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 s how the linear behavior predicted by Kohlrausch's law. Classical electrolyt e theory states that such deviations are to be expected due to the finite s ize of the ions. This paper uses the Pitts equation to account for the effe ct of ionic size on the ionic strength dependence of mobilities in CZE. Exp erimental 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 d escribed by mu- = mu(0) - Az (I-1/2/(1 + 2.4I(1/2))), where the constant in the denominator is empirically determined. Infinite dilution mobilities (m u(0)) determined by extrapolation of mobility data to zero ionic strength b ased on this expression yielded excellent agreement (100.3 +/- 3.3%) with:l iterature values for 14 compounds in a variety of buffers. The Pitts equati on provides a reasonable estimate of the constant A for solutes up to a cha rge of -5. However, this constant also depends on temperature and the natur e of the buffer counterion, presumably due to ion association. Thus it is m ost appropriate to determine the constant A empirically for a given buffer system.