Determination of the Recombination Coefficient in Electrolytic Solutions from Impedance Spectroscopy Measurements

The dependence of the actual/effective ionic concentration on the nominal salt concentration is experimentally investigated for aqueous solutions of NaCl and KCl. The actual ionic bulk density is determined by means of the impedance spectroscopy technique using two approaches for the data analysis: A first approach scrutinizing the real part's plateau of the cell's electric impedance spectrum versus the bulk density of dopant when the impedance is written in the series representation. A second one via extraction of the ionic density from simultaneously fitting of the real and imaginary parts of the impedance spectrum by using the Poisson-Nernst-Planck model. The validity of this procedure reposes on the hypothesis that both the diffusion coefficient and the dielectric constant are independent from the salt concentration in the explored range of concentrations. Our experimental results indicate that a first-order reaction describes well the dissociation phenomenon. The dissociation-association ratio of the reaction has been calculated. Deviations from linearity appear at rather low concentrations of the order of mM.