462 PRACTICAL PHYSIOLOGY 



By analysis, therefore, we learn that uric acid contains two urea 

 molecules (which are split off at different stages of oxidation) united 

 together by a chain of carbon atoms (represented by the oxalic acid). 

 It is therefore a diureide. By Synthesis. Diureides are usually pre- 

 pared by the condensation of hydroxy acids with urea, and in the case 

 of uric acid, lactic acid is the hydroxy acid used, or still better, a 

 substitution product of it, viz., tri-chlor-lactamide. By heating urea 

 with this body the following reaction ensues : 



Cl 



HNH C1-C-C1 HN C H 



I >CO. 



CO CH(OH) HN^H 



HNH CO-NH 2 (urea) 



(urea) (trichlorlactamide) 



The groups printed in thick type unite to form uric acid (after 

 Hopkins). 



Estimation of Uric Acid. Hopkins' Method. Principle. This has 

 already been described in the elementarj^ course (p. 259). 



Reagents and Solutions necessary. 1. Pure ammonium chloride. (Must 

 be entirely soluble in water and must not contain any iron salt.) 



2. Pure ammonium sulphate. When the uric acid is to be estimated 

 by titrating it against potassium permanganate, it is absolutely 

 necessary that all traces of chlorine be removed from the ammonium 

 urate precipitate. This is done by washing it with an ammonium 

 sulphate solution (9 parts saturated solution + 1 part water). 



3. A twentieth normal solution of potassium permanganate made by 

 dissolving 1-581 gr. potassium permanganate in 1 litre of distilled 

 water. 1 c.c. of this solution = 3'75 mg. uric acid. 



Determination. Weigh out 30 grammes powdered ammon. chlor. and 

 add it to 100 c.c. of urine, stirring briskly until all the salt is dissolved. 

 Now add a few drops of strong ammonia and allow the mixture to 

 stand until the precipitate of ammonium urate has settled at the 

 bottom of the beaker, and the supernatant fluid is quite clear. Then 

 filter the supernatant fluid through a small filter, and wash the 

 beaker with a saturated ammon. chloride solution so as to remove 

 all traces of the precipitate, the washings being also passed through 

 the filter paper. Finally, wash the precipitate itself with saturated 

 ammon. chloride solution and, when the last washings have drained 

 through, puncture the apex of the filter paper with a pointed glass 

 rod, and wash the precipitate into the beaker in which the precipi- 

 tation was performed by means of a fine jet of water which has 



