NDLI: Mass transport in a boundary layer and in an ion exchange membrane: Mechanism of concentration polarization and water dissociation

Content Provider	Springer Nature Link
Author	Tanaka, Yoshibu
Copyright Year	2012
Abstract	In an unforced flowing NaCl solution in bulk, gravitational or electro convection supplies ions from bulk toward the membrane surface through a boundary layer. In a boundary layer formed on an anion exchange membrane, the convection converts to migration and diffusion and carries an electric current. In a boundary layer formed on a cation exchange membrane, the convection converts to migration and carry an electric current. In a forced flowing solution in bulk, the boundary layer thickness is reduced and gravitation or electro convection is disappeared. An electric current is carried by diffusion and migration on the anion exchange membrane and by migration on the cation exchange membrane. Ion transport in a boundary layer on the cation exchange membrane immersed in a NaCl solution is more restricted comparing to the phenomenon on the anion exchange membrane. This is due to lower counter-ion mobility in the boundary layer and the restricted water dissociation reaction in the membrane. The water dissociation reaction is generated in an ion exchange membrane and promoted due to the increased forward reaction rate constant. However, the current efficiency for the water dissociation reaction is generally low. The intensity of the water dissociation is more suppressed in the strong acid cation exchange membrane comparing to the phenomenon in the strong base anion exchange membrane due to lower forward reaction rate constant in the cation exchange membrane. In the strong acid cation exchange membrane, the intensity of electric potential is larger than the values in the strong base anion exchange membrane. Accordingly, the stronger repulsive force is developed between ion exchange groups (SO 3 • groups) and co-ions (OH$^{−}$ ions) in the cation exchange membrane, and the water dissociation reaction is suppressed. In the strong base anion exchange membrane, the repulsive force between ion exchange groups (N$^{+}$(CH$_{3}$)$_{3}$ groups) and co-ions (H$^{+}$ ions) is relatively low, and the water dissociation reaction is not suppressed. Violent water dissociation is generated in metallic hydroxides precipitated on the desalting surface of the cation exchange membrane. This phenomenon is caused by a catalytic effect of metallic hydroxides. Such violent water dissociation does not occur on the anion exchange membrane.
Starting Page	665
Ending Page	681
Page Count	17
File Format	PDF
ISSN	10231935
Journal	Russian Journal of Electrochemistry
Volume Number	48
Issue Number	7
e-ISSN	16083342
Language	English
Publisher	SP MAIK Nauka/Interperiodica
Publisher Date	2012-07-12
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	ion exchange membrane electrodialysis mass transport boundary layer concentration polarization water dissociation Electrochemistry Physical Chemistry
Content Type	Text
Resource Type	Article
Subject	Electrochemistry

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