CH. XIII .1 URIC ACID. 341 



the phosphoric acid and then transfer to a narrow 25 cc. cylinder. 

 Wash out with water to make a total volume of 20 cc. Mix well and 

 estimate the creatinine by the method given in Ex. 402. 2 cc. of 

 the solution correspond to i cc. of the urine. 



F. Uric Acid. 



Most of the methods employed at present are based on one of two main 

 principles. The first is on Hopkins' ammonium chloride method ; the other 

 is colorimetric with Folin's reagent. The author's experience with all modifi- 

 cations of the latter has been so unfavourable that it has been reluctantly 

 abandoned. It is possible that the difficulty of obtaining reliable chemicals 

 accounts for many of the troubles, but the greatest care in this respect has not 

 been rewarded with success. 



Hopkins' is the standard method. It requires skill and practice to get 

 good results, but it is absolutely reliable. The modification of it introduced 

 by Folin and Schaffer is a concession to the unskilful manipulator, but it has 

 the disadvantage of an allowance of 3 mgms. for the ammonium urate not 

 precipitated by ammonium sulphate. The author humbly suggests that this 

 is an averaging of results, for comparisons with Hopkins' original method and 

 also with the modification described below, seem to indicate that with certain 

 specimens of urine the allowance should be smaller or greater than this. It is 

 always the same for a given specimen of urine, suggesting that some unknown 

 factor affects the solubility of ammonium urate under the conditions of the 

 experiment. 



It is an objection to Hopkins' method that the result cannot be obtained 

 rapidly, as the solution must be allowed to stand over-night for the whole of 

 the uric acid to crystallise out. This inconvenience is also a feature of the 

 Folin-Schaffer modification. Many attempts have been made to titrate the 

 original precipitate of ammonium urate, but they have not been very suc- 

 cessful owing to the difficulty of removing the chlorides which also titrate with 

 the permanganate in acid solution. It is generally stated that the addition of 

 manganese sulphate prevents the action of the chlorides. By accidentally 

 using magnesium sulphate on one occasion the author was led to investigate 

 carefully the extent to which the presence of chlorides interfere with a correct 

 result. It was found that moderate concentrations have no effect, owing to 

 the great rate at which uric acid is oxidised compared to the low velocity 

 of the reaction between chlorides and permanganate. As a result of a con- 

 siderable amount of work the modification described below was elaborated. 

 The final result can be obtained in i hours, and in the hands of the author 

 agrees to i mgm. per 100 cc. with that of Hopkins' original method. It has 

 been regularly used in class work for the past 4 years, and presents little 

 difficulty to the average student. But it must be admitted that it has not 

 been tested with a large number of pathological urines, and for that reason it is 

 possible that in certain cases it will only yield approximate results. There is 

 no a priori reason why it should fail more than other methods. 



406. Uric acid (Cole's modification of Hopkins' method). 



Principle. The urine is treated with colloidal iron to remove an unknown 

 substance that is precipitated by ammonium chloride. The filtrate is treated 

 with solid ammonium chloride and made strongly alkaline with ammonia. 



