CHEM-GUIDE: Electrode potential, standard electrode potential,standard hydrogen electrode and its applications

Electrode potential

It is important to understand the development of charges at the electrodes. When a strip of a metal M is placed in a solution of its ions Mⁿ⁺, a metal - metal ion electrode is obtained. The possible processes that can occur at the electrodes are:
The metal ion Mⁿ⁺ collides with the electrode and undergoes no change.
A metal ion Mⁿ⁺ collides with the electrode, gains n electrons and gets converted into a metal atom M (i.e., the metal ion is reduced).
A metal atom on the electrode M may lose n electrons to the electrode, and enter the solution as Mn+, (i.e., the metal atom is oxidised).
The above processes affect the electrodes as follows:
When neither the metal atom (M), nor the metal ion Mn+ undergo any change no charge is developed on the electrode relative to the solution.
If the metal has relatively high tendency to get oxidized, its atoms will lose electrons readily and form Mn⁺ ions, which go into the solution. The electrons lost from the electrode get accumulated on the metal electrode and the electrode acquires a slight negative charge with respect to the solution. The Mn⁺ ions from the solution will take up electron to become M atoms. After some time, an equilibrium is established as:
Such an equilibrium results in separation of charges (negative on the electrode with respect to the solution).
Similarly, if the metal ions have relatively greater tendency to get reduced due to electron deficiency, they will take electrons from the electrode. As a result, separation of charges occurs and a net positive charge will be developed on the electrode with respect to the solution.
Fig: 8.7 - Concept of electrode potential
So, an electrode develops a charge depending upon the nature of the process, which is favoured at the electrode. Due to separation of charges between the electrode and the solution, an electrical potential is set up between metal electrode and its solution. The electrical potential difference between the metal and its solution is known as electrode potential (or half-cell potential). Thus, the electrode potential is a measure of tendency of an electrode in a half-cell to gain or lose electrons. Electrode potentials are denoted by the letter E and are of two types:
Oxidation Potential
The tendency of an electrode to lose electrons or to get oxidized is called its oxidation potential.
Reduction Potential
The tendency of an electrode to gain electrons or to get reduced is called its reduction potential.
It is evident that the oxidation potential is the reverse of reduction potential. For example, reduction potential of Zn is -0.76 volts, its oxidation potential is +0.76 volts. According to the present convention, the half-cell reactions are always written as reduction half reactions and their potentials are represented as reduction potentials.

Standard electrode potential
It is the potential developed when the pure metal is in contact with its ions at one molar concentration at a temperature of 25^oC or 298 K.
Example: When a Zn rod of any length is dipped in 1M ZnSO₄ solution, standard electrode is formed and the potential developed is called standard zinc electrode potential (E_oZn). The standard zinc electrode is represented as Zn/Zn⁺² (IM).
In case of a gas electrode, the standard electrode potential (E^o) is defined as the potential developed at the interface of the gas and solution containing its own ions when an equilibrium is established between the gas at a pressure of 760 mm of Hg and the ions in solution of unit concentration.
When the H₂ gas at a pressure of 1atm is bubbled through HCl of 1 M std H₂ electrode is formed and the potential developed is called std hydrogen electrode potential (E^oH₂) whose magnitude is considered to be 0.
The standard H₂ electrode is represented as Pt, H₂ / H⁺(760 mm of Hg)/ (IM)
The magnitude of the standard electrode potential is independent of temperature since it depends only on the concentration of the ions.

Standard hydrogen electrode
Standard Hydrogen Electrode (S.H.E or N.H.E.)
A hydrogen electrode in which pressure of hydrogen gas is maintained at 1 atm, and the concentration of H⁺ ions in the solution is 1M is called a standard hydrogen electrode (SHE). As the potential of a standard hydrogen electrode is taken as 0.00 V at all temperatures. This electrode is used as a primary reference electrode for measuring the potential of all other electrodes.
The potential of hydrogen electrode depends upon,
Concentration of H⁺ ions in solution.
Pressure of the hydrogen gas.
Thus, the potential of an electrode relative to a standard hydrogen electrode at 298 K, and 1 atm pressure when the concentration of the ion taking part in the electrode reaction is 1 mol L^-¹ is called the standard electrode potential.
The electrode potential of an electrode can be determined by connecting its half-cell with standard hydrogen electrode. As the electrode potential of the standard hydrogen electrode is assigned zero, the electrode potential of the metal electrode as determined with respect to the standard or normal hydrogen electrode is called electrode potential (E).