Conductivity and Diffusion Characteristics of Heterogeneous Cation- and Anion-Exchange Membranes in Solutions of Malic and Citric Acids

The conductivity and diffusion characteristics of heterogeneous cation-exchange (MK‑40, Ralex CMHPES) and anion-exchange (MA-41, Ralex AMHPES) membranes produced in Russia and abroad have been studied in solutions of fruit acids. It has been shown that the conductivity of the cation-exchange membranes is higher than that of the corresponding citric and malic acids solutions due to a higher concentration of hydrogen ions in the membrane phase. The conductivity of the anion-exchange membranes is significantly lower than that of the cation-exchange membranes, which is associated with a low mobility of organic anions. An atypical descending dependence of the specific conductivity of the anion-exchange membranes on the concentration of organic acids is discovered, caused by an increase in the concentration of low-mobility multiply charged organic ions in the membrane phase. The diffusion permeability of the anion-exchange membranes in organic acids solutions is much higher compared to the cation-exchange membranes due to the contribution of molecular acid diffusion and the high mobility of the hydrogen cation, the transfer of which limits diffusion in the case of anion-exchange membranes. A study of reagent-free pH correction of a model malic acid solution using bipolar electrodialysis revealed the optimal process conditions (1–1.5 A/dm2) and demonstrated that energy consumption is comparable to that known in the literature for similar processes.