484 A MANUAL OF PHYSIOLOGY 



9. Uric Acid (i) 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 hydroxide changes 

 the yellow to violet. The purple -red substance is murexide or 

 ammonium purpurate. It is also formed by the action of nitric 

 acid and ammonia on theobromine (dimethylxanthin), an alkaloid 

 in cocoa, and theine or caffeine (trimethylxanthin), an alkaloid in 

 tea and coffee, which are also purin derivatives (p. 441). 



(2) Quantitative Estimation (a] 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 precipitate 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 hydro- 

 chloric 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 off 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. 477). 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. 



Or, instead of being estimated by weighing, the uric acid crystals 

 may, after having been washed with the cold distilled water as 

 described, till free from chlorides, be washed again with hot water 

 off the filter (which need not in this case be a weighed one) into a 

 capsule. The capsule is filled up with distilled water, i c.c. of 10 per 

 cent, sodium carbonate solution added, and the contents heated to 

 boiling to dissolve the uric acid. The solution is allowed to cool, 

 and then emptied into an Erlenmeyer's flask with a mark roughly 

 corresponding to 100 c.c. The solution is made up to 100 c.c. with 

 distilled water. A burette is filled with standard potassium perman- 

 ganate solution (a twentieth-normal solution made by dissolving i -581 

 grammes of the permanganate in a litre of water). Twenty c.c. of 

 strong sulphuric acid are poured into the flask, which is then shaken. 

 The permanganate is now run in with constant shaking of the flask. 

 At first the pink colour produced where the drops of permanganate 

 fall into the liquid disappears at once without spreading through the 

 liquid. When a certain amount has been run in, however, the whole 

 liquid becomes pink, although the colour soon disappears. This 

 indicates that enough of the permanganate has been added. Each 

 c.c. of the permanganate used is equivalent to 0-00375 gramme uric 



