190 QUANTITATIVE ANALYSIS. 



heat just to boiling, filter, rinse out the flask with hot water, 

 and wash the precipitate a few times with hot water. 1 The 

 filtrate must be entirely clear and colorless. If it is quite 

 dark-colored, it must at once be poured back on the filter 

 before beginning the washing, and this must be repeated 

 until it runs through clear or almost clear. If only a very 

 little silver sulphide has passed through, this may be tem- 

 porarily neglected. Evaporate the filtrate first over a free 

 flame, then on the water-bath to a few cubic centimeters, add 

 about five to eight drops of hydrochloric acid, and let stand 

 till next day: the uric acid crystallizes and is usually only 

 slightly colored. 



If any silver sulphide separates during the evaporation, 

 it may be filtered off. It is advisable, however, to do this 

 at a time when the volume of the fluid has not been reduced 

 more than one-half, as otherwise loss from the precipitation 

 of the uric acid may occur. It is now necessary to deter- 

 mine the quantity of the uric acid. For this purpose dry a 

 small filter in a watch-glass apparatus or in a weighing-tube 

 (open) at 110-115, and weigh the whole apparatus (closed). 

 Bring the entire amount of the uric acid on the filter, using 

 a part of the filtrate for the rinsing. When all the uric acid 

 is on the filter, wash with a small quantity of water until a 

 portion of the filtrate ceases to give any turbidity with nitric 

 acid and silver nitrate solution. The filtrate and the wash- 

 water are collected and measured. If possible, the volume 

 of the filtrate and wash-water should not amount to more 

 than 50-60 cc. Then wash twice with absolute alcohol and 

 once with ether, place the filter in the watch-glass apparatus 

 or in the weighing-tube, dry (open) and weigh (closed). 

 The difference between the two weights is uric acid. 



In calculating the results it is usual to apply a correction 



1 It is always advisable to examine the silver sulphide under the micro- 

 scope to detect any uric acid that may be mixed with it. 



