Standard hydrogen electrode
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The potential difference developed between the two terminals is measured using a potentiometer.
The direction of the flow of electric current in the external circuit is identified using a galvanometer.
This enables us to identify the positive and the negative terminal of the set up as the current flows from positive terminal to negative terminal.
This in turn will help us mark the anode and the cathode electrodes due to the fact that the electrons will flow from anode to cathode.
Ecell = Ecathode – Eanode
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In case of Daniel cell At the anode:
Oxidation---------------- loss of electrons.
Zn-- > Zn 2 + + 2e-
At cathode,
Reduction-------------gain of electrons.
Cu 2 + + 2e--- > Cu
The net reaction of this cell is the sum of two half-cell reactions. Zn( s) + Cu 2 +( aq)-- > Zn 2 +( aq) + Cu( s)
Emf of the cell = Ecell = Ecathode – Eanode
= 0.34 V –(- 0.76) V = 1.10 V( Measured Emf of Cu is 0.34 V and that of Zn is 0.76 V).
Standard hydrogen electrode
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The electrode is connected to a standard hydrogen electrode( SHE) to constitute a cell
o It consists of a platinum electrode coated with a layer of platinum black.