Comment on "A broad frequency range dielectric spectrometer for colloidal suspensions: cell design, calibration, and...

Recently, a simple analytical model was derived from the standard electrokinetic theory to deal with electrode polarization in low frequency dielectric measurements [Hollingsworth and Saville, J. Colloid Interface Sci. 257 (2003) 65-76]. Comparisons were made between dielectric constant-frequency relationships extracted from data on electrolyte solutions using the analytical model and the well-known RC-circuit analog. Electrolyte spectra interpreted with the analytical approach remained smooth (and constant) down to much lower frequencies than was the case with the RC model. It was also shown how the RC model arises naturally from the analytical model as an asymptotic expansion in inverse powers of the electrode spacing. Numerical calculations indicated substantial differences between the two models at low frequencies due to higher order terms omitted in the RC model. As it turns out, the comparison contained a numerical error. Here we revisit the methodology to show that although the two formulations disagree, they do so (in a numerical sense) only at much smaller electrode separations than those used in the aforementioned example. The purpose of this Letter is to correct the numerical error and show, explicitly, how the RC-circuit analog coefficients are related to the asymptotic expansion at low frequencies.