Functions and the Definition of pH

Since logarithms are given to the base 10 it is easy to convert the p-function mentally to the actual activity to obtain an order of magnitude estimate. 

Since p-functions refer to the activity of one ion in an electrolyte solution and this solution contains at least two ions, there is a fundamental problem with its experimental measurement. Consider for example the measurement of the pH of an HCl solution using the cell 

The problem can be examined in another way using a cell whose potential is exactly defined by thermodynamics, namely the system 

H2 (1 atm) I xHCl, H2O AgCl Ag The potential of this cell is given by 

Although the concentrations of the H and CP ions are exactly equal, their activities are not necessarily equal. Furthermore, there is no way to determine the individual ion activities separately. As discussed earlier in this chapter, thermodynamic data for this cell are analyzed in terms of the mean ionic activity and mean ionic activity coefficient y . 

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