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 x 10(-8) M Ca(2+) and a good potential reproducibility of 148.9 +/- 1.0 mV in 10(-4) M CaCl(2).