Standard hydrogen electrode
o
o
o
o
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
o
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
o
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 .