CH. XIII.] 



URIC ACID. 



343 



the urate is filtered off either through a plain paper, or more rapidly 

 by moderate suction through a paper and paper pulp supported on a 

 perforated plate (20 to 25 mm. diameter) resting in a funnel, as 

 shewn in the accompanying figure. The filter is prepared by cutting 

 a piece of filter paper rather larger than the disc, placing this in 

 position, moistening it and applying suction by a pump. Any 

 creases round the edge are flattened out by the point of a pencil. 

 The pressure being released 

 a little paper pulp is poured 

 on to the disc and allowed 

 to settle and then gentle 

 suction applied. The pulp 

 will completely seal the 

 cracks between the disc and 

 the funnel. It is advisable 

 to cut another circle of 

 paper, smaller than the 

 first, and to place it on the 

 centre of the pulp. It 

 prevents the latter being 

 washed away during filtra- 

 tion. Filter the supernatant 

 fluid first, taking care not 

 to disturb the bulk of 

 the precipitate. Do not 

 use too high a pressure, as this drives the amorphous ammonium 

 urate into a cake which renders the subsequent washing very slow. 

 The pressure can be regulated by use of the apparatus shewn on 

 page 74. If this is not available, a T-piece can be used, as shewn 

 in fig. 45a. One limb of this is connected to the pump and the other 

 is fitted with a piece of pressure tubing and a spring clip, by means 

 of which the pressure can be instantaneously released. When the 

 main mass of the fluid has passed through, transfer the bulk of the 

 precipitate, but do not suck quite dry. Wash out the precipitate 

 remaining in the beaker with the ammonium sulphate solution on to 

 the filter and start suction again. Do this twice more, finally 

 sucking the precipitate dry. The object of the washing is to remove 

 as much ammonium chloride as possible from the precipitate, paper 



Fig. 45A. 



