Volta potential  [Volta potential between two metals and measurement of Volta potential]

Volta potential (also called Volta potential difference, or contact potential difference, or outer potential difference, Δψ, delta psi) in electrochemistry, is the electrostatic potential difference between two points ("1" and "2") in the vacuum:

• point "1" close to the surface of metal M₁;
• point "2" close to the surface of metal M₂ (or electrolyte);

where M₁ and M₂ are two metals that are in contact and in thermodynamic equilibrium.

The Volta potential is named after Alessandro Volta.

Volta potential between two metals

When two metals are electrically isolated from each other, an arbitrary potential difference may                                                                exist between them. However, when two different neutrally charged metals are brought into                                                                    electrical contact (even indirectly, say, through a long wire), electrons will flow from the metal                                                                                                          with a higher Fermi level to the metal with the lower Fermi level until the Fermi levels in the                                                                            both phases are equal. Once this has occurred, the metals are in thermodynamic equilibrium                                                                         with each other (the actual number of electrons that passes between the two phases is                                                                                usually small). Just because the Fermi levels are equal, however, does not mean that                                                                               the electric potentials are equal. The electric potential outside each material is controlled by                                                                     its work function, and so dissimilar metals can show an electric potential difference even at equilibrium.

The Volta potential is not an intrinsic property of the two bulk metals under consideration,                                                                                                        but rather is determined by work function differences between the metals' surfaces. Just like                                                                             the work function, the Volta potential depends sensitively on surface state, contamination, and so on.

Measurement of Volta potential (Kelvin probe)

The Volta potential can be significant (of order 1 volt) but it cannot be measured directly by an ordinary voltmeter                                              . A voltmeter does not measure vacuum electrostatic potentials, but instead the difference in                                                                     Fermi level between the two materials, a difference that is exactly zero at equilibrium.

The Volta potential however corresponds to a real electric field in the spaces between and around                                                                       the two metal objects, a field generated by the accumulation of charges at their surfaces.                                                                               The total charge  over each objects' surface depends on the capacitance  between the                                                                           two objects, by the relation  where  is the Volta potential. It follows therefore                                                                           that the value of the potential can be measured by varying the capacitance between the materials                                                                    by a known amount (e.g., by moving the objects further from each other), and measuring the                                                                         displaced charge that flows through the wire that connects them.

The Volta potential difference between a metal and an electrolyte can be measured in a similar fashion.                                                                                             The Volta potential of a metal surface can be mapped on very small scales by use of a Kelvin probe                                                                  force microscope. In this case the capacitance change is not known—instead, a compensating                                                                    DC voltage is added to cancel the Volta potential so that no current is induced by the change in                                                                  capacitance. This compensating voltage is the negative of the Volta potential.