Lifetime of ion-selective electrodes based on charged ionophores

The lifetime of solvent polymeric ion-selective electrodes (ISEs) is limited by leaching of the membrane components into the sample solutions. In this article, leaching of charged ionophores is discussed. Because of the electroneutrality principle, the loss of the charged ionophore into the sample must be accompanied by parallel transport of an ion of the opposite charge sign 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 that the cotransported or ion-exchanging ions are primary or interfering ions forming complexes with the ionophore. A general theory that allows quantification of ionophore lipophilicities and a discussion of changes in the membrane composition and selectivity with time is presented. A high complex stability and high analyte concentrations diminish the rate of ionophore loss into the sample if a charged ionophore is coextracted from the membrane into the sample together with an analyte ion of opposite charge. On the other hand, if the charged ionophore has the same charge sign as the ion that it binds, a large binding constant and high analyte concentrations enhance ionophore leaching into the sample. The model is applied to interpret results for an electrically charged ionophore, for which selectivity changes as a function of the leaching time were observed and the lipophilicity was determined with potentiometric measurements. Using the lipophilicities of neutral ionophores, as described previously, and the lipophilicities of charged ionophores, as described here, a direct comparison of the expected leaching rates of charged and neutral ionophores has become possible.