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electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an out ...
, the Galvani potential (also called Galvani potential difference, or inner potential difference, Δφ, delta phi) is the electric potential difference between two points in the bulk of two phases. These phases can be two different
solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...
s (e.g., two
metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...
joined together), or a solid and a
liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, ...
(e.g., a metal
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
submerged in an
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 ...
). The Galvani potential is named after
Luigi Galvani Luigi Galvani (, also ; ; la, Aloysius Galvanus; 9 September 1737 – 4 December 1798) was an Italian physician, physicist, biologist and philosopher, who studied animal electricity. In 1780, he discovered that the muscles of dead frogs' legs ...
.


Galvani potential between two metals

First, consider the Galvani potential between two metals. When two metals are electrically isolated from each other, an arbitrary voltage difference may exist between them. However, when two different metals are brought into electronic contact, electrons will flow from the metal with a lower voltage to the metal with the higher voltage until the
Fermi level The Fermi level of a solid-state body is the thermodynamic work required to add one electron to the body. It is a thermodynamic quantity usually denoted by ''µ'' or ''E''F for brevity. The Fermi level does not include the work required to remove ...
of the electrons in the bulk of both phases are equal. The actual numbers of electrons that passes between the two phases is small (it depends on the capacitance between the objects), and the occupancies of the electron bands are practically unaffected. Rather, this small increase or decrease in charge results in a shift in all the energy levels in the metals. An electrical double layer is formed at the interface between the two phases. The equality of the electrochemical potential between the two different phases in contact can be written as: :\overline_j^ = \overline_j^ where: *\overline is the electrochemical potential *j denotes the species which are the carrier of electric current in the system (which are electrons in metals) *(1) and (2) denote phase 1 and phase 2, respectively. Now, the electrochemical potential of a species is defined as a sum of its chemical potential and the local electrostatic potential: :\overline_j = \mu_j + z_j F \phi where: *μ is the
chemical potential In thermodynamics, the chemical potential of a species is the energy that can be absorbed or released due to a change of the particle number of the given species, e.g. in a chemical reaction or phase transition. The chemical potential of a speci ...
*z is the electrical charge carried by a single charge carrier (unity for electrons) *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 ...
*Φ is the electrostatic potential From the two equations above: :\phi^ - \phi^ = \frac where the difference on the left-hand side is the Galvani potential difference between the phases (1) and (2). Thus, the Galvani potential difference is determined entirely by the chemical difference of the two phases; specifically by the difference of the chemical potential of the charge carriers in the two phases. The Galvani potential difference between an electrode and electrolyte (or between other two electrically conductive phases) forms in an analogous fashion, although the chemical potentials in the equation above may need to include all species involved in the electrochemical reaction at the interface.


Relation to measured cell potential

The Galvani potential difference is not directly measurable using voltmeters. The measured potential difference between two metal electrodes assembled into a cell does not equal the difference of the Galvani potentials of the two metals (or their combination with the solution Galvani potential) because the cell needs to contain another metal-metal interface, as in the following schematic of a
galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
: :M(1) , S , M(2) , M(1)' where: *M(1) and M(2) are the two different metals, *S denotes the electrolyte, *M(1)' is the additional metal (here assumed to be the metal (1)) that must be inserted into the circuit to close it, *the vertical bar, , , denotes a phase boundary. Instead, the measured cell potential can be written as: :E^ - E^ = \left(\phi^ - \phi^ - \frac \right) - \left(\phi^ - \phi^ - \frac \right) = \left(\phi^ - \phi^\right) - \left(\frac \right) where: *E is the potential of a single electrode, *(S) denotes the electrolyte solution. From the above equation, two metals in electronic contact (i.e., under electronic equilibrium) must have the same electrode potential. Also, the electrochemical potentials of the electrons within the two metals will be the same. However, their Galvani potentials will be different (unless the metals are identical). Moreover, if define \pi, the ''electric potential'' (or the ''electromotive potential in'' , as :\pi=-\frac+\phi, which is effectively negative of the reduced electrochemical potential of electrons given in units of volts. It is noted that what one experimentally measures using an inert metallic probe and a voltmeter is \pi.


See also

* Absolute electrode potential *
Electrode potential In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ...
* ITIES (interface between two immiscible electrolyte solutions) * Volta potential * Donnan potential


References

{{Reflist Electrochemical concepts Electrochemical potentials