Time characteristics of electromotive force in single-membrane cell for stable and unstable conditions of reconstructing of concentration boundary layers

Abstract

The stability of concentration boundary layers near membrane (CBLs), arising after turning off mechanical stirring of KCl solutions in single-membrane cell with polymer membrane placed in horizontal plane, was studied by measurement of time changes of electromotive force of single-membrane cell (EMF). In the case when solution with low concentration and low density ( C l) was over the membrane and the solution with higher concentration and higher density ( C h) was under the membrane (configuration A) the stable recreation of CBLs by diffusion was observed, which was manifested by gradual and monotonous decrease of EMF after turning off mechanical stirring. The results of the carried out experiment show that character of time changes of EMF for A configuration depends on the quotient of concentrations on the membrane C h/ C l at the initial moment. Besides, for configuration A the pulsations of EMF were not observed in the whole studied range of concentrations. In the case of opposite location of solutions in relation to the membrane (configuration B) it was stated that for C h/ C l ≤ 10 time changes of EMF were similar to time changes of EMF in configuration A. For C h/ C l > 10 time characteristics of EMF for A and B configurations were different. For B configuration and C h/ C l ≥ 100 the pulsations of EMF, arising in a short time after turning off mechanical stirring, were observed. The pulsations of EMF can be connected with hydrodynamic instability in CBLs. Increase of C h/ C l caused increase of amplitude and decrease of period of EMF pulsations. Besides, for C h/ C l > 1000 damping of EMF pulsation was observed, greater for higher C h/ C l. The interpretation of experimental data was made by means of the Kedem–Katchalsky model, diffusion equation and concentration Rayleigh number.