A New Discovery\non the Potential Stability of Ion-Selective\nMembranes: The Poison from Tetrahydrofuran

Ion-selective electrodes (ISEs) have widespread use in\nthe fields\nof clinical and environmental analyses. Tetrahydrofuran (THF) is the\nmost used solvent for the preparation of modern ISEs, equipped with\nion-selective membranes (ISMs). Until now, the influence of impurities\nin THF toward potentiometric instability of ion-selective membrane\nbased ISEs was probably associated with the presence of either residual\nwater or peroxide. To address this issue, most literature recommends\nredistilling THF prior to use in the preparation of the potentiometric\nmembranes. Current study reveals that the actual THF impurity that\nis responsible for potential instability in the ISM includes products\nfrom the oxidation of THF, which contains the hydroxyl group and possibly\ncarbonyl group with a boiling point of above 200 °C. The density\nfunctional theory calculation supported pathway of the chemical reaction\nof THF oxidation, hence, the chemical structure of the uncertain impurities\nwas predicted. The underlying reason for the deteriorating potential\nstability of the ISEs is proposed as the significant hydrophilicity\nof these impurities that affect the partitioning of the ion sensing\ncomponents in the membrane, thus enhancing the leaching of the membrane\ncomponents from the membrane phase. This finding explains why redistillation\nof aged THF is advised.