120 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



practical exercises is the principal test for uric acid. The test has 

 received the name on account of the resemblance of the colour to the 

 purple of the ancients, which was obtained from certain snails of the 

 genus Murex. 



Another reaction that uric acid undergoes (though it is not appli- 

 cable as a test) is that on treatment with certain oxidising reagents 

 urea and oxalic acid can be obtained from it. It is, however, doubtful 

 whether a similar oxidation occurs in the normal metabolic processes 

 of the body (see p. 113). 



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 like 

 sodium ; an acid urate is one in which only one atom of hydrogen is 

 thus replaced. The formulae would be — 



C5H4N403=uric acid 

 C5H3NaN403:=acid sodium urate 

 C5H2Na2N403=normal sodium urate 



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

 of urates, which, as we have already stated, is called the lateritious 

 deposit. 



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

 10 grains (0*5 to 0*75 gramme) daily. 



The best method for determining the quantity of uric acid in the 

 urine is that of Hopkins. Ammonium chloride in crystals is added 

 to the urine until no more will dissolve. This saturation completely 

 precipitates all the uric acid in the form of ammonium urate. After 

 standing for two hours the precipitate is collected on a filter, washed 

 with saturated solution of ammonium chloride, and then dissolved in 

 weak alkali. From this solution the uric acid is precipitated by 

 neutralising with hydrochloric acid. The precipitate of uric acid is 

 collected on a weighed filter, dried and weighed, or titration may be 

 performed with potassium permanganate (see Advanced Course). 



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

 When the kidneys are removed uric acid continues to be formed and 

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

 liver has been removed from birds, and uric acid is then hardly formed 

 at all, its place being taken by ammonia and lactic acid. It is there- 

 fore probable that ammonia and lactic acid are normally synthesised 

 in the liver to form uric acid. 



The principal conditions which lead to an increase of uric acid in 

 the urine are — 



