TERNARY MUTUAL DIFFUSION-COEFFICIENTS OF ZNCL2-KCL-H2O AT 25-DEGREES-C BY RAYLEIGH INTERFEROMETRY

Mutual diffusion coefficients and densities were measured for aqueous ZnCl2-KCl mixtures at 25° by using free-diffusion Rayleigh interferometry and pycnometry, respectively. The ZnCl2 concentrations were fixed at 1.5 mol-dm-3, whereas those of KCl were 0.5, 1.25, 2.0, or 4.0 mol-dm-3. This corresponds to a half charged zinc-chlorine storage battery at various suporting electrolyte concentrations. The main-term coefficient of ZnCl2 only varies by 10% with KCl concentration, whereas that of KCl varies by about 22%. The ZnCl2 cross-term coefficient remains small and positive; in contrast the KCl cross-term coefficient goes through a maximum and is negative at high and low KCl concentrations. At KCl concentrations of 0.5 and 4.0 mol-dm-3, solutions with the KCl Δc≈0 are statically and dynamically (diffusively) unstable at the top and bottom of the boundary. Evaluation of the parameters of the non-linear least-squares solution to the diffusion equation is difficult for the 1.25 mol-dm-3 KCl case, since this system has nearly equal eigenvalues in its diffusion coefficient matrix. © 1990 Plenum Publishing Corporation.