Liquid junction potential (shortly LJP) occurs when two solutions of

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

s of different concentrations are in contact with each other. The more concentrated solution will have a tendency to

diffuse Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

into the comparatively less concentrated one. The rate of diffusion of each

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...

will be roughly proportional to its speed in an

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field f ...

, or their ion mobility. If the

anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

diffuse more rapidly than the

cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, they will diffuse ahead into the dilute solution, leaving the latter negatively charged and the concentrated solution positively charged. This will result in an

electrical double layer A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The D ...

of positive and negative charges at the junction of the two solutions. Thus at the point of junction, a

potential difference Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to mo ...

will develop because of the ionic transfer. This potential is called liquid junction potential or

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

potential which is non-equilibrium potential. The magnitude of the potential depends on the relative speeds of the 'ions' movement.

Calculation

The liquid junction potential cannot be measured directly but calculated. The

electromotive force In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transd ...

(EMF) of a

concentration cell In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nern ...

with transference includes the liquid junction potential. The EMF of a concentration cell without transport is: :

E_\mathrm = \frac \ln \frac

where

a_1

and

a_2

are activities of HCl in the two solutions,

R

is the

universal gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment pe ...

T

is the temperature and

F

is the

Faraday constant In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of SI ...

. The EMF of a concentration cell with transport (including the

ion transport number In chemistry, ion transport number, also called the transference number, is the fraction of the total electric current carried in an electrolyte by a given ionic species : :t_i = \frac Differences in transport number arise from differences in el ...

) is: :

E_\mathrm = 2t_M\frac \ln \frac

where

a_2

and

a_1

are activities of HCl solutions of right and left hand electrodes, respectively, and

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: :

E_\mathrm = E_\mathrm - E_\mathrm = (2t_M - 1)\frac \ln \frac

Elimination

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. The most common method of eliminating the liquid junction potential is to place a

salt bridge In electrochemistry, a salt bridge or ion bridge is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell (voltaic cell), a type of electrochemical cell. It maintains electrical neutrality within the in ...

consisting of a saturated solution of

potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...

(KCl) and

ammonium nitrate Ammonium nitrate is a chemical compound with the chemical formula . It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is p ...

(NH₄NO₃) with

lithium acetate Lithium acetate (CH3COOLi) is a salt of lithium and acetic acid. It is often abbreviated as LiOAc. Uses Lithium acetate is used in the laboratory as buffer for gel electrophoresis of DNA and RNA. It has a lower electrical conductivity and can ...

(CH₃COOLi) 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 whole of the current across the boundary. The efficiency of KCl/NH₄NO₃ is connected with the fact that in these salts, the transport numbers{{Clarify, date=June 2013 of anions and cations are the same.

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
Diffusion Ions Physical chemistry Electrochemistry Electrochemical potentials

Calculation

Elimination

See also

References

External links