Liquid junction potential
Encyclopedia
Liquid junction potential occurs when two solutions of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated one. The rate of diffusion of each ion
will be roughly proportional to its speed in an electric field. If the anion diffuses more rapidly than the cation, it will diffuse ahead into the dilute solution, leaving the latter negatively charged
and the concentrated solution positively charged. So an electrical double layer of positive and negative charges will be produced at the junction of the two solutions. So at the point of junction, a potential difference
will develop because of the ionic transfer
. This potential is called liquid junction potential or diffusion potential. The magnitude of the potential depends on the relative speeds of the ions.
(EMF) of a concentration cell
with transference includes the liquid junction potential.
where a1 and a2 are activities of HCl in the two solutions, R is the Universal Gas Constant, T is the temperature and F is Faraday's Constant.
where a2 and a1 are activities of HCl solutions of right and left hand electrodes respectively and tM be transport number of Cl–
(KCl) and Ammonium nitrate (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 of anion and cation are the same.
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
will be roughly proportional to its speed in an electric field. If the anion diffuses more rapidly than the cation, it will diffuse ahead into the dilute solution, leaving the latter negatively charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
and the concentrated solution positively charged. So an electrical double layer of positive and negative charges will be produced at the junction of the two solutions. So at the point of junction, a potential difference
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
will develop because of the ionic transfer
Ionic transfer
Ionic transfer is a term which can be used to describe the transfer of ions from one liquid phase to another.This is related to the phase transfer catalysts which are a special type of liquid-liquid extraction which is used in synthetic chemistry....
. This potential is called liquid junction potential or diffusion potential. The magnitude of the potential depends on the relative speeds of the ions.
Calculation
The liquid junction potential can not be measured directly but calculated. The Electromotive forceElectromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
(EMF) of a concentration cell
Concentration cell
A concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same material differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst Equation. A concentration cell produces a voltage as it attempts to reach...
with transference includes the liquid junction potential.
- Ewithout transference = RT/F . ln(a2/a1)
where a1 and a2 are activities of HCl in the two solutions, R is the Universal Gas Constant, T is the temperature and F is Faraday's Constant.
- Ewith transference = tM RT/F . ln (a2/a1)
where a2 and a1 are activities of HCl solutions of right and left hand electrodes respectively and tM be transport number of Cl–
- Liquid Junction potential = Ewith transference– Ewithout transference = (tM – 1) RT/F . ln (a2/a1)
Elimination of liquid junction potential
The liquid junction potential interferes with the exact measurement of EMF of a chemical cell. So it should be eliminated or at least its effect should be minimized. The general practice to eliminate the liquid junction potential is to place a salt bridge consisting of usually a saturated solution of Potassium chloridePotassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
(KCl) and Ammonium nitrate (NH4NO3) with Lithium acetate
Lithium acetate
Lithium acetate is a salt of lithium and acetic acid.- 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 be run at higher speeds than can gels made from TAE buffer...
(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 of anion and cation are the same.