Prediction of electrophoretic mobilities. 3. Effect of ionic strength in capillary zone electrophoresis

Plots of mobility versus the square root of ionic strength (I(1/2)) do not show the linear behavior predicted by Kohlrausch's law. Classical electrolyte theory states that such deviations are to be expected due to the finite size of the ions. This paper uses the Pitts equation to account for the effect of ionic size on the ionic strength dependence of mobilities in CZE. Experimental mobilities for carboxylates, phenols, and sulfonates of -1 to -6 charge in aqueous buffers ranging from 0.001 to 0.1 M ionic strength were described by μ(-) = μ(0) - Az (I(1/2)/(1 + 2.4I(1/2))), where the constant in the denominator is empirically determined. Infinite dilution mobilities (μ(0)) determined by extrapolation of mobility data to zero ionic strength based on this expression yielded excellent agreement (100.3 ± 3.3%) with literature values for 14 compounds in a variety of buffers. The Pitts equation provides a reasonable estimate of the constant A for solutes up to a charge of -5. However, this constant also depends on temperature and the nature of the buffer counterion, presumably due to ion association. Thus it is most appropriate to determine the constant A empirically for a given buffer system.