Junction Potentials in Galvanic Cells

The measured potential differs because ions in the two half-cell electrolyte solutions have different mobilities and diffuse at unequal rates across the boundary. This creates a charge separation at the interface, generating an additional potential called the liquid junction potential. The total measured emf includes both the Nernst potential and this junction potential.

A liquid junction potential is a small potential difference that develops at the interface between two electrolyte solutions due to different ion diffusion rates. Since ions have different mobilities, they move at different speeds across the boundary, creating unequal charge separation. This additional potential must be accounted for in accurate electrochemical measurements.

A salt bridge contains agar gel mixed with an electrolyte like concentrated potassium chloride. Potassium and chloride ions have similar mobilities, so they diffuse at comparable rates. This balanced diffusion minimizes the junction potential at both ends of the salt bridge, causing them to largely cancel each other out and improve measurement accuracy.

No, salt bridges can minimize but not eliminate liquid junction potentials. Although the junction potentials at both ends of the salt bridge largely cancel each other due to similar ion mobilities, a small residual potential remains. This is why junction potentials are considered significant for accurate electrochemical work.

Ion mobilities determine the diffusion rates of ions across electrolyte solution boundaries. When ions have different mobilities, they move at unequal speeds, creating uneven charge distribution at the interface. This charge separation generates the liquid junction potential. Selecting ions with similar mobilities, as in salt bridges, reduces this effect.

The total emf of a cell with a liquid junction equals the sum of the Nernst potential and the liquid junction potential. The Nernst equation calculates the potential under thermodynamic equilibrium conditions without a junction. Adding the junction potential accounts for the real-world charge separation that occurs when different electrolyte solutions meet.

Junction potentials are important because they contribute to the total measured emf and can introduce significant errors in electrochemical work. Although small, they affect measurement accuracy in applications of emf measurements. Understanding and minimizing junction potentials through salt bridges ensures reliable and precise electrochemical data.