Measurement of the Standard Electrode Potential Standard Hydrogen Electrode


Standard Hydrogen Electrode

Reference Electrode

It is not possible to measure the absolute value of the single electrode potential directly. Only the difference in potential between two electrodes can be measured experimentally.  It is, therefore, necessary to couple the electrode with another electrode whose potential is known. This electrode is termed as reference electrode. The commonly used reference electrode is Standard Hydrogen Electrode (SHE) or also called as Normal Hydrogen Electrode (NHE).

Standard Hydrogen Electrode, (SHE)

Ø  It con­sists of a small platinum strip coated with platinum black as to adsorb hydrogen gas.
Ø  The platinum strip is placed in an acid solution which has H+ ion concentration 1 M.
Ø  A platinum wire is welded to the platinum strip and sealed in a glass tube as to make contact with the outer circuit through mercury.
Ø  Pure hydrogen gas at one atmospheric pressure is constantly bubbled through the solution.
Ø  The hydrogen electrode thus obtained forms one of two half-cells of a voltaic cell. When this half-cell is connected with any other half-cell, a voltaic cell is constituted. The hydrogen electrode can act as cathode or anode with respect to other electrode.
Ø  When it acts as anode the cell reaction that occurs at this electrode is
                         H2   2H+ + 2e-
Ø  When it acts as cathode the cell reaction that occurs at this electrode is  
                                                           2H+ + 2e →  H2
The temperature of the cell is maintained at 250C. By international agreement the standard hydrogen electrode is arbitrarily assigned a potential of exactly ± 0.000 Volt.

Determination of Standard Electrode Potential of Zn/Zn2+ Electrode

An Electrochemical Cell Is Setup in which a zinc rod is dipped in 1 M zinc Sulphate solution. This half-cell is combined with a standard hydrogen electrode through a salt bridge. The deflection of the voltmeter indicates that current is flowing from hydrogen electrode to metal electrode or the electrons are moving from zinc rod to hydrogen electrode. Hence the zinc electrode acts as an anode and the hydrogen electrode acts as cathode .in this case the reading (EMF) by the voltmeter is 0.76 V. The cell can be represented as     
                                             Zn + 2H+   →  Zn2+ +H2      
 Calculations      
The EMF of the cell is 0.76 volt
ECell = EoAnode + EoCathode
0.76 =  EoAnode + 0          or                EoAnode = +0.76 V
Hence oxidation potential of zinc is 0.76 V and reduction potential of zinc will be – 0.76 V

Determination of Standard Electrode Potential of Cu2+/Cu, Electrode:

An Electrochemical Cell Is Setup in which a copper rod is dipped in 1 M copper Sulphate solution. This half-cell is combined with a standard hydrogen electrode through a salt bridge. The deflection of the voltmeter indicates that current is flowing from copper electrode to hydrogen electrode or the electrons are moving from hydrogen rod to copper electrode. Hence the copper electrode acts as an cathode and the hydrogen electrode acts as anode. In this case the reading (EMF) by the voltmeter is 0.34 V. The cell can be represented as    
                                                Cu2+  + H2 → 2H+ + Cu
Calculations      
The EMF of the cell is 0.34 volt
E Cell = Eo Anode + Eo Cathode
0.34 = 0.0 +  Eo Cathode          or                Eo cathode= +0.34 V
                  Hence reduction potential of copper is 0.34 V and oxidation potential of copper will be – 0.34V.


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