Determination of Water
Uptake of Polymeric Ion-Selective
Membranes with the Coulometric Karl Fischer and FT-IR-Attenuated Total
Reflection Techniques
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Abstract
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 Ca<sup>2+</sup>-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 CaCl<sub>2</sub>, 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 Ca<sup>2+</sup>-selective membranes in DIW,
whereas the water uptake was lower in 0.1 M electrolyte solutions.
Symmetrically contacted Ca<sup>2+</sup>-selective SR membranes had
much lower water uptake in 0.1 M CaCl<sub>2</sub> (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