Optimal frequency range for the measurement of A.C. conductivity in aqueous solutions

Abstract

The measurement of electrolytic conductivity is widely applied as a control parameter for different types of aqueous solutions. In order to obtain absolute methods this measurement has recently undergone a critical revision, several criteria have been established defining the experimental chain leading to conductivity values traceable to the SI units. In this work the attention is focussed, under a.c. regime, on the electrical behaviour versus frequency of the cell-solution system, which is represented through a lumped parameter model comprehending parasitic effects. Conductivity has been measured using an a.c.-bridge in the frequency range 12 Hz-200 kHz on solutions with very different ionic content: from ultrapure water to 0.1 m KCl. Results have been compared under the prospect of the reported electrical model. The model correctly represents the electrical behaviour versus frequency of the samples. It has been observed that stray effects at the electrode-solution interface and in the bulk solution differently affect the measurement when dilution is varied. A frequency range can be identified where parasitic effects are minimised and the measured electrical properties confidently can be attributed to the solution ionic content. This is valid for any measurement conditions even if when dilution level increases the valid range decreases in width and drifts towards low frequency values.

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