554 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Uric Acid. 



In 1892 it was shown by Hopkins that uric acid could be com- 

 pletely precipitated from urine as ammonium urate by saturating the 

 urine with ammonium chloride. Its amount was estimated by convert- 

 ing this into uric acid by the action of hydrochloric acid, dissolving the 

 latter in sodium carbonate and titrating with standard permanganate 

 solution. Very accurate results are obtained by Folin and Schaffer's 

 method, which is in reality a shortened Hopkins' method. It is the 

 one most commonly employed. 



For this method the following reagents are required : 



(1) Uranium acetate mixture (see p. 614). 



(2) 10 per cent, ammonium sulphate solution. 



(3) '05 N potassium permanganate solution made by dissolving 

 1*581 gm. pure potassium permanganate in I litre of water (l c.c. = 

 0*00375 gm. uric acid). 



Procedure. 



200 c.c. of urine are measured out with a pipette into a 500 c.c. 

 flask and 50 c.c. of the uranium acetate mixture are added. The 

 solutions are mixed and allowed to stand for about half an hour so as 

 to allow the precipitate to settle. This precipitate contains a mucoid 

 substance which, if not thus removed, renders the subsequent filtration 

 and washing of the ammonium urate precipitate very slow. The 

 supernatant liquid is filtered off through a dry filter into a dry vessel ; 

 125 c.c. (= 100 c.c. urine) of this are measured out with pipettes into 

 a beaker ; 5 cc. of concentrated ammonia are added and mixed and 

 it is allowed to stand covered with paper for 12-24 hours. 



The supernatant liquid is carefully decanted upon a filter, the pre- 

 cipitate of ammonium urate is washed on to the filter with 10 per cent, 

 ammonium sulphate solution and washed once or twice with the same 

 reagent to remove the chlorides as completely as possible. 



The filter is removed from the funnel, opened and with a fine stream 

 of water the ammonium urate precipitate is washed into a beaker or 

 flask. To the ammonium urate precipitate, suspended in about 100 

 c.c. of water, 15 c.c. of concentrated sulphuric acid are added and it is 

 titrated at once without cooling with -05 N potassium permanganate 

 until a pink colour is first seen throughout the solution (cf. p. 292). 



The result is calculated as follows : 



i c.c. 'c-sN KMnO 4 = 0*00375 gm. uric acid, 

 .*. x c.c. = x x 0*00375 gm. uric acid, 



but since ammonium urate is slightly soluble a correction of 3 mgm. 

 for every 100 c.c. of urine used must be added. The result is : 



100 c.c. urine contain x x "00375 + -003 gm. uric acid, from which 

 the amount in the 24 hours' quantity is calculated. 



