Measurement of Electrode Potential
Galvanic cell is a cell in which chemical energy is converted into electrical energy.
This electrical energy is used to run gadgets such as inverters.
And for a device to run on a particular cell, we need to know the exact energy provided by that cell or battery.
So, for finding the exact energy provided by a cell we have to measure the electromotive force.
This electromotive force is the potential difference between the two half cells.
But we need to remember a point here: that individual half cells can never exist and so, we can not measure the individual electrode potential.
It is always the potential difference that is measured.
But if we want to measure the individual electrode potential at the electrode-electrolye interface, there is a way to do it.
If we know the potential of one electrode, then the potential of the other can be determined with respect to it.
For that we have something called: standard hydrogen electrode(SHE).
Before we explain how the SHE is used, let us look the construction of SHE.
SHE consists of a platinum square plate coated with finely divided platinum black.
The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it.
Here, the pressure of hydrogen gas is 1 bar and the concentration of hydrogen ion in the solution is 1 M.
Standard hydrogen electrode(SHE) can be represented by the equation
$Pt(s)âˆ£H_{2}(g)âˆ£H_{+}(aq)$
And is assigned a zero potential at all temperatures corresponding to the equation:
Now we will see how to find individual electrode potential with respect to SHE.
We will measure the electrode potential of the individual half cells of a Daniell cell here.
We will start by measuring the electrode potential corresponding to the anodic half cell.
For that, we take zinc electrode as the cathode and try to measure the reduction potential with respect to SHE.
The standard hydrogen electrode is taken as the anode.
The cell notation then becomes:
Now, when the emf of this cell is measured, we get the value as
$âˆ’0.76V$
.
The EMF of this cell can be represented by the equation :
$E_{cell}=E_{R}âˆ’E_{L}=E_{Zn_{2+}/Zn}âˆ’E_{H_{+}/H_{2}}$
Since the electrode potential of the left hand side, that is, SHE is
$0V$
, we get
$E_{R}=E_{Zn_{2+}/Zn}=âˆ’0.76V$
Here, negative value of standard reduction potential indicates that zinc is oxidised by hydrogen ions.
So, in a Daniell cell this forms part of the anodic half cell.
Let us now move on to measure the electrode potential of the other half cell of the Daniell cell.
So, if we want to find the electrode potential corresponding to the equation:
$Cu_{2+}+2e_{âˆ’}â†’Cu$
We can again do it with respect to the SHE.
In this case, the measured EMF is
$0.34V$
.
And if we calculate the standard electrode potential of
$E_{Cu2+/Cu}$
, we get the value
$0.34V$
.
Positive value of standard electrode potential indicates that
$Cu_{2+}$
ions are more easily reduced than hydrogen ions.
Since, zinc is more easily oxidised and copper more readily reduced, the complete cell reaction in a Daniell cell is:
The cell notation for this is
At this point we can also calculate the potential difference or EMF of the complete cell.
Similarly, we can find the electrode potential of any other half cell with respect to the standard hydrogen electrode.
Revision
Individual half cells can not exist and hence, electrode potential of individual half cell can not be measured.
If we want to measure the electrode potential of the individual half we measure it with respect to a standard electrode.
The standard electrode used to measure the potential of individual half cell is standard hydrogen electrode.
Standard hydrogen electrode electrode is assigned a value
$OV$
.
The end.