PRACTICAL EXERCISES 427 



precipitated by a current of carbon dioxide. The uric acid is set 

 free by boiling the precipitate with dilute hydrochloric acid, and is 

 deposited as a colourless crystalline powder on cooling. 



(2) Qualitative Test for Uric Acid Murexide Test. A small 

 quantity of uric acid or one of its salts is heated with a little dilute 

 nitric acid. The colour of the residue left by evaporation becomes 

 yellow, and then red, and on the addition of ammonia changes to 

 deep purple-red. Potassium or sodium hydrate changes the yellow to 

 violet. The purple-red substance is murexide or ammonium purpurate, 

 which is also formed by the action of nitric acid and ammonia on 

 theobromine (dimethylxanthin), the alkaloid of cocoa, and theine or 

 caffeine (trimethylxanthin), the alkaloid of tea and coffee. 



(3) Quantitative Estimation (a) by Precipitation and Weighing. 

 Uric acid is precipitated like grains of cayenne pepper on the sides 

 and bottom of the vessel in which urine, strongly acidulated with 

 pure hydrochloric acid, is allowed to stand for forty-eight hours. 

 By collecting the crystals from a measured quantity of urine (say 

 200 c.c. with 10 c.c. hydrochloric acid added) on a small weighed 

 fii.'er, washing the precipitate on the filter with as small a quantity of 

 water as possible (not more than 30 c.c.), drying at 110 C, and 

 weighing, an estimate may be made of the amount of uric acid present 

 (Heintz). Notwithstanding that the pigment carried down with the 

 uric acid is added to the weight of the latter, this method gives 

 results somewhat too small, as a portion of the uric acid is left in 

 solution. A better method is the following : 



(b) Hopkins s Method of Estimation of Uric Acid. Add about 25 

 grammes of ammonium chloride to 100 c.c. of urine in a beaker. 

 Stir well to dissolve all the salt. Then add 2 c.c. of strong ammonia. 

 Let the mixture stand till the precipitate of ammonium urate which 

 is formed has entirely settled to the bottom and the liquid above it 

 is clear. Then filter through a small filter-paper. Wash the pre- 

 cipitate on the filter twice with saturated solution of ammonium 

 chloride from a wash-bottle. Then wash the precipitate into a 

 porcelain capsule with hot water from a wash-bottle with a fine jet, 

 unfolding the filter-paper over the capsule so as to get it all off. 

 For this purpose not more than 20 to 30 c.c. of water should 

 generally be necessary ; but if much more has been used the excess 

 should be got rid of by evaporation on a water bath. Pour about 

 i c.c. of strong hydrochloric acid into the capsule, heat to boiling, 

 and then allow the capsule to stand in the cold till the uric acid 

 crystallizes out. The time required for this is 2 to 12 hours, being 

 shorter the lower the temperature. The crystals are filtered oft 

 through a small weighed filter, the filtrate being received in a 

 graduated cylinder and measured. The crystals are then washed on 

 the filter with cold distilled water, till the washings come through 

 the filter free from chlorides, as tested by silver nitrate (p. 421). 

 The filter with the crystals is dried in the oven and weighed. To 

 the weight of the uric acid thus obtained i milligramme must be 

 added for each 15 c.c. of the filtrate (the mother liquor) collected in 

 the graduated cylinder. 



