Different Types of Electrodes and Electrode Potentials

Types of Electrodes

Metal in contact with its ions:

Whether a given electrode acts as anode or cathode depends upon the other electrode with which it forms an electrochemical cell. So it is necessary to define both oxidation and reduction potentials for an electrode.

• Oxidation Electrode

M/Mⁿ⁺ is written as:

M – ne^- → Mⁿ⁺ (1.O M) (oxidation electrode)

and its potential is called as oxidation potential and at standard state is represented as E⁰ (M/Mⁿ⁺)

• Reduction Electrode

Mⁿ⁺/M is written as:

Mⁿ⁺ (1.O M) + ne^- → M (reduction electrode)

and its potential is called as reduction potential and at standard state is represented as E⁰ (M/Mⁿ⁺).

• Gaseous electrode:

Similarly, CI₂/Cl^- electrode is prepared "by passing Cl₂ gas and taking HCl (aq) or KCl (aq) as electrolyte.

Its electrode potentials are represented as:

E⁰ (H₂ /H⁺): 1/2 H₂ (g) – 1e → H⁺ (oxidation potential)

E⁰ (H⁺/ H₂): H⁺ + 1e → 1/2 H₂ (g) (reduction potential)

Similarly, CI₂/ Cl^- electrode is prepared "by passing Cl₂ gas and taking HCl (aq) or KCl (aq) as electrolyte.

Electrode Potentials:

The electrode potentials of half cells can then be measured by combining them with the standard reference electrodes in a cell arrangement. this is the hydrogen electrode half-cell at a concentration of H⁺ ions equal to 1.O M and H₂(g) at I atm pressure kept at 25 C (298 K). It is also known as SHE (standard hydrogen electrode) 'or NHE (normal hydrogen electrode). Its potential is taken as 'O' volts.

E⁰ (H₂ /H⁺) = O = E⁰ (H⁺/ H₂)

Now other half cells can be divided into two categories: One which will act as anode and others which will act as cathode in a cell arrangement with SHE. Cu electrode (half-cell) acts as cathode with SHE i.e. as Cu²⁺/Cu [Cu²⁺ (1.O M) + 2e ---+ Cu]. The experimental measurement of EMF value for this cell arrangement give O.34 volts.

E⁰ (Cu²⁺ /Cu) = +O.34 volts

The oxidation potential of Cu half-cell is just the negative of this value.

E⁰ (Cu / Cu²⁺) = -O.34 volts

Rules for assigning sign (+ve or -ve) to electrode potentials:

+ve means natural tendency to show that nature. e.g. oxidation potential of active metals like Na, Mg, AI, Zn, Fe etc is given a +ve sign, as they by nature get oxidized. Also reduction potential of less active metals like Cu, Ag etc. is given a +ve sign, as they by nature get reduced.

E⁰ (Cu²⁺/Cu) = + O.34 volts

Standard Emf of A Cell (E⁰):

E⁰ = difference in potentials of two half- cell

 a: anode c: cathode ox.: oxidation

In a different way:

 a: anode c: cathode Red.: reduction

Construction of a cell & writing cell representation:

Compare the reduction potentials of two electrodes. The electrode whose reduction potential has greater positive value is made the cathode and the other electrode is made the anode.

Note:

1. Constructing the cell in this manner will always give a positive EMF otherwise sometime if an electrode is chosen wrongly, the EMF comes out to be negative. In that case you should reverse the electrodes i.e., make anode the cathode and Vice-versa.
2. It is now standardised to use the definition of EMF in terms of standard reduction potentials
   i.e.