Origin of pH-glass electrode potentials and development of pNa-responsive glasses

Generally accepted theories about the origin of the pH response of glasses are based on the assumption that hydrogen ions do not cross the glass membrane. Based on our previous findings that there exist two types of hydrogen ions in glasses, one that is hydrogen bonded and mobile and the other is free of hydrogen bonding and substantially immobile, we proposed that the original presence of mobile hydrogen ions in glasses should be required for the glasses to exhibit the pH response. This proposition was experimentally proved. This indicates that any cations (M), if they are mobile in glasses, should show the pH response. The mobilities of the mobile hydrogen ions in pH-selective glasses were evaluated to be ca.10(3) times higher than those of alkali ions. Thus, the essential condition for glasses to exhibit the pNa response should be that the mobility of hydrogen ions in the glasses should be kept as low as possible compared to that of Na+ ions. This suggestion was experimentally verified for a 20Na(2)O-20Al(2)O(3)-60SiO(2) glass in which Nat ions are naturally mobile and H+ ions are much less so. (C) 2000 The Electrochemical Society. S0013-4651(99)04-084-7. All rights reserved.