9.5
Junction Potentials in Galvanic Cells
The Nernst equation, derived under the assumption of thermodynamic equilibrium, calculates the electromotive force (emf) as the sum of potential diffe…
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, EJ.
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.
View the full transcript and gain access to JoVE Core videos
Q1: Why does the measured potential in a galvanic cell differ from the Nernst equation prediction?
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.
Q2: What is a liquid junction potential and how does it form?
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.
Q3: How does a salt bridge minimize junction potential in galvanic cells?
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.
Q4: Can salt bridges completely eliminate liquid junction potentials?
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.
Q5: What role do ion mobilities play in creating junction potentials?
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.
Q6: How does the total emf of a cell with a liquid junction relate to the Nernst equation?
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.
Q7: Why are junction potentials important in electrochemical measurements?
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.