The determination of activities and activity coefficients from cell potentials

17 THE DETERMINATION OF ACTIVITIES AND ACTIVITY COEFFICIENTS FROM CELL POTENTIALS 

Once an accurate value of has been obtained for a cell, then the potential measurements yield values of the activity coefficients directly. Consider the cell 

According to Eq. (17.55), the potential of the cell does not depend on the individual ion activities but on the product an+ aci- As it turns out there is no measurable quantity that depends on an individual ion activity. Consequently, we replace the product h+ Uqi- by a . Since in HCl, m = m, we have a = (y+ m) this reduces Eq. (17.55) to 

Having determined by the extrapolation described in Section 17.16, we see that the 

The measurement of cell potentials is the most powerful method of obtaining values of activities of electrolytes. Experimentally it is, in many cases at least, much easier to handle than measurements of colligative properties. It has the additional advantage that it can be used over a wide range of temperatures. Although cell potentials can be measured in nonaqueous solvents, the electrode equilibria often are not as easily established so that the experimental difficulties are much greater. 

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