168 URIC ACID. 



dish with a few drops of strong nitric acid and evaporated carefully 

 to dryness, by preference on a water-bath. The residue thus obtained 

 will, if uric acid is present, be of a yellow or more frequently red 

 colour, which turns to a brilliant reddish purple on exposure to the 

 vapours of ammonia. On the subsequent addition of a drop of caustic 

 soda the colour is changed to a reddish blue. This disappears on 

 warming, whereas the similar colour obtained by the above process 

 from guanin does not. This is an important means of distinguishing 

 between the two substances. 



The test depends on the formation of murexid, which is the acid ammonium salt 

 of purpuric acid, the acid itself being unknown in the free state. Uric acid is 

 decomposed when heated with nitric acid, yielding alloxan and then alloxantin ; by 

 the action of ammonia the latter is converted into murexid (NH 4 ) C 8 H 4 N S 6 + H 2 0. 



The murexid test is so striking and characteristic that it suffices 

 completely for the identification of uric acid. The following tests may 

 be applied in confirmation if required, but not for the purposes of initial 

 detection. 



ii. Schiff's reaction 1 . The substance is dissolved in sodium 

 carbonate, and a drop is then placed on filter paper previously 

 moistened with nitrate of silver. A yellow or almost black coloura- 

 tion, due to the formation of metallic silver by reduction of its nitrate, 

 is at once obtained. 



iii. When a solution of uric acid in caustic soda is boiled with a 

 small amount of Fehl ing's fluid, reduction occurs with production of a 

 greyish precipitate of urate of cuprous oxide. If the copper salt is in 

 excess red cuprous oxide is obtained. 



Estimation of uric acid in solutions (urine}. The accurate quanti- 

 tative determination of uric acid is a matter of some difficulty ; for 

 details some standard works (quoted sub urea) should be consulted. It 

 will suffice to indicate here the principles of the more usually employed 

 methods. 



i. Salkowski-Ludwig method*. When an ammoniacal solution of 

 nitrate of silver is added to a solution of uric acid, to which an 

 ammoniacal mixture of magnesium chloride and ammonium chloride has 

 been previously added, the uric acid is precipitated as a magnesio-silver 

 salt. This is collected, washed and decomposed by sodium or potassium 

 hydrosulphide, whereupon the uric acid passes again into solution as a 

 urate of the alkali. On the addition of an excess of hydrochloric acid 

 to this solution the urate is decomposed, uric acid separates out and is 

 collected and weighed. 



1 Ann. d. Chem. u. Pharm. Bd. cix. (1859), S. 65. 



2 Ludwig, Wien. med. Jahrb. 1884, S. 597. Cf. Camerer, Zt. f. Biol. Bd. xxvu. 

 (1890), S. 153. 



