How Does One Measure the Dielectric Constant of Ionic Solutions

How Does One Measure the Dielectric Constant of Ionic Solutions  

There is much to think about here. If one wishes to measure the dielectric constant of a liquid, not a conducting ionic solution, one simply uses an alternating current (ac) bridge containing a capacitor in one of the arms. Then the capacitance is measured in the presence of the liquid, the dielectric constant of which is to be measured, and then without it, i.e., in the presence of air. Since the dielectric constant is near to unity in the latter case, this gives rise to knowledge of the dielectric constant of the liquid because the capacitance of the cell in the bridge arm increases as the dielectric constant increases. 

However, when the liquid is a conducting solution, this approach breaks down. The conductance of the solution contributes to the impedance of the cell, which (depending on frequency) may no longer be overwhelmingly capacitative. Hence, the dielectric constant of a conducting liquid cannot be simply measured, because of the conductive components of the impedance. 

Two approaches can be used to avoid this difficulty. In the one, which is best used for frequencies well below the relaxation frequency of water, one measures the force between two plates that have the conducting liquid between them. This force is independent of the conductance of the liquid or the direction of the field. If dis the distance between the plates, it is easy to show that the force is 

Studies of the dielectric constant of solutions and the relaxation times of water in the presence of ions have been refined since the 1980 s and indeed difficulties do turn up if one looks at data from measurements over large frequency ranges. The variation of the dielectric constant with frequency has been studied particularly by Winsor and Cole, who used the Fourier transform of time domain reflectometry to obtain dielectric constants of aqueous solutions and the relaxation times in them. Their frequency ranges from over 50 MHz to 9 GHz. 

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