Interfering Ions
Because of the ion-exchange nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange centers of the glass and to distort the linear dependence of the measured electrode potential on pH or other electrode function. In some cases it is possible to change the electrode function from one ion to another. For example, some silicate pNa electrodes can be changed to pAg function by soaking in a silver salt solution.
Interference effects are commonly described the semiempirical Nicolsky-Eisenman equation (also known as Nikolsky-Eisenman equation), an extension to the Nernst equation. It is given by
where E is the emf, E0 the standard electrode potential, z the ionic valency including the sign, a the activity, i the ion of interest, j the interfering ions and kij is the selectivity coefficient. The smaller the selectivity coefficient, the less is the interference by j.
To see the interfering effect of Na+ to a pH-electrode:
Read more about this topic: Glass Electrode
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