Liquid junction potential

Liquid junction potential (shortly LJP) occurs when two solutions of electrolytes of different concentrations (e.g. 1.0 M HCl and 0.1 M HCl) are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated one. Furthermore, the diffusion fluxes of each ion are usually not equal. In the preceding example H⁺ ions, due to their higher electrical mobility (or alternatively, their higher diffusion coefficient), will move faster than the Cl^- ions. In this case the dilute solution will acquire a positive charge on its side of the liquid junction (because H+ cations diffuse faster than Cl- anions), while the concentrated solution will become negatively charged. This charge separation creates an electric field at the liquid junction, and this field contributes to the potential difference between reference electrodes immersed in the two solutions.

Liquid junction potential also develops between two solutions of different compositions, even their concentrations are the same. This is also because the diffusion coefficients of the different ions are not the same, in general.

This additional liquid junction potential (also known as diffusion potential) is a non-equilibrium potential (and, thus, cannot be calculated thermodynamically), but it can achieve a steady-state when, the speed of ion migration in the electric field balanced the speed of ions' diffusion.

The diffusion potential is small in solutions, when the cation and anion mobilities (or, equivalently their diffusion coefficients) are similar. This is also equivalent to saying, that in such solutions the ion transport numbers for anions and cations are the same.

The two most often used salts with near-similar diffusion coefficients of cation and anion are: KCl and NaNO₃. ^[1]

Calculation

The liquid junction potential cannot be measured directly but calculated. The electromotive force (EMF) of a concentration cell with transference includes the liquid junction potential.

The EMF of a concentration cell without transport is:

${\displaystyle E_{\mathrm {nt} }={\frac {RT}{F}}\ln {\frac {a_{2}}{a_{1}}}}$

where ${\displaystyle a_{1}}$ and ${\displaystyle a_{2}}$ are activities of HCl in the two solutions, ${\displaystyle R}$ is the universal gas constant, ${\displaystyle T}$ is the temperature and ${\displaystyle F}$ is the Faraday constant.

The EMF of a concentration cell with transport (including the ion transport number) is:

${\displaystyle E_{\mathrm {wt} }=2t_{M}{\frac {RT}{F}}\ln {\frac {a_{2}}{a_{1}}}}$

where ${\displaystyle a_{2}}$ and ${\displaystyle a_{1}}$ are activities of HCl solutions of right and left hand electrodes, respectively, and ${\displaystyle t_{M}}$ is the transport number of Cl⁻.

Liquid junction potential is the difference between the two EMFs of the two concentration cells, with and without ionic transport:

${\displaystyle E_{\mathrm {lj} }=E_{\mathrm {wt} }-E_{\mathrm {nt} }=(2t_{M}-1){\frac {RT}{F}}\ln {\frac {a_{2}}{a_{1}}}}$

Minimization of liquid junction potentials

The liquid junction potential interferes with the exact measurement of the electromotive force of a chemical cell, so its effect should be minimized as much as possible for accurate measurement.

As stated earlier, he magnitude of the liquid junction potential depends on the relative speeds of the moving ions. K⁺ cations and Cl^- anions have similar values of diffusion coefficients in many solvents. For this reason KCl solutions are often used in salt bridges to minimize the liquid junction potential. ^{[2] [3]}

Ammonium nitrate (NH₄NO₃) solutions have also been used in salt bridges, ^[4] particularly when the system under investigation cannot tolerate chloride ions.

The most common practical method of eliminating the liquid junction potential is to place a salt bridge consisting of a saturated solution of potassium chloride (KCl) or ammonium nitrate (NH₄NO₃) between the two solutions constituting the junction. When such a bridge is used, the ions in the bridge are present in large excess at the junction and they carry almost the all the charges, that are responsible for the development of a liquid junction potential across the boundary. Furthermore, in the most common design of a salt bridge the liquid junction potentials between the bridge and the two connected solutions subtract from each other, thus further minimizing the non-thermodynamic contributions to the measured voltage.

See also

• Concentration cell
• Ion transport number
• ITIES
• Electrochemical kinetics

References

• Advanced Physical Chemistry by Gurtu & Snehi
• Principles of Physical Chemistry by Puri, Sharma, Pathania

External links

• J. Phys. Chem. Elimination of the junction potențial with glass electrode

• Open source Liquid Junction Potential calculator

• Junction Potential Explanation Video

1. May, Peter M.; May, Eric F. (November 2025). "Junction potentials in electrochemical cells with transference: A review and prescription to end over 70 years of sleepwalking". Electrochimica Acta. 539: 147087. doi:10.1016/j.electacta.2025.147087.
2. Longhi, Paolo.; D'Andrea, Fernando.; Mussini, Patrizia R.; Mussini, Torquato.; Rondinini, Sandra. (1990-05-15). "Verification of the approximate equitransference of the aqueous potassium chloride salt bridge at high concentrations". Analytical Chemistry. 62 (10): 1019–1021. doi:10.1021/ac00209a011. ISSN   0003-2700.
3. Basili, Alessandro; Mussini, Patrizia R.; Mussini, Torquato; Rondinini, Sandra; Sala, Barbara; Vertova, Alberto (1999-09-01). "Transference Numbers of Alkali Chlorides and Characterization of Salt Bridges for Use in Methanol + Water Mixed Solvents". Journal of Chemical & Engineering Data. 44 (5): 1002–1008. doi:10.1021/je9900979. ISSN   0021-9568.
4. Iwunze, M. O.; Nnodimele, R. A. (January 1986). "An Evaluation of Cassava Starch as a Gelling Material for Electrochemical Salt Bridges". Starch - Stärke. 38 (6): 193–194. doi:10.1002/star.19860380605. ISSN   0038-9056.