Chemical
potential
Electrode potential
Electric potential
Redox potential
Standard potential
Thermodynamic potential
The chemical potential mA of a substance A in a mixture of substances A, B, C... is defined by the equation:
where G is the free enthalpy of the mixture, T the Absolute temperature (degrees Kelvin), p the pressure, and nA is the quantity of substance A considered, and where nB , nC are the quantities of the other components of the mixture.
The electrode potential is equal to the electromotive force of the cell made up of an electrode and the standard hydrogen electrode whose potential is, by convention, equal to zero. Indeed, the potential difference of the cell (DE) is written as
DE = E2 - E1
where E1 is the potential
of electrode 1 and E2 is the potential of electrode 2.
If electrode 1 is the standard hydrogen electrode, E1 = 0
(by convention),
we have : E2 = DE which is an experimentally
measurable quantity.
In electrical circuit, the practical unit of electric potential is the volt.
The system made up of a conducting metal rod dipping into a solution of a conducting electrolyte is called a "half cell" or electrode.
The potential difference at the interface between metal and the solution is called the "electrochemical potential of the electrode".
A thermodynamic definition of the "electrochemical potential" can be written in a similar way to that for the chemical potential. When a system exchanges electric work Welec with the external medium, the free electochemical enthalpy Gelec can be defined according to the relation:
dGelec = dG + dWelec
The electrochemical potential mA is defined by:
This is the name given to the potential of an electrode obtained by dipping an electrode into the solution and where the electrolyte can exist in different oxidation states. It is determined by measuring of the electromotive force of the cell made up of this electrode and the standard hydrogen electrode whose potential is by convention equal to zero.
This is the potential of a redox couple (OX/RED) measured when the activities of the oxidised and reduced forms of the couple are equal to 1.
The Nernst potential for a OX/RED couple when OX = RED is given by
E = E° + RT / nF ln ( [OX] / [RED] )
where E° is the standard potential of the redox couple and [OX], [RED] signify the concentration of the respective species.
This is a function whose minimum gives the equilibrium state of a system subject to constraints, e.g. free enthalpy G (temperature and constant pressure) and free energy F (constant temperature and volume).