Types of Electrodes | Electrochemistry Class 12

Types of Electrodes

Half-Cells, Reference Electrodes & Nernst Equation

1. What is an Electrode?

An electrode (or half-cell) consists of a metallic or non-metallic conductor in contact with an electrolyte. The potential difference developed at the interface is called the Electrode Potential.

2. Metal-Metal Ion Electrode

A metal rod dipped in a solution containing its own ions (e.g., Zinc rod in $ZnSO_4$).

                    Representation: $M(s) | M^{n+}(aq)$
                    
Half-Cell Reaction (Reduction): $M^{n+}(aq) + ne^- \rightleftharpoons M(s)$

Nernst Equation:

$$ E = E^\circ - \frac{0.0591}{n} \log \frac{1}{[M^{n+}]} $$

Examples: $Zn|Zn^{2+}$, $Cu|Cu^{2+}$, $Ag|Ag^+$.

3. Gas Electrode

Since gases are non-conductors, an inert metal (like Platinum) is used to establish electrical contact and adsorb the gas.

Standard Hydrogen Electrode (SHE)

Used as the Primary Reference Electrode. Its potential is arbitrarily assigned as 0.00 V at all temperatures.

Representation: $Pt(s) | H_2(g, 1 \text{ bar}) | H^+(aq, 1 M)$
Reaction: $2H^+(aq) + 2e^- \rightleftharpoons H_2(g)$
Formula: $E_{SHE} = -0.0591 \times pH$ (if $P_{H_2}=1$ bar).

Other examples: Chlorine Electrode ($Pt | Cl_2 | Cl^-$).

4. Metal-Insoluble Salt-Anion Electrode

A metal coated with its sparingly soluble salt, dipped in a solution containing a common anion. These are stable and used as Secondary Reference Electrodes.

A. Calomel Electrode

Structure: Mercury ($Hg$) in contact with Mercurous Chloride ($Hg_2Cl_2$, Calomel) and $KCl$ solution.
Notation: $Hg(l) | Hg_2Cl_2(s) | Cl^-(aq)$
Reaction: $Hg_2Cl_2(s) + 2e^- \rightleftharpoons 2Hg(l) + 2Cl^-(aq)$
Potential: Depends only on $[Cl^-]$. For Saturated KCl, $E = +0.2415 V$.

B. Silver-Silver Chloride Electrode

Notation: $Ag(s) | AgCl(s) | Cl^-(aq)$
Reaction: $AgCl(s) + e^- \rightleftharpoons Ag(s) + Cl^-(aq)$

5. Redox Electrode

An inert metal (Pt) dipped in a solution containing ions of the same element in two different oxidation states.

                    Notation: $Pt | Fe^{2+}(aq), Fe^{3+}(aq)$
                    
Reaction: $Fe^{3+}(aq) + e^- \rightleftharpoons Fe^{2+}(aq)$

$$ E = E^\circ - 0.0591 \log \frac{[Fe^{2+}]}{[Fe^{3+}]} $$

Other examples: Quinhydrone Electrode ($Q, QH_2$).

6. Ion Selective Electrode

Glass Electrode: Used for pH measurement. It has a thin glass membrane sensitive to $H^+$ ions.
Potential: $E = E^\circ - 0.0591 \times pH$.

Practice Quiz

Test your knowledge on Electrode Types.

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