A new discovery on the potential stability of ion-selective membranes: the poison from tetrahydrofuran

Abstract

Ion-selective electrodes (ISEs) have widespread use in the fields of clinical and environmental analyses. Tetrahydrofuran (THF) is the most used solvent for the preparation of modern ISEs, equipped with ion-selective membranes (ISMs). Until now, the influence of impurities in THF towards potentiometric instability of ion-selective membrane based ISEs was prob-ably associated with either presence of residual water or peroxide. To address this issue, most literature recommends to re-distil THF prior to the use in preparation of the potentiometric membranes. Current study reveals that the actual THF impu-rity that is responsible for potential instability in the ISM includes products from the oxidation of THF, which contains hy-droxyl group and possibly carbonyl group with boiling point of above 200 °C. The density functional theory supported pathway of chemical reaction of THF oxidation, hence the chemical structure of the uncertain impurities was predicted. The underlying reason for the deteriorating potential stability of the ISEs is proposed as the significant hydrophilicity of these impurities that affect the partitioning of the ion sensing components in the membrane and thus enhancing the leaching of the membrane components from membrane phase. This finding explains why re-distillation of aged THF is advised.