We have studied water contained in the perfluorinated sulfonic acid (denoted as RSO 3 H) membrane by FT-IR spectroscopy. The IR spectra of the membrane, which absorbed water quickly even from the air, changed so drastically during dehydration that they showed clearly two isosbestic points in the 4000-1500 cm -1 region. Analysis of the IR spectral changes made it clear that water molecules exist mostly as H 3 O + in equilibrium with RSO 3and RSO 3 H in the membrane. The IR change accompanying dehydration was found to result from the reaction as RSO 3 -+ H 3 O + f RSO 3 H + H 2 O(evaporating out). The IR spectrum of H 3 O + was clearly separated, which shows a broad OH stretching absorption with two unseparated peaks around 3370 and 3120 cm -1 and an OH deformation absorption at about 1720 cm -1 . The spectrum is distinctively different from that of water separated, coexisting with H 3 O + , RSO 3 -, and RSO 3 H in the membrane, which shows OH stretching and deformation absorptions at about 3480 and 1628 cm -1 , respectively.

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