Potentiometric Measurements in SECM

This approach is, of course, only possible with solid-state ISE and only with those rapidly electrochemically regenerated. Unless removed by purging, oxygen is always present in the solution and alleviates the need for addition of a redox mediator that could alter the rest potential of the antimony electrode and interfere with the pH. Moreover, the normalization of the approach curve with the limiting current in the bulk removes the need for the knowledge of the bulk concentration and diffusion coefficient of oxygen. 

In the absence of redox mediator, the method described previously is not applicable. However, an alternative approach based on conductimetry is possible when the tip does not inhibit the passage of current. An ac impedance 

5 Solution resistance vs distance curves for Ag/AgCl micropipette electrode over (a) Pt and (b) Teflon targets. Solution, 0.1 M KC1 applied voltage 50 mV between the two Ag/AgCl electrodes micropipette diameter, 20 gm (RG = 10). (+ + +) Experimental data ( ) theoretical curves. (From Ref. 77.) 

6 (a) Setup for distance measurements using Ag/AgCI micropipette elec- 

The total resistance between the probe and the secondary electrode, R xJ d), is calculated from the applied voltage and the measured current. As discussed previously, this resistance is the sum of two terms 

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