ION-PAIRING AND ACID DISSOCIATION-CONSTANTS IN POLY(VINYL CHLORIDE)-BASED ION-SELECTIVE ELECTRODE MEMBRANES

Ion pairing and acid dissociation constants, as well as limiting conductances, have been estimated in ion-selective electrode membranes based on a 33 wt% poly(vinyl chloride) (PVC), 66 wt% dioctyladipate matrix using the Fuoss equation of conductance for weakly dissociable species. Dissociation constants of 1.66, 3.51, and 0.69 x 10(-5) were determined for NaClO4, C6H5COOK (KBz), and benzoic acid (HBz), respectively, in a membrane matrix with 1% valinomycin and 0.01% KB(C6H5)4 present as additives. The first two values are consistent with tight ion pairing in a low dielectric solvent and indicate no special role for PVC in ion solvation. The weak acid dissociation is 4 to 5 orders of magnitude higher than expected; HBz also shows unusual interactions with the membrane matrix at low added concentrations. The effect of HBz and methylbenzoate (MeBz) on surface charge transfer resistance and membrane bulk resistance shows that HBz is dissociating to form ions, and a possible mechanism for the unusual interactions has been deduced. MeBz caused a significant increase in surface charge transfer resistance, indicating a mechanism by which membrane permeation by lipophilic, neutral components of biological samples could degrade sensor performance.