Estimate liquid junction potential for a binary electrolyte or a 3 M KCl bridge using concentrations, electrolyte type and temperature. The separately labeled legacy concentration-potential mode retains the original 25°C formula.

Henderson estimates use ideal concentrations and fixed 25°C ion mobilities. Concentrated solutions, including 3 M KCl, can differ substantially. Sign: side 2 minus side 1.

25°C is the mobility-reference temperature; other temperatures only scale RT/F.

Liquid Junction Potential Formula

For a binary electrolyte with constant transport numbers, the junction potential of side 2 relative to side 1 is:

LJP = (R × T / F) × (tₘinus / abs(zₘinus) - tₚlus / zₚlus) × ln(C2 / C1)

Variables:

  • LJP is the junction potential of side 2 minus side 1; the formula gives volts, multiplied by 1000 for millivolts.
  • C1 is the ion concentration of the first solution (mol/L)
  • C2 is the ion concentration of the second solution (mol/L)

R is the gas constant, T is absolute temperature, F is the Faraday constant, t is the ion transport fraction and z is the ion charge number. The Henderson estimate assumes ideal concentrations and constant ion mobilities. The original 59 × log10(C1/C2) mV formula is retained only as a labeled legacy concentration potential at 25°C; it omits ion transport and is not a general junction potential.

What is Liquid Junction Potential?

Liquid junction potential is the potential difference that develops when two ionic solutions of different concentrations come into contact. It is an important factor in electrochemistry and is considered when measuring the potential of electrochemical cells, especially in the use of reference electrodes. The liquid junction potential arises due to the differing rates at which ions migrate across the junction, leading to charge separation and a potential difference.

How to Calculate Liquid Junction Potential?

The following steps outline how to calculate the Liquid Junction Potential.

  1. First, determine the ion concentration of the first solution (C1) in moles per liter (mol/L).
  2. Next, determine the ion concentration of the second solution (C2) in moles per liter (mol/L).
  3. Choose the electrolyte and temperature, then apply the binary junction formula above using its ion transport fractions.
  4. Finally, calculate the Liquid Junction Potential (LJP) in millivolts (mV).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Ion concentration of the first solution (C1) = 0.1 mol/L

Ion concentration of the second solution (C2) = 0.01 mol/L. For KCl at 25°C with the listed fixed mobilities, side 2 minus side 1 is approximately −1.1047 mV. The legacy concentration-potential mode instead gives 59 mV.