Junction Potentials in Galvanic Cells

The Nernst equation predicts the cell potential under non-standard conditions.

However, in many galvanic cells, the measured potential often differs from the value predicted by the Nernst equation.

To understand this, let's consider a galvanic cell containing two half-cells with different electrolyte solutions. When they come into contact, ions begin to diffuse across the boundary. Since ions have different mobilities, they move at different rates.

This unequal movement of ions creates a small charge separation at the interface between the two solutions. This develops an additional potential called the liquid junction potential, E_J.

As a result, the measured emf includes both the Nernst potential and the junction potential.

To reduce this effect, a salt bridge is used. The salt bridge typically contains agar gel mixed with an electrolyte like concentrated potassium chloride.

Since potassium and chloride ions have similar mobilities, they diffuse at similar rates. Their diffusion helps minimize the junction potential and improves measurement accuracy.