THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 83 



Table 7. Composition of Mixtures Giving pH Values at 20°C at 

 Intervals of 0.2 {Continued) 



KH2PO4, NaOH mixtures 



Boric acid, KCl, NaOH mixtures 



Notes. Overlapping members of the above series should be checked for consis- 

 tency; i.e., phthalate "5.8" to "6.2" should match phosphates of the same pH num- 

 bers when tested with bromcresol purple; likewise for phosphate and borate "7.8" and 

 "8.0" when tested with cresol red. 



According to more recent assumptions used in standardization, the pH values given 

 in the above table are too low about 0.03-0.04 unit of pH. 



containers, concentration of indicator in each of the solutions, and radiant 

 power incident upon the systems under comparison. Also, any inherent 

 color in the unknown must be compensated by an equivalent amount in 

 the optical path through the standard. These conditions are met by 

 selecting clear, unscratched tubes of uniform bore, glass thickness, and 

 color, by having the same concentrations of indicator in the unknown and 



