Measurement of h-ion concentration 



451 



The pH values have been calibrated with solutions of known 

 H. I. C. prepared according to directions given by Clark and Lubs 

 (1917) and cross checked where the indicators overlap. 



Keeping qualities of the " standard colors." The standard color 

 solutions have been kept for three months by observing the fol- 



TABLE 3 



Composition of "Standard Colors" prepared with brom thymol blue 0.02 per cent 

 watery solution — range pH 6.4 to 7.6* 



* The useful range of color of this indicator, according to calibration with 

 phosphate NaOH mixture, should begin with pH 6.2, instead of pH 6, as given 

 by Clark and Lubs. 



TABLE 4 



Composition of "standard colors" prepared with phenol red 0.04 V er cen ^ watery 

 solution — range pH 7 to pH 8.2 



Yellow 



Lighter yellow 

 Pink yellow 

 More pink yellow 

 Beginning red 

 Slightly more red 

 Red 



pH 



7.0 



7.2 

 7.4 

 7.6 



7.S 



s.o 



8.2 



lowing rules in their preparation. New test tubes were washed 

 in warm water (hot water produced a coating on the inside of 

 some of the tubes) dried and filled with the respective w NaOH 

 and 0.1 per cent HCl freshly prepared with fresh distilled water. 

 The tubes were then set up and the indicator solutions added, as 



