9fi Mr. F. G. Hopkins. [June 16, 



added after saturation with AmCl. The urate, plus phosphate, may 

 now be filtered off within ten minutes, and the uric acid liberated in 



the usual way. 



In describing the process above, standing for two hours is recom- 

 mended where AmCl is added alone. In a very large percentage of 

 urines, however, if not in all, one hour will be found enough. 



(c.) The final determination of the separated uric acid may, of 

 course, be carried out by means of weighing on a tared filter paper, 

 or on a Ludwig's glass-wool filter. 



Personally, however, I greatly prefer to filter the crystals through 

 a small filter of the kind sold by Schleicher and Schiill, and called by 

 them " Gehartete " filter paper. From these the uric acid may be 

 very easily washed, without loss, into a weighed glass basin. The 

 washings are then evaporated to dryness on the water-bath, the 

 residue dried at 110 C., and weighed. 



A Volumetric Method. Recent experience has convinced me of the 

 utility of permanganate solutions (vide Sutton's ' Volumetric Analysis,' 

 4th ed., p. 89) for the accurate titration of urates; so much so 

 that I believe uric acid, once separated, by any method, from ad- 

 mixture with other organic substances, may be better titrated than 

 weighed. 



At temperatures ranging round 60 C. a perfectly determinate 

 reaction occurs, involving the instantaneous decolorisation of the 

 permanganate ; and, though oxidation continues on standing greatly 

 beyond the limits of this reaction, a perfectly definite end-point is 

 obtainable, giving results which will be found extremely accurate. 



The uric acid separated, as above, from 100 c.c. of urine is, after 

 washing, dissolved by warming with a minimum of Na 2 C0 3 . The 

 solution is then cooled to about 15 C., made up to 100 c.c., trans- 

 ferred to a flask, rapidly mixed with 20 c.c. of pure strong sulphuric 

 acid, and then immediately titrated with one-twentieth normal potas- 

 sium permanganate solution. The addition of 20 per cent, of sulph- 

 uric acid produces just such a temperature as is required for the 

 reaction. The end-point is marked by the first appearance of a per- 

 manent pink flush. On standing, the colour continues to disap- 

 pear, but this slow decolorisation is in marked contrast with the 

 instantaneous nature of the first reaction. Careful experiments with 

 pure urates have convinced me that uric acid may be thus estimated 

 to within less than half a milligramme. 



One c.c. of twentieth normal permanganate of potassium, made by 

 dissolving T578 grammes of the salt in 1 litre of distilled water, is 

 equal to 0'00375 gramme uric acid. 



In the ' Guy's Hospital Reports ' for 1891 1 have described a method 

 in which the ammonium urate precipitate, obtained by saturation 

 with AmCl, is directly titrated with permanganate. As this precipi- 



