ADVANCED CHEMICAL PHYSIOLOGY 



285 



M 



flask to the 500 c.c. mark, and mix well. By means of a Duboscq or other 

 suitable colorimeter determine the depth of liquid required to give in 

 daylight an intensity of colour exactly equal to that given by a depth of 

 8 mm. of a solution containing 24-55 gms. pure potassium bichromate 

 per litre. (If pure creatinine zinc chloride is available a better standard 

 is prepared by dissolving 1 -6106 gms. of the salt in water in a litre flask, 



N 

 add 100 c.c. -y HC1 and make up to 1,000 c.c. ; 1 c.c. contains 1 mg. 



creatinine. ) The readings of the colorimeter, of which several should be 

 taken, should be completed within twenty minutes of the dilution, as 

 the reaction liquid 

 frequently fades. 

 The zero of both 

 sides of the colori- 

 meter should be 

 tested, and it is 

 as well to test the 

 use of the colori- 

 meter by employ- 

 ing the standard 

 solution on both 

 sides before 

 determining the 

 creatinine. The 

 readings in the 

 creatinine deter- 

 mination should 

 not differ by more 

 than 0-3 mm. If 

 the average read- 

 ing is less than 5 

 mm., the urine 

 should be care- 

 fully diluted and 

 another deter- 

 mination made ; 

 if above 13 mm., 

 20 c.c. urine in- 

 stead of 10 c.c. 

 should be em- 

 ployed. 



The result of 



FIG. 197. The Duboscq Colorimeter. 



the determination is calculated from the formula 



x = 



10 x 8-1 

 a 



Where x is the quantity of creatinine in milligrammes in the volume of 

 urine employed, and a is the colorimeter reading in millimetres. The 

 amount of creatinine is inversely proportional to the colorimeter reading. 

 The formula depends on the fact that, when 10 mg. of pure creatinine 

 was employed for a determination, the colorimeter reading, against 

 8 mm. of standard bichromate, was 8-1 mm. 



To get rid of aceto-acetic acid, if present. Graham and Poulton have 

 shown that aceto-acetic acid is converted by heat into acetone which 



