The Water Uptake of Plasticized Poly(vinyl chloride) Solid‐Contact Calcium‐Selective Electrodes

A hyphenated method based on FTIR‐ATR and electrochemical impedance spectroscopy has been applied to simultaneously measure the water uptake, changes in the bulk resistance and potential of plasticized poly(vinyl chloride) (PVC) based Ca 2+ ‐selective coated‐wire (CaCWE) and solid‐contact electrodes (CaSCISEs). Most of the water uptake of the ion‐selective membranes (ISMs) used in both electrode types took place within the first 9 h in 10 −3  M CaCl 2 showing good correlation with the stabilization of the individual electrode potentials. The bulk resistance of the ISMs of the CaCWEs and the CaSCISEs with poly(3‐octylthiophene) (POT) as the solid‐contact (SC) increased most during the first 18 h in 10 −3  M CaCl 2 . The increase in the resistance was found to be related to the exchange of K + for Ca 2+ in the ISM and the formation of the Ca 2+ ‐ionophore (ETH 5234) complex having a lower diffusivity than the free K + ions. In contrary to previously published results on silicone rubber based SCISEs and poly(methyl methacrylate):poly( n ‐decyl methacrylate) membranes containing POT, the plasticized PVC‐based CaSCISEs with POT as the SC had a higher water uptake than the CaCWEs. The CaSCISEs had a detection limit of 2×10 −8  M Ca 2+ and a good potential reproducibility of 148.9±1.0 mV in 10 −4  M CaCl 2 .