A modular guide to electrochemical gradients, ion activity, and EMF under non-standard conditions

1. Introduction

Not all electrochemical cells rely on different metals or redox couples. In concentration cells, the electrodes are identical, and the voltage arises purely from differences in ion concentration. These cells demonstrate how chemical potential gradients drive spontaneous electron flow until equilibrium is reached.

Applications:

• pH sensors and ion-selective electrodes
• Battery diagnostics (concentration effects during discharge)
• Biological membrane potentials

Concentration cells are a natural extension of electrochemical principles and are quantified using the Nernst equation.

References:

• LibreTexts – Concentration Cells
• Khan Academy – Nernst Equation

2. Concept of a Concentration Cell

A concentration cell has:

• Identical electrodes (e.g., Zn/Zn²⁺)
• Different electrolyte concentrations (e.g., 0.1 M vs. 1.0 M Zn²⁺)

Electrons flow from the dilute side to the concentrated side to equalise chemical potential. The cell EMF is driven entirely by the concentration gradient, not the intrinsic electrode potential.

3. The Nernst Equation

The Nernst equation adjusts the electrode potential for non-standard conditions:

Where:

• E = electrode potential
• E° = standard potential
• R = 8.314 J mol⁻¹ K⁻¹
• T = temperature (K)
• n = electrons transferred
• F = 96,485 C/mol
• Q = reaction quotient

At 25 °C, the equation simplifies:

In a concentration cell, E° = 0, and Q reflects the ratio of ion concentrations.

4. Example Calculation

For a Zn²⁺ concentration cell:

The positive EMF indicates a spontaneous electron flow from dilute to concentrated solution.

5. Reaction Quotient (Q)

In concentration cells:

• Q < 1 → EMF positive → spontaneous
• Q > 1 → EMF negative → non-spontaneous
• Q = 1 → EMF = 0 → equilibrium

This helps predict cell behaviour over time.

6. Real-World Applications

1. pH Measurement: Glass electrodes use H⁺ concentration gradients.
2. Ion-Selective Electrodes: Detect Na⁺, K⁺, Cl⁻ via tailored membranes.
3. Biological Membranes: Ion gradients generate potentials for nerves and muscles.
4. Battery Diagnostics: Concentration differences affect voltage during discharge.

References:

• LibreTexts – Nernst Equation

7. Summary

• Concentration cells produce EMF from ion gradients, even with identical electrodes
• The Nernst equation allows the calculation of electrode potentials under non-standard conditions
• EMF depends on Q, temperature, and the number of electrons transferred
• These principles underpin sensors, biological systems, and battery monitoring

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