LEAFLET IX 



THE MEASUREMENT OF pH, TITRATABLE ACIDITY, 

 AND OXIDATION-REDUCTION POTENTIALS^ 



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 approximates the logarithm of the reciprocal of the corre- 

 sponding hydrogen ion activity. The activity of any substance is 

 virtually the product of that substance's molar concentration and a 

 factor, called the activity coefficient. 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 prac- 

 tical 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. 



The Hydrogen electrode method. This is regarded as the basic 

 experimental method whereby the various other methods are stand- 

 ardized. It consists in the measurement of the potential difference 

 (emf) established under conditions of maximum work between the 

 "hydrogen half-cell", or "hydrogen electrode", and a calomel or 

 other half-cell which is employed as a working standard. 



The hydrogen half-cell consists of a suitable vessel provided with (a) a platinum 

 foil electrode, coated with platinum-black, which is immersed or intermittently dipped 

 in the solution to be measured, and (6) an inlet and outlet for oxygen-free hydrogen to 

 saturate both solution and electrode at atmospheric pressure. 



A convenient reference half-cell is the "saturated calomel electrode" which consists 

 of a vessel containing a layer of purified mercury covered with a paste of calomel 

 (HgoCy, mercury, and saturated KCl solution; the calomel paste is layered with crys- 

 tals of KCl, and the rest of the vessel is filled with saturated KCl solution which has 

 been saturated with calomel. A platinum wire provides the electrical lead to the 

 mercury of the calomel cell, and a siphon containing saturated KCl solution provides 

 liquid junction with the solution to be measured in the hydrogen half-cell. 



In the normal hydrogen half-cell, which provides the standard of potential for all 

 measurements of potential in electrochemistry, the hydrogen partial pressure is one 



iThis 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. 



1X48-2 



