282 



PHYSIOLOGICAL CHEMISTRY. 



This acid occurs normally in the urine of both the carnivora and 

 herbivora but is more abundant in the urine of the latter. It is 

 formed by a synthesis of benzoic acid and glycocoll which takes 

 place in the kidneys. The average excretion of an adult man 

 for 24 hours under normal conditions is about 0.7 gram. Hippuric 



FIG. 92. 



ACID. 



acid crystallizes in needles or rhombic prisms (see Fig. 92, above) 

 the particular form depending upon the rapidity of crystallization. 

 Pure hippuric acid melts at 187 C. The most satisfactory method 

 for the isolation of hippuric acid from the urine in crystalline 

 form is that proposed by Roaf (see p. 283). It is easily soluble 

 in alcohol or hot water, and only slightly soluble in ether. The 

 output of hippuric acid is increased in diabetes owing probably to the 

 ingestion of much protein and fruit. It is decreased in fevers 

 and in certain kidney disorders where the synthetic activity of the 

 renal cells is diminished. Hippuric acid may be determined quan- 

 titatively by means of Dakin's methods (see p. 383). 



EXPERIMENTS ON HIPPURIC ACID. 



i. Separation from the Urine, (a) First Method. Render 500- 

 looo c.c. of urine of the horse or cow 1 alkaline with milk of lime, 



1 If urine of the horse or cow is not available human urine may serve the 

 purpose fully as well provided means are taken to increase it's content of hippuric 

 acid. This may be conveniently accomplished by ingesting 2 grams of ammonium 

 benzoate at night. The fraction of urine passed in the morning will be found 

 to have a high content of hippuric acid. The ammonium benzoate is in no way 

 harmful. 



