302 PHYSIOLOGICAL CHEMISTRY 



overlook is that the rubber tubing used for connections needs to 

 be rinsed with water before being used the first time, and, later 

 also, if the tubing has been idle for any length of time. The 

 talcum powder with which the inner and outer surface of rub- 

 ber tubing is coated is probably the source of the trouble in the 

 case of new rubber tubing. It is probably contaminated with 

 ammonia. 



Determination of Preformed Creatinine. Transfer 25 (or 

 50) c.c. of a saturated solution of purified picric acid to a 

 small, clean flask, add 5 (or 10) c.c. of 10% sodium hydroxide, 

 and mix. Transfer 10 c.c. of blood filtrate to a small flask or 

 to a test tube, transfer 5 c.c. of the standard creatinine solution 

 described below to another flask, and dilute the standard to 

 20 c.c. Then add 5 c.c. of the freshly prepared alkaline picrate 

 solution to the blood filtrate, and 10 c.c. to the diluted creatinine 

 solution. Let stand for 8 to 10 minutes and make the color 

 comparison in the usual manner, never omitting first to ascer- 

 tain that the two fields of fhe colorimeter are equal when both 

 cups contain the standard creatinine picrate solution. The 

 color comparison should be completed within 15 minutes from 

 the time the alkaline picrate was added; it is therefore never 

 advisable to work with more- than three to five blood filtrates at 

 a time. 



When the amount of blcod filtrate available for the creatinine . 

 determination is too small to permit repetition it is of course 

 advantageous or necessary to start with more than one standard. 

 If a high creatinine should be encountered unexpectedly without 

 several standards ready, the determination can be saved by- 

 diluting the unknown with an appropriate amount of the alka- 

 line picrate solution using for such dilution a picrate solution 

 first diluted with two volumes of water so as to preserve equal- 

 ity between the standard and the unknown in relation to the 

 concentration of picric acid and sodium hydroxide. 



One standard creatinine solution, suitable both for creatinine 

 and for creatine determinations in blood, can be made as fol- 



