PHYSIOLOGICAL CHEMISTRY 429 



necessary. Chemically hippuric acid is benzoyl glycine C 6 H 5 . CO. NH. 

 CH 2 . COOH. It may be synthesised by the action of benzoyl chloride 

 on glycine, and decomposes to benzoic acid and glycine on heating with 

 strong hydrochloric acid. 



The presence of hippuric acid in urine is due to aromatic substances 

 in the food, which are oxidised to benzoic acid in the body and excreted 

 in combination with glycine. A vegetable diet is particularly rich in 

 these aromatic substances. Hence the large quantity of hippuric acid 

 in the urine of herbivorous animals. 



Creatinin. The chemical relationships of this body have already 

 been discussed (see Muscle, p. 352). Urine always contains creatinin. 

 On a creatinin-free diet the amount excreted per diem is remarkably 

 constant for a given individual. So much so that a determination of 

 creatinin in a urine of twenty-four hours from a person whose creatinin 

 output is known affords a test as to whether the urine has been properly 

 collected. Creatin is not normally present in urine unless large quan- 

 tities of creatin are taken in the diet. It, however, makes its appear- 

 ance in inanition and in carcinoma of the liver. 



Tests for Creatinin. 



EXPERIMENT VI. Weyl's Reaction. To five c.c. of urine are added 

 four or five drops of a very dilute solution of sodium nitro-prusside, so 

 that the original colour of the urine remains unchanged. If a weak 

 solution of caustic soda be now added drop by drop a ruby-red colour 

 results, quickly changing to yellow. If an excess of acetic acid be 

 added and the solution boiled, a greenish blue colour results, and after 

 standing some time a blue sediment (Prussian blue) settles to the 

 bottom of the tube. 



Acetone gives a similar colour with the nitro-prusside and alkali, but 

 it does not change to yellow on standing, and turns reddish purple on 

 the addition of acetic acid. 



Creatinin possesses, to a certain extent, the power of reducing 

 metallic oxides in alkaline solution, and this must be remembered as a 

 possible source of fallacy in testing for dextrose. 



EXPERIMENT VII. Jaffa's Test. Add a few drops of a saturated 

 solution of picric acid in water and a few drops of caustic soda 10 per 

 cent, solution to about 5 c.c. urine. A red colour is produced owing to 

 the formation of picramic acid. 



Estimation of Creatinin. Folin's Method. For this purpose the urine 



must be free from aceto-acetic acid and hydrogen sulphide, and must contain not 

 more than traces of acetone. Measure 10 c.c. urine with a pipette into a 500 c.c. 

 graduated flask. Add 15 c.c. saturated aqueous picric acid solution (about 1*2 

 per cent.) and 5 c.c. 10 per cent, caustic soda solution. Mix and allow to stand 



