Limiting non-Nernstian calibrations of ion-selective electrodes

Abstract

The relationship between the e.m.f., E, of an ion-selective electrode and the concentration, C, of determinand ceases to be logarithmic when C is small and eventually a limit is reached in which the relationship is linear; E=q+rC. The values of q and r are derived theoretically for the cases where the cause of the deviation from the logarithmic relationship is (i) the solubility of a salt incorporated in the electrode itself or (ii) the presence of determinand in reagents added to the sample solution. In case (i), q=E⁰+klnH and r=yk/(x+y)H, where H=[(x/y)^yK]^1/(x+y), k=RT/zF being the Nernst slope factor and K the solubility product of the salt A_xB_y incorporated in the A-selective electrode. Allowance can be made for the effect of a complexing agent on the solubility. In case (ii), when the reagent blank, b, is large compared with both the determinand concentration and the solubility of the electrode components, q=E⁰+klnb and r=k/b.