204 THE JOURNAL OF PHARMACOLOGY. 



5. Method of Hopkins. 

 This method is based on the fact that a complete precipitation of uric 

 acid, in the form of ammonium urate, results, when urine is saturated with 

 ammonium chloride. The ammonium urate thus obtained may be decom- 

 posed with hydrochloric acid, uric acid separating in the form of 

 crystals. The crystals are then weighed. Or the precipitate of am- 

 monium urate may be washed with a saturated solution of ammonium 

 sulphate until free from chlorides, after which it is decomposed with 

 hydrochloric acid and the crystals so obtained dissolved in a small 

 quantity of water with the aid of sodium carbonate. The mixture, after 

 addition of concentrated sulphuric acid is titrated with N/20 potassium 

 permanganate solution. Other ammonium salts besides ammonium 

 chloride may be employed, and complete saturation is not an essential. 

 In general, the addition of 10-15 per cent, of such salts suffices. A num- 

 ber of modifications of this method are in general use. 

 Solutio7is required : 



N/20 Potassium permanganate solution which has been carefully stand- 

 ardized with N/io oxalic acid solution. 



1.578 grammes of potassium permanganate are dissolved in enough 

 distilled water to measure i liter. To standardize the solution proceed 

 as follows : 



Place 10 cubic centimeters of the N/io oxalic acid solution in a beaker, 

 add a little water and sulphuric acid and warm to 60° C. Then add the 

 potassium permanganate solution until a permanent pink color results. 

 This should require 20 cubic centimeters of the latter solution. If a lesser 

 quantity (say 19 cc.) were required, the factor of the potassium perman- 

 ganate must be calculated. 



Example : 10 cubic centimeters of N/ 10 oxalic acid solution required 19 

 cubic centimeters of the potassium permanganate solution. Then 20 ^ 19 = 

 1.0526, which represents the factor of the potassium permanganate solu- 

 tion. The number of cubic centimeters of this potassium permanganate 

 solution used in any operation must be multiplied by 1.0526. The result 

 gives the number of corresponding cubic centimeters of a N/20 potassium 

 permanganate solution. 

 Execution of method : 



(a) Modification of Ritter : 100 cubic centimeters of urine are saturated 

 with ammonium chloride, set aside for at least two hours with occasional 

 stirring. The resulting precipitate of ammonium urate is washed with a 

 saturated solution of ammonium sulphate to free from chlorides, and is 

 then washed into a flask by means of a little hot water. Sodium carbonate 

 (free from chlorides) is then added until solution is efiected. Dilute to 100 

 cubic centimeters and add 20 cubic centimeters of concentrated sulphuric 

 acid. Titrate at once with the N/20 potassium permanganate solution. 



