Solvent-Independent pH and Electrode Potential Scales

The activity of hydrogen ions in a solution, a (h j, such a widely used concept, cannot be measured directly because it pertains to the theimodynanuc quantity of an individual ion. Therefore, pH =-logaof the solution mnst be determined operationally in terms of the method employed for its measurement. It is related to internationally agreed-on and defined standards. The pH is generally measured electrochemically in a cell such as  

Several operational standard buffers have been defined and their pH values assigned by the use of cells without liquid junctions according to Covington et al. and Kristensen et al. [9, 10]. The primary standard, 0.05m aqueous potassium hydrogen phthalate (KHPh) was established. One implementation of the cell nsed for assignment of the reference standard pH is  

This expression, however, involves the activity coefficient of an individual ion, that cannot be measured but is assigned conventionally (the Bates-Guggenheim convention) as  

These considerations, however, do not apply to aqneous solutions beyond the above-stated limits of pH and concentrations and to nonaqueons and mixed aqueous-nonaqueous solutions, for which the nominal hydrogen ion activity, that is, the pH, is required. Sea water is one example of aqueous solutions for which special treatment is needed and highly saline brines is another one. Standard sea water of salinity 0.35% has an ionic strength of about 0.7 m and at 25°C has, because of this, pH=pH(dilute aqueous solu-tions)+0.076 as shown by Bates [12], provided that the cell is standardized with the same electrodes against the standard buffer used in dilute aqueous solutions described above. 

As commonly practiced, cells with liquid junctions to the reference electrode, for example, a saturated calomel electrode (SCE), are employed in pH measurements. Therefore, the liquid junction potential occurring for the standardizing bnffer measurement, E., and that in a highly saline water or brine, subscript x, the pH of which is to be measured, E., must be taken into account. A term (E. -E.)(F/RTln(10)) must be added on the right-hand side of Equation 8.2, both E. terms of which are not well known but are not expected to cancel out entirely in the difference. On rearranging, we get  

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