Understanding Liquid Junction Potential

The question describes a situation where two different concentrations of Hydrochloric acid (HCl) are used as electrolytes and are in contact. When two solutions of different concentrations or compositions are brought into contact, there is a tendency for ions to diffuse from the region of higher concentration to the region of lower concentration.

In the case of HCl, both $\text{H}^+$ and $\text{Cl}^-$ ions will diffuse. However, these ions usually have different mobilities (speeds at which they move through the solution). For example, $\text{H}^+$ ions are known to have very high mobility compared to most other ions, including $\text{Cl}^-$ ions.

Because $\text{H}^+$ ions move faster than $\text{Cl}^-$ ions, they diffuse across the interface more quickly from the concentrated side to the dilute side. This difference in diffusion rates leads to a slight separation of charge at the interface: the more dilute solution tends to accumulate a slight positive charge (due to the faster $\text{H}^+$ ions arriving first), while the more concentrated solution tends to develop a slight negative charge (due to the slower departure of $\text{Cl}^-$ ions).

This charge separation creates an electrical potential difference across the interface between the two solutions. This potential difference is known as the liquid junction potential. It is an additional source of potential difference in an electrochemical cell when the electrolytes in the two half-cells are different or have different concentrations and are in direct contact.

Evaluating the Options

• Salt bridge: A salt bridge is used to connect two half-cells to complete the circuit and maintain electrical neutrality by allowing ion migration. It minimizes the liquid junction potential, but it is not the potential difference itself.
• Reducing agent: A reducing agent is a substance that causes reduction in another substance while getting oxidized itself. This relates to redox reactions, not directly to the potential difference arising from concentration differences at an electrolyte interface.
• Oxidising agent: An oxidising agent is a substance that causes oxidation in another substance while getting reduced itself. Like a reducing agent, this relates to redox reactions and not to the interface potential described.
• Liquid junction potential: As explained above, this is the potential difference that arises at the interface between two electrolyte solutions of different concentrations or compositions due to the unequal mobilities of ions diffusing across the boundary. This perfectly matches the scenario described in the question.

Therefore, when two different concentrations of Hydrochloric acid are used as electrolytes in contact, the additional source of potential difference at the interface is an example of a liquid junction potential.