Anomalous conductivity zones in electrophoresis. II. Theory for three‐ion systems and for changes in pH

The theory developed in the previous paper is extended to cover a system containing three ions. If an “inactive” electrolyte whose ions are equally affected by a retarding medium is added to a “zone‐former” for which this is not the case, the main effect is to reduce the integrated concentration changes at the interfaces. Where a buffer contains two ions carrying charges of the same sign, the theory predicts a progressive change of pH within a zone of altered concentration. Other sources of pH change are also considered. Where a buffer includes an uncharged weak acid or base, changes in ionic concentration at an interface will lead to changes in pH. Other changes may arise from the migration of water ions, whose contributions are normally neglected. This is not justified where there are large discontinuities in conductivity. Expressions are derived for the changes in pH under different circumstances. In some cases no steady‐state is possible and there are accelerating shifts of pH at the boundaries of a zone. This could have important consequences in practical electrophoresis.