CREATININE 45 



and allow the mixture to stand for five minutes. During this 

 interval pour a little N/2 potassium bichromate solution into 

 each of the two cylinders of the colorimeter (Duboscq) and care- 

 fully adjust the depth of the solution in one of the cylinders to 

 the 8 mm. mark. A few preliminary colorimetric readings may 

 now be made with the solution in the other cylinder, in order 

 to insure greater accuracy in the subsequent examination of the 

 solution of unknown strength. Obviously the two solutions of 

 potassium bichromate are identical in color and in their examina- 

 tion no two readings should differ more than 0.1 to 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 accurate than the 

 succeeding readings. In time as one becomes proficient in the 

 technic it is perfectly safe to take the average of the first two 

 readings. 



At the end of the five-minute interval already mentioned, the 

 contents of the 500 c.c. flask are diluted to the 500 c.c. mark, 

 the bichromate solution is thoroughly rinsed out of one of the 

 cylinders and replaced with the solution thus prepared and a 

 number of colorimetric 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 unknown strength. 



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

 of pure creatinine, when brought 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 N/2 

 solution of potassium bichromate. Bearing this in mind the 

 computation is readily made by means of the following proportion 

 in which y represents the number of millimeters of the solution 

 of unknown strength equivalent to the 8 mm. of the potassium 

 bichromate solution: 



y : 8.1:: 10 : x (mg. of creatinine in the quantity of urine 

 used). 



