Measured and predicted ternary diffusion coefficients for concentrated aqueous LiCl + KCl solutions over a wide range of compositions

Abstract

Taylor dispersion and differential refractometry are used to measure mutual diffusion (chemical interdiffusion) in ternary LiCl + KCl + H₂O solutions at 25 °C. Ternary diffusion coefficients are reported for 23 different compositions at total electrolyte concentrations of 0.5, 1.0, 2.0 and 3.0 mol dm⁻³. The results show that LiCl concentration gradients drive substantial coupled flows of KCl, but only weak flows of LiCl are produced by KCl gradients. In dilute solutions, aqueous K⁺ ions are nearly twice as mobile as Li⁺ ions and KCl diffuses significantly more rapidly than LiCl. In concentrated LiCl solutions, however, KCl diffuses more slowly than LiCl. The measured diffusion coefficients are in close agreement with values predicted by a model of mixed electrolyte diffusion which includes electrostatic coupling, ionic hydration, solvent counterflow, and changes in the viscosity and the thermodynamic driving forces for diffusion. The average magnitude of the difference between the measured and predicted diffusion coefficients is 0.03×10⁻⁵ cm² s⁻¹.