Potentiometric sensors diffusion potential

More recently, in the determination of cyanide with a metallic silver-wire electrode Frenzel ei al.[38] have shown that relatively non-selective potentiometric sensors may be used to make selective determinations by enhancing the selectivity using gas-diffusion separations. Potential interferences from sulfide, sulfite and nitrite which may form interfering gaseous species were removed by a pre-oxidation with permanganate and dichromate. 

If more than a single type of ion contributes to the measured potential in Equation 5.4, the potential can no longer be used to quantify the ions of interest. This is the interference in a potentiometric sensor. Thus, in many cases, the surface of the active electrode often incorporates a specific functional membrane which maybe ion-selective, ion-permeable, or have ion-exchange properties. These membranes tend to selectivity permit the ions of interest to diffuse or migrate through. This minimizes the ionic interference. 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

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