THE ORGANIC CONSTITUENTS OF THE URINE. 239 



responding amount of uric acid calculated. The gravimetric method 

 is probably the most accurate. The titration method presupposes 

 that the composition of magnesium-silver urate is constant, viz., that 

 it contains one molecule of uric acid for every atom of silver ; but 

 this has not been definitely established. 



For clinical purposes, however, the titration method may be em- 

 ployed, as the error which is thus involved is probably only slight. 



METHOD. Two hundred and fifty c.c. of urine, which should 

 present a specific gravity approximating 1.020, are treated with 50 

 c.c. of ammoniacal magnesia mixture. (This is prepared by dis- 

 solving 100 grammes of magnesium chloride in water, adding a cold 

 saturated solution of ammonium chloride in excess, and enough 

 strong ammonia to impart a decided odor. If the mixture is not 

 clear, more of the ammonium chloride solution is added, and it is 

 finally diluted with water to the 1000 c.c. mark.) After filtering 

 off the phosphates, which are precipitated by the magnesium 

 mixture, 250 c.c. of the filtrate, corresponding to 200 c.c. of urine, 

 are treated with 20 c.c. of an ammoniacal 3 per cent, silver nitrate 

 solution. In this manner the uric acid, as also the xanthin bases, 

 .are precipitated as silver salts, and are allowed to settle. A few 

 cubic centimeters of the clear supernatant fluid are then examined 

 to ascertain whether a sufficient quantity of the silver solution has 

 been added. To this end, it is only necessary to acidify with con- 

 centrated nitric acid, when a precipitate of silver chloride should 

 occur. If this does not happen, the specimen is again rendered 

 alkaline with ammonia, poured back, and the entire volume treated 

 with a little more of the silver solution. 



The gelatinous precipitate is filtered off with the aid of a 

 suction-pump, washed free from chlorides and silver with weak 

 ammonia- water, and transferred to a beaker. Twenty c.c. of a 

 solution of sodium sulphide, diluted with an equal volume of water, 

 .are brought to the boiling-point, and then immediately added. 

 (The sulphide solution is prepared by dissolving 10 grammes of 

 chemically pure sodium, hydrate in 1000 c.c. of distilled water; one- 

 half of this volume is saturated with hydrogen sulphide and mixed 

 with the other half.) A little more boiling water is added to the 

 mixture, which is kept on the water-bath for a while. As soon 

 as the supernatant fluid is perfectly colorless the solution is allowed 

 to cool. The sulphide of silver is then filtered off and washed with 

 hot water. Filtrate and washings are then acidified with hydro- 

 chloric acid and evaporated to about 15 c.c. 



After adding a little more hydrochloric acid the solution is 

 allowed to stand for twenty-four hours, when the uric acid is 

 filtered off, washed with water, then with alcohol, and dried at 

 115 C. For every 10 c.c. of the mother-liquor and water used in 

 the final washing, 0.00048 gramme is finally added to the result, 

 to allow for the trifling amount of uric acid which remains in 

 .solution. 



