Turbulent light scattering fluctuation spectra near a cation electrodialysis membrane

Abstract

Light scattering spectra from polystyrene latex spheres near a cation exchange membrane in 0.020 M HCl and 0.020 M NaCl solutions (identical solutions both sides of membrane) have been measured. For HCI, results were as follows: spectra on the depleted side showed evidence for turbulent flow at and above the critical current density (CCD). Spectra on the concentrate (cathodic) side showed similar spectra above approximately twice the CCD, although of lower intensity. The spectra in all cases were Gaussian, with halfwidth 10 to 20 Hz, except for a sharp cutoff in the 15–30 Hz range. The dependence on current was relatively weak beyond the point that the characteristic spectrum appeared, suggesting that once turbulence developed fully, its characteristic features in the region probed by this experiment were not qualitatively changed by further energy input. Weak light scattering was observed at currents below those at which these spectra were found. For NaCl, depleted side spectra generally were different; they began at currents above approximately twice the CCD, and were generally proportional to f −2, up to 40 ± 10 Hz, where they ran into background. This is characteristic of Lorentzian spectra. Gaussian spectra were sometimes found with NaCl, however. On the concentrate side, at least six to seven times the CCD was needed for nonzero difference spectra, and these resembled weak Gaussian spectra. This evidence supports the attribution of electrical noise spectra to turbulence on the depleted side, both for NaCl and HCl (except for the diffusion component of the electrical noise).