592 THE URINE [CH. XXXIX. 



formation of purpurate of ammonia. On the addition of potash the 

 colour becomes bluer. 



Another reaction that uric acid undergoes (though it is not 

 applicable as a test) is, that on treatment with certain oxidising 

 reagents urea and oxalic acid can be obtained from it. Alloxan 

 (C 4 H 2 N 2 4 ) or allantoin (C^^Og) are intermediate products. It 

 is, however, doubtful whether a similar oxidation occurs in the 

 normal metabolic processes of the body. 



Uric acid is dibasic, and thus there are two classes of urates 

 the normal urates and the acid urates. A normal urate is one in 

 which two atoms of the hydrogen are replaced by two of a monad 

 metal such as sodium ; an acid urate is one in which only one atom of 

 hydrogen is thus replaced. The formulae would be 



C 5 H 4 N 4 3 = uric acid. 

 C 5 H 3 NaISr 4 3 = acid sodium urate. 

 C 5 H 2 Na 2 N 4 3 = normal sodium urate. 



The acid sodium urate is the chief constituent of the pinkish deposit 

 of urates, which often occurs in urine, and is called the lateritious 

 deposit. 



If uric acid is represented by H.,U, the normal urates may be represented by 

 M 2 U, and the acid urates by MHU. Bence Jones, and later Sir W. Roberts, 

 considered that the urates actually occurring in urine are what are termed quadri- 

 urates MHU.H 2 U. There is no doubt that such compounds do not really exist; 

 they are merely mixtures of acid urate, MHU, and free uric acid, H 2 U. 



The quantity of uric acid excreted by an adult varies from 7 to 

 10 grains (0'5 to 0'7o gramme) daily. 



The best method for estimating uric acid in urine is that of 

 Hopkins (Folin's modification). The urine is made alkaline with 

 ammonia and saturated with ammonium sulphate. This precipitates 

 all the uric acid as ammonium urate; the precipitate is collected on 

 a filter, and washed free from chlorides with a 10 per cent, solution 

 of ammonium sulphate ; it is then rinsed into a flask with hot water ; 

 when cold, sulphuric acid is added. It is then titrated at 60 C. 

 with a standard solution of potassium permanganate until a perma- 

 nent rose colour is obtained. From the amount of the permanganate 

 solution used to obtain the end point, the quantity of uric acid 

 present is calculated. Uric acid may also be determined colori- 

 metrically by the blue colour it strikes with phospho-tungstic acid 

 (Folin and A. B. Macallum, jua). 



Origin of Uric Acid. Uric acid is not made by the kidneys; 

 when these organs are removed, uric acid continues to be formed, 

 and accumulates in the organs, especially in the liver and the spleen. 

 After extirpation of the liver in birds (in which animals uric acid is 

 such an important katabolite), the formation of uric acid practically 

 ceases, and its place is taken by ammonia and lactic acid, and the 



