CHAPTER IV 



The Measurement of pH, Titratable Acidity, 

 and Oxidation-reduction Potentials^ 



Barnett Cohen^ 



THE MEASUREMENT OF pH 



Originally, pH was defined as the logarithm of the reciprocal of the 

 hydrogen-ion concentration. However, certain assumptions regarding 

 indeterminate factors enter the theoretical treatment of any method of 

 measuring this quantity. It is now recognized that the pH scale is 

 standardized on a basis that is arbitrary with respect to a small and 

 indeterminate uncertainty, although any pH number closely approxi- 

 mates the logarithm of the reciprocal of the corresponding hydrogen-ion 

 activity. The activity of any substance is virtually the product of that 

 substance's molar concentration and a factor called the activity coeffi- 

 cient. This factor expresses the departure from that behavior which 

 would obtain were there no van der Waals and Coulomb (attraction and 

 repulsion) forces operating. 



The common methods for the measurement of pH are of two types: 

 (1) potentiometric and (2) colorimetric. The theoretical and practical 

 aspects of the subject are treated extensively in the monograph by Clark 

 (1928). 



Potentiometric Methods 



The several potentiometric methods to be cited depend upon the fact 

 that the pH of a solution suitably incorporated in a so-called half-cell is 

 proportional to the electric potential difference established between this 

 half-cell and some reference half-cell used as a standard. 



^ This presentation is confined to the brief description of general procedures that 

 may be applied in the bacteriological laboratory. For theoretical discussions and the 

 elaboration of detail, the reader should consult the texts, monographs, and original 

 references cited. 



* Deceased. 



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