Measurements of pH with a glass electrode that reveal a strong dependence on the ion pair of the background electrolyte and on the salt concentration are presented. Although such phenomena are well known, they are inexplicable in the classical theory based on the Nernst equation. It is shown that this (Hofmeister) effect can be understood once neglected ionic dispersion potentials acting between ions and the glass-water interface are taken into account. At high concentrations, those of interest in biology (greater than or equal to 0.1 M), co-ions, are shown to play a profound and previously overlooked role in pH changes near glass-water interfaces.