Lifetime of ion-selective electrodes based on charged ionophores

The lifetime of solvent polymeric ion-selective electrodes (ISEs) is limite d by leaching of the membrane components into the sample solutions. In this article, leaching of charged ionophores is discussed. Because of the elect roneutrality principle, the loss of the charged ionophore into the sample m ust be accompanied by parallel transport of an ion of the opposite charge s ign into the sample or by ion exchange with a sample ion of the same charge sign. Because ionic sites of high lipophilicity are available, the loss of ionic sites is, in general, not a concern. Therefore, it is assumed here t hat the cotransported or ion-exchanging ions are primary or interfering ion s forming complexes with the ionophore, A general theory that allows quanti fication of ionophore lipophilicities and a discussion of changes in the me mbrane composition and selectivity with time is presented. A high complex s tability and high analyte concentrations diminish the rate of ionophore los s into the sample if a charged ionophore is coextracted from the membrane i nto the sample together with an analyte ion of opposite charge. On the othe r hand, if the charged ionophore has the same charge sign as the ion that i t binds, a large binding constant and high analyte concentrations enhance i onophore leaching into the sample. The model is applied to interpret result s for an electrically charged ionophore, for which selectivity changes as a function of the leaching time were observed and the lipophilicity was dete rmined with potentiometric measurements. Using the lipophilicities of neutr al ionophores, as described previously, and the lipophilicities of charged. ionophores, as described here, a direct comparison of the expected leachin g rates of charged and neutral ionophores has become possible.