238 THE URINE. 



reliable results and in its place the method of Hopkins or of 

 Ludwig-Salkowski is now almost exclusively used (see below). 



Quantitative Estimation. HOPKINS' METHOD. This method 

 furnishes results which are as accurate as those obtained with the 

 older method of Ludwig-Salkowski, and has, above all, the advan- 

 tage of greater simplicity. It is based upon the fact that uric acid 

 can be completely precipitated from the urine by the addition of cer- 

 tain ammonium salts. Insoluble acid ammonium urate thus results, 

 which is transformed into the free acid, and this estimated either 

 gravimetrically or by titration with a solution of potassium per- 

 manganate of known strength. According to Folin's most recent 

 modification of the original method, we may proceed as follows : 



FOLIN'S METHOD. To precipitate the uric acid, and also to re- 

 move 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 Schiill 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 

 washed into a beaker, after opening the filter, using about 100 c.c. 

 of water. Fifteen c.c. of concentrated sulphuric acid are then added 

 and the solution is titrated at once with a one-twentieth normal solu- 

 tion of potassium permanganate. Toward the end of the titration 

 Folin suggests to add the permanganate in portions of two drops at 

 a time, until tine first trace of rose is apparent throughout the entire 

 fluid. Each cubic centimeter of the reagent corresponds to 0.00375 

 gramme of uric acid. A final correction of 0.003 gramme for every 

 100 c.c. of urine employed is necessary, owing to the slight degree 

 to which ammonium urate is soluble. 



LUDWIG-SALKOWSKI METHOD. Tin's is based upon the forma- 

 tion of insoluble magnesium silver urate when urine is treated with 

 ammoniacal magnesia mixture and subsequently with an ammoniacal 

 solution of silver nitrate, while the chlorides remain in solution. 

 The double salt is then decomposed and the uric acid obtained as 

 such, or the amount of silver is ascertained by titration and the cor- 



