Can temperature be used to tune the selectivity of membrane ion-selective electrodes?

The selectivity coefficients, KIJpot, of ion-selective electrodes (ISEs) have been fundamentally related to thermodynamic parameters, but yet, the effect of temperature on KIJpot has not been studied. We describe a new approach to fine-tune the selectivity of ISEs based on the effect of temperature on selectivity coefficients. This effect was quantified for ion exchangers as well as neutral carrier ionophores. Potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate was used as an ion exchanger in a poly(vinyl chloride) membrane. The corresponding electrode showed an inverse relationship between log KNa+,K+pot and temperature. Additionally, sodium-selective electrodes using monensin, monensin methyl ester (MME), and monensin decyl ester (MDE) were evaluated as models of electrodes based on neutral carriers. The electrode based on monensin showed an enhancement in the selectivity for sodium over potassium by half an order of magnitude as the temperature increased from 20 to 50 degrees C, while the electrodes based on MME or MDE showed a very small change in selectivity. This can be explained in light of changes in the formation constants between the ions and ionophores with temperature. The theory of the effect of temperature on selectivity for both ion exchangers and neutral carrier ionophores is also discussed.