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

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

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 convent ...

will be roughly proportional to its speed in an

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

, 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 Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

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

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) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical ''transducer ...

(EMF) of a concentration cell 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, temperature ...

T

is the temperature and

F

is the

Faraday constant In physical chemistry, the Faraday constant (symbol , sometimes stylized as ℱ) is a physical constant defined as the quotient of the total electric charge () by the amount () of elementary charge carriers in any given sample of matter: it ...

. 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 an essential laboratory device discovered over 100 years ago. It contains an electrolyte solution, typically an inert solution, used to connect the Redox, oxidation and reduction Half cell, ...

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 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, but does not form hydrates. It is predominantly us ...

(NH₄NO₃) with lithium acetate (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