392 PHYSIOLOGICAL CHEMISTRY. 



X. Creatinine. 



Folin's Colorimetric Method. This method is based upon the 

 characteristic property possessed alone by creatinine, of yielding a 

 certain definite color-reaction in the presence of picric acid in 

 alkaline solution. The procedure is as follows : Place 10 c.c. of 

 urine in a 500 c.c. volumetric flask, add 15 c.c. of a saturated solu- 

 tion of picric acid and 5 c.c. of a 10 per cent solution of sodium 

 hydroxide, shake thoroughly and allow the mixture to stand for 

 5 minutes. During this interval pour a little f potassium bichro- 

 mate solution 1 into each of the two cylinders of the color- 

 imeter (Duboscq's) and carefully adjust the depth of the 

 solution in one of the cylinders to the 8 mm. mark. A few pre- 

 liminary colorimetric readings may now be made with the solution 

 in the other cylinder, in order to insure greater accuracy in the sub- 

 sequent examination of the solution of unknown strength. Obvi- 

 ously the two solutions of potassium bichromate are identical in 

 color and in their examination no two readings should differ more 

 than 0.1-0.2 mm. from the true value (8 mm.). Four or more 

 readings should be made in each case and an average taken of all 

 of them exclusive of the first reading, which is apt to be less ac- 

 curate than the succeeding readings. In time as one becomes pro- 

 ficient in the technique it is perfectly safe to take the average of 

 the first two readings. 



At the end of the 5-6 minute interval already mentioned, the con- 

 tents of the 500 c.c. flask are diluted to the 500 c.c. mark, the bichro- 

 mate solution is thoroughly rinsed out of one of the cylinders and 

 replaced with the solution thus prepared and a number of colorimet- 

 ric readings are immediately made. 



Ordinarily 10 c.c. of urine is used in the determination by this 

 method but if the content of creatinine is above 15 mg. or below 

 5 mg. the determination should be repeated with a volume of 

 urine selected according to the content of creatinine. This variation 

 in the volume of urine according to the content of creatinine. is quite 

 essential, since the method loses in accuracy when more than 15 

 mg. or less than 5 mg. of creatinine is present in the solution of 

 unknow r n strength. 



Calculation. By experiment it has been determined that 10 mg. 

 of pure creatinine, when b/ought into solution and diluted to 500 

 c.c. as explained in the above method, yields a mixture 8.1 mm. 

 of which possesses the same colorimetric value as 8 mm. of a 



1 This solution contains 24.55 grams of potassium bichromate to the liter. 



