TITRATION OF THE BICARBONATE CONTENT 



137 



In order to obtain a carbonate-free alkali solution, use is made 

 of the well-known expedient of first dissolving the NaOH in an 

 equal weight of water. Sodium carbonate is insoluble in such 

 a concentrated alkali solution and settles to the bottom. 5.5 c.c. 

 of the clear supernatant solution diluted to 5 liters yields an approx- 

 imately 0.02 N solution which is standardized by titration with 

 neutral red against 0.02 N HC1. In performing the titration it 

 is preferable to run the acid into the alkali, thus titrating from 

 the yellow alkaline color to the acid red. The color change in this 

 direction occurs without the time lag observed when alkali is 

 added to acid. 



The Standard Solutions of pH 7.2 and 7.4- The standard 

 solutions of pH 7.2 and 7.4 may be made as follows: M/20 

 solutions being obtained. The M/5 KH 2 PC>4 contains 27.23 gm. 

 of KH 2 P0 4 per liter. 



They may also be made by Sorensen's method from 

 and Na2HP04 as follows: the phosphate concentration being 



M/15. 



If crystalline Na2HPC>4 2H 2 is used instead of the anhydrous 

 Na2HP(>4, the amounts are increased to 8.66 and 9.67 gms. for 

 the solutions of pH 7.2 and 7.4 respectively. 



The plasma solution becomes somewhat turbid, but the tur- 

 bidity partially clears up as the end-point is approached, and 

 the latter can be determined within one drop of the 0.02 N NaOH. 



