Anomalous Water Transport across Cation-Exchange Membranes under an Osmotic Pressure Difference in Mixed Aqueous Solutions of Hydrochloric Acid and Alkali Metallic Halide.

Abstract

Water transport under an osmotic pressure difference across a cation-exchange membrane was measured in 0.005molkg-1 HCl+MCl(mc)/membrane/MCl(mc) system, where M in MCl represents alkali metal and mc is the molality of the electrolyte solutions ranging from 0 to 0.1molkg-1. Hydrocarbonsulfonic acid-type cation-exchange membrane Aciplex K-101 was used. The volume flux observed in the system at mc≠0 was always larger than that in the system at mc=0, although the apparent molality differences across the membrane of the systems were same. Bell-shaped dependence of the osmotic volume flux was observed on mc. In general, the order of the magnitude of the volume flux was LiCl>NaCl>KCl. Electroosmotic volume flux across the membrane was also measured at 0.01molkg-1 to estimate the reduced transport number of water τ0. The values of τ0F represent the amount of transported water followed by the flow of one mole of the monovalent cations. The values of τ0F were 1.7, 26.4, 18.0, and 12.8 for H+, Li+, Na+, and K+, respectively. The anomalous osmotic properties observed in the mixed aqueous solutions of HCl and alkali metallic halide would be due to the displacement of water caused by the counter transport of proton and alkali metallic ion across the membrane in addition to the normal osmotic volume flux.