonium phosphate in them. This is, however, impossible, as the 

 phosphates were precipitated by Salkowski previously, and the urine 

 allowed to stand for twenty-four hours before filtration to ensure their 

 complete separation. Salkowski's results were confirmed by Maly, 

 who found that if in the presence of salts of calcium, magnesium, 

 potassium, and ammonium, a solution of a urate be precipitated by 

 silver nitrate, the precipitate contains these metals as urates as well as 

 silver urate. 



As a test of the accuracy of Haycraft's method, I examined samples 

 of various urines both by his method and by Salkowski's method, 

 which is universally acknowledged to be the most reliable, and the 

 accuracy of which has been proved by experimental evidence. This 

 method consists in taking 250 c.c. of urine, adding 50 c.c. of magnesia 

 mixture to precipitate phosphates, and then adding to 240 c.c. of the 

 filtrate (which are equivalent to 200 c.c. of the urine) silver nitrate 

 to precipitate the uric acid. This precipitate of silver urate is 

 decomposed by sulphuretted hydrogen after being suspended in water. 

 The liquid is then acidified, filtered hot, and evaporated to small bulk, 

 and the uric acid allowed to crystallise out. These crystals are then 

 dried and weighed. The following results were obtained : — 



z 2 



