Chapter 1:- Concept of Electrode Potential

B.sc 2nd year Book
(Page 5)

# Electrode Potential

When a strip of metal is immersed in its salt solution a net negative or positive charge is developed on the metal with respect to its solution. The negative or positive charge developed on the metal produces electrical potential difference between the metal and its solution. This electrical potential difference is called ‘electrode potential’. In other words, ‘the potential of any electrode is the potential difference between it and the electrolyte surrounding the electrode’.

Or the tendency of an electrode to lose or gain electrons when it is in contact with its own solution is called ‘electrode potential’. The tendency to lose electrons means the tendency to get oxidized and the potential difference developed is called ‘oxidation electrode potential (Eox). Similarly, a tendency to gain electrons means a tendency to get reduced and the potential difference is called ‘reduction electrode potential’ (Ered). Thus, the electrode potential is a measure of the tendency of an electrode in a half-cell to gain or lose electrons. The unit of electrode potential is Volt (V). It depends on the nature of the metal, the concentration of the metallic ions in the solution, and also on the temperature of the solution.

When the ions are at unit activity(IM) and the temperature is 25 °C (298 K)  the potential difference is called ‘standard electrode potential(E°’)’.

The measurement of single electrode potential(metal-metal ion electrode) is very difficult because oxidation half-reaction or reduction half-reaction cannot take place alone. However, it can only be measured by using some electrodes as the reference electrode. The reference electrode used is a standard or normal hydrogen electrode(SHE or NHE) because the electrode potential of a standard hydrogen electrode has been assigned a value of zero volt (E°=0.0 V). For this purpose, constitute a galvanic cell by connecting the given electrode with the reference electrode through a salt bridge. Connect the electrodes with each other through a voltmeter and note the reading if any deflection occurs in the voltmeter.

Calomel electrode is also used as a reference electrode to determine the various single electrode potentials as shown in figure1.06 and 1.07.

Thus, ‘standard electrode potential of a conductor(metal) may be defined as a potential difference in volts developed in a cell consisting of two electrodes in which the pure metal is in contact with a molar solution of one of its ions and the NHE’. Thus, the determination of the standard electrode potential of zinc, the cell representation is shown as-

Zn, Z2+ (1M) || H+(c=1) ; H2(1atm), Pt,  E° Zn2+ /Zn= -0.76 V
oxidation electrode .   reduction electrode.

Similarly, for determining the standard electrode potential of copper, the cell representation is shown as-

Pt, H2(1atm) ; H+(c=1M) || Cu2+ (c=1M), Cu ; E° Cu2+ /Cu= +0.34 V
oxidation electrode.                 reduction electrode.

According to the latest convention of sign, the electrode at which reduction takes place with respect to standard hydrogen electrode has reduction potential which is given a positive sign and the electrode at which oxidation takes place with respect to standard hydrogen electrode has oxidation potential, it will have a negative sign.

Types of Electrode potential
Electrode potentials are of two types :

(i) Oxidation Electrode Potential: The oxidation electrode potential is the potential of the electrode at which oxidation reaction takes place and hence is a measure of the tendency of the electrode in a half-cell to get oxidized or to lose electrons. It is represented as (Eox)elec or EM/Mn+ .  For example standard oxidation electrode potentials of Zn(s)/Zn2+(aq) and Cu(s)/Cu2+ (aq) electrodes are + 0.76 V and -0.34 V respectively.

Oxidation : Zn(s)—————————-Zn2+ (aq)  +  2e, E°Zn/Zn2+ = + 0.76 V

Oxidation : Cu(s)—————————-Cu2+ (aq)  +  2e, E°Cu/Cu2+ = -0.34 V

(ii) Reduction Electrode Potential: The reduction electrode potential is the potential of the electrode at which reduction reaction takes place and hence is a measure of the tendency of the electrode in a half-cell to get reduced or to gain electrons. It is represented as (Ered)elec or EMn+ /M.  For example, standard reduction electrode potentials of Zn2+ (aq)/Zn(s) and Cu2+ (aq)/Cu(s) electrodes are -0.76 V and +0.34 V respectively.

Reduction : Zn2+ (aq) + 2e–  ———————> Zn(s),  E°Zn2+ /Zn  = -0.76 V

Reduction : Cu2+ (aq) + 2e  ———————>Cu(s)  +  E°Cu2+ /Cu = +0.34 V

Relation between Oxidation potential and Reduction potential of a given Electrode:

Oxidation potential of an electrode is numerically equal to the reduction potential of the same electrode with opposite sign, i.e. (E°ox)elec = – (E°red)elec  or –(E°ox)elec  = (E°red)elec. For example, E°Cu/Cu2+  = -0.34 V and E°Cu2+/Cu = + 0.34 V.