Determination of Water Uptake of Polymeric Ion-Selective Membranes with the Coulometric Karl Fischer and FT-IR-Attenuated Total Reflection Techniques

The water uptake of plasticized poly­(vinyl chloride) (PVC) and silicone rubber (SR) based calcium-selective membranes which are commonly used in solid-contact and coated-wire ion-selective electrodes (SC-ISEs and CWEs) was quantified with the oven based coulometric Karl Fischer (KF) technique. Two different membrane types were studied: (1) the plasticized PVC or SR (RTV 3140) membrane matrix without other added membrane components and (2) the full Ca²⁺-selective membrane formulation consisting of the membrane matrixes, potassium tetrakis­[3,5-bis­(trifluoromethyl)­phenyl]­borate and calcium ionophore IV (ETH 5234) or calcium ionophore I (ETH 1001). The membranes were contacted for 24 h either asymmetrically from one side or symmetrically from both sides with deionized water (DIW) or 0.1 M solutions of CaCl₂, KCl, or NaCl. It was found that the water uptake was higher for symmetrically contacted membranes. The highest water uptake (0.15–0.17 wt %) was obtained for the plasticized PVC based Ca²⁺-selective membranes in DIW, whereas the water uptake was lower in 0.1 M electrolyte solutions. Symmetrically contacted Ca²⁺-selective SR membranes had much lower water uptake in 0.1 M CaCl₂ (0.03 wt %) than their plasticized PVC counterparts (0.1 wt %). However, the (noncontacted) SR membranes contained initially much more water (0.09–0.15 wt %) than the PVC membranes (0.04–0.07 wt %). Furthermore, in good accordance with the KF measurements, it was verified with FT-IR-attenuated total reflection (ATR) spectroscopy that the water content at the substrate/membrane interface and consequently in the whole membrane was influenced by the electrolyte solution

Pre-Polarized Hydrophobic Conducting Polymer Solid-Contact Ion-Selective Electrodes with Improved Potential Reproducibility

Electrically conducting polymers (ECPs) are one of the most popular types of materials to interface ion-selective membranes (ISMs) with electron-conducting substrates to construct solid-contact ion-selective electrodes (SCISEs). For optimal ion-to-electron transduction and potential stability, the p-doped ECPs with low oxidation potentials such as PPy need to be generally in their conducting form along with providing a sufficiently hydrophobic interface to counteract the aqueous layer formation. The first criterion requires that the ECPs are in their oxidized state, but the high charge density of this state is detrimental for the prevention of the aqueous layer formation. We offer here a solution to this paradox by implementing a highly hydrophobic perfluorinated anion (perfluorooctanesulfonate, PFOS^–) as doping ion by which the oxidized form of the ECP becomes hydrophobic. The proof of concept is shown by using polypyrrole (PPy) films doped with PFOS^– (PPy-PFOS) as the solid contact in K⁺-selective SCISEs (K⁺-SCISE). Prior to applying the plasticized poly­(vinyl chloride) ISM, the oxidation state of the electrodeposited PPy-PFOS was adjusted by polarization to the known open-circuit potential of the solid contact in 0.1 M KCl. We show that the prepolarization results in a hydrophobic PPy-PFOS film with a water contact angle of 97 ± 5°, which effectively prevents the aqueous layer formation under the ISM. Under optimal conditions the K⁺-SCISEs had a very low standard deviation of <i>E</i>⁰ of only 501.0 ± 0.7 mV that is the best <i>E</i>⁰ reproducibility reported for ECP-based SCISEs