CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 411
11.9 Electrode polarization
During electrolysis, the products of electrochemical reactions are discharged at the electrodes.
These products cause the formation of a galvanic cell whose potential (electromotive force)
suppresses the electrolysis (this is another example of Le Chatelier’s principle, see8.5). This
phenomenon is calledelectrode polarization, the respective electrode potential is called the
polarization voltage,Ep. Ohm’s law then acquires the form
E−Ep=I R. (11.128)It is obvious that the electrolysis starts at voltages higher than the polarization voltage.
Example
During the electrolysis of hydrochloric acid, hydrogen is discharged at the cathode and chlorine
at the anode. A galvanic cell originates formed by a hydrogen anode, chlorine cathode, and
a hydrogen chloride solution. Electrolysis starts only when the voltage of the source is higher
than the potential (electromotive force) of the given cell (approximately 1.36 V under laboratory
conditions).In addition, polarization voltage may be due to concentration changes during electrolysis,
see11.2.5. An electrolyte concentration cell originates, see11.8.13, whose potential also
suppresses the electrolysis. In this connection we speak aboutconcentration polarization
while in the above example it waschemical polarization.
It is necessary to distinguish between the polarization voltage and thedecomposition
voltage, the real voltage of the source at which undisturbed electrolysis starts to proceed. The
difference between the decomposition and polarization voltages is calledovervoltage. It is
caused by complicated kinetic phenomena during electrolysis.