THE TITRATION OF ORGANIC ACIDS IN URINE 23 



the end-point is approached, sufficient water is added to the 

 titrated solution to make its volume equal to that of the 60 c.c. 

 standard solution used in a similar tube as a color control. 



In comparing the color of the titrated solution with that in the 

 standard, it is convenient during the titration to hold the two tubes 

 side by side between the thumb and fingers, the tube containing 

 the urine being the one held nearer to the tips of the fingers where 

 it can be easily shaken as the 0.2 N acid is run in from the burette. 



Sometimes it is desirable to use a similar technique for the 

 phenolphthalein end-point also. In this case a tube of urine 

 filtrate to which no phenolphthalein is added serves as a standard. 



Calculation. From the volume of 0.2 N HC1 used to titrate 

 from the end-point of phenolphthalein to that of the tropaeolin 00, 

 the amount, usually 0.7 c.c. is subtracted which is utilized in a 

 similar titration of a control determination in which water is sub- 

 stituted for the urine. The volume of 0.2 N HC1 thus corrected 

 represents the approximate organic acid content of the urine 

 sample, plus the creatine and creatinine, and an amount of amino- 

 acids ordinarily negligible. 



In order to calculate the results in terms of c.c. of 0.1 N organic 

 acid per liter, the figure representing the c.c. of 0.2 N HC1 used in 

 the titration is multiplied by 80 (by i ff i = 40 in order to transfer 

 figure from 25 c.c. to 1000 c.c. of urine, and by 2 to change from 

 0.2 N to 0. IN terms). 



Correction for Creatinine. A 0.1 N solution of creatinine (11.32 

 mg. per c.c.) titrates in the above determination as a 0.1 N solution 

 of organic acid. Therefore, in order to correct for the creatinine, 

 the c.c. of 0.1 N organic acid per liter calculated from the above 

 titration may be diminished by 



mg. creatinine per liter urine , mg. creatinine N per liter urine 

 11.32 ~^2~ 



The simplest way is to subtract the creatinine correction 

 directly from the c.c. of 0.2 N acid used in the titration, and mul- 

 tiply the difference by 80. In this case the correction is ^o as 

 great as the above; i.e., c.c. correction = 



mg. creatinine per liter urine mg. creatinine N per liter urine 



906 336 



