Quantitative enrichment of trace levels of ions by electrodialysis

The effect of ionic strength on the rate of ion transport across ion-exchange membranes under either constant potential or constant current conditions in electrodialysis precludes direct application of this method to quantitative enrichment studies. Addition of an excess of electrolyte to a sample to normalize the ionic strength permits either dialysis mode to be employed. The rate of ion transport into a receiver electrolyte becomes directly proportional to concentration over at least 2.5 orders-of-magnitude. The concentration range for which linear enrichment is achieved for cations extends down to the 10^-7 M level with 30-min dialyses into a mixed 0.1 M MgCl₂—0.1 M HCl receiver. Alternatively, an internal standard approach can be used; however, the difficulty in matching the behaviour of membrane transport numbers as a function of ionic strength can limit the application of this approach.