THE MEASUREMENT OF pH 



1x43-13 



TABLE 4 



DROP-RATIO COLOR STANDARDS FOR pH DETERMINATIONS 



*If a little more accuracy is desired one may use a 1 ml. pipet graduated in tenths and 

 use the specified number of tenths of a milliliter instead of drops in preparing these 

 tubes. In that case each tube should be brought up to a total volume of 10 ml. instead 

 of 5 ml. 



TABLE 5 



DATA FOR DETERMINING PH VALUE BY THE DROP-RATIO METHOD 



Data as to stock solutions 



*If the standards are prepared by the method suggested in the footnote to Table 4 

 (i. e., measuring the indicator in tenths of 1 ml. and diluting to 10 ml.) it is well to use 

 N/10 instead of N 20 NaOH to assure proper strength. The exact concentration or 

 the exact number of drops used is of no great importance. 



fUse approximately N/20 HCl (or N/10 if the method is modified as indicated in 

 the footnote to Table 4) except in the case of cresol red and thymol blue. In the case of 

 these two indicators a weaker acid must be used. Gillespie recommends 2 percent 

 H2KPO41 or in the case of thymol blue no acid need be used, water alone having a suffi- 

 ciently high pH value to bring out the full acid color. 



