In this article, we will discuss the back emf or polarization potential in electrolysis.
Electrolysis is
an electrochemical process in which an electric current flowing through a
solution of an electrolyte results in chemical changes. Electrolysis is carried
out in an apparatus called an electrolytic cell
or voltameter.
What is Back EMF in Electrolysis?
In an electrolytic cell or voltameter, when an external
direct voltage is applied across cell terminals, an electromotive force
(EMF) is built up between each electrode and the electrolyte that resists the
applied direct voltage. This resisting or opposing emf is called back emf or polarization potential of the electrolyte.
It is generally denoted by the symbol Eb. The back emf in electrolysis is produced because of
the coating of electrodes of the voltameter by the products of the electrolysis
process. This effect is referred to as polarization,
and hence, the back emf is termed as polarization potential as well.
In simple terms, we can define the back emf as under:
The emf developed in
the electrolytic cell that opposes the applied external direct voltage is
referred to as the back emf of the
electrolyte.
The different electrolytes have different values of the back
emf. For example, such acids and alkalis that evolve oxygen and hydrogen have
the value of back emf equal to about 1.7 volts. For other electrolytes, the value of back emf depends upon the salt. For normal
electrolytic solutions, the value of back emf varies from 0.5 to 2 volts.
Voltage Equation for Electrolysis
The voltage equation for electrolysis is given by,
Where, V is the applied external direct voltage, Eb is the back emf, and IRe is the voltage drop in the electrolyte.
From this equation, it is clear that in
order to carry out the process of electrolysis at an appreciable rate, the
external direct voltage (V) must be equal to Eb + IRe. In
other words, if
Then, the process of electrolysis will
not take place.
Hence, this is all about the back emf in
electrolysis and voltage equation for the electrolytic process.
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