Page 157 - Analytical Electrochemistry 2d Ed - Jospeh Wang

142 POTENTIOMETRY

Such a potential arises whenever the membrane separates two solutions of different
ion activity (Figure 5-2). The ion-recognition (binding) event generates a phase
boundary potential at the membrane±sample interface. Another phase boundary
potential is developed at the inner surface of the membrane (at the membrane±®lling
solution interface). The membrane potential corresponds to the potential difference
across the membrane. The resulting potential of the ion-selective electrode, which
re¯ects the unequal distribution of the analyte ions across the boundary, is generally
monitored relative to the potential of a reference electrode. Since the potential of the
reference electrode is ®xed, and the activity of the ion in the inner solution is
constant, the measured cell potential re¯ects the potential of the ISE, and can thus be
related to the activity of the target ion in the sample solution. Ideally, the response of
the ISE should obey equation (5-3):

E ˆ K ‡ 2:303RT=z F† log a 5-3†
i i
where E is the potential, and z and a are the ionic charge and activity, respectively,
i i
of the ion. The constant K includes all sample-independent potential contributions,
which depend upon various factors (in¯uenced by the speci®c design of the ISE).
Equation (5-3) predicts that the electrode potential is proportional to the logarithm of
the activity of the ion monitored. For example, at room temperature a 59.1 mV
change in the electrode potential should result from a 10-fold change in the activity
of a monovalent ion (z ˆ 1). Similar changes in the activity of a divalent ion should
result in a 29.6 mV change of the potential. A 1 mV change in the potential

FIGURE 5-2 Membrane potential re¯ects the gradient of activity of the analyte ion in the
inner and outer (sample) solutions.