> Oxidation occurs when a chemical species looses an electron.

> Reduction occurs when a chemical species gains an electron.

For example, the reaction

may be decomposed into two half reactions

In this reaction, iron is oxidized and hydrogen is reduced.

> An oxidant species promotes oxidation and a reductant promotes reduction.

In the above reaction, the proton is the oxidant or oxidation agent while iron (II) is the reductant or reducing agent.

The propensity for a species to undergo oxidation or reduction, as well as the propensity of a particular reaction, can be given by

All of these are quantitative measures of propensity of the reaction and do not take into account the kinetics or rate at which a reaction can occur.

Chemical systems that are at equilibrium are characterized by these quantities throughout the volume.

Chemical systems that are not at equilibrium may still be characterized by these quantities, but each species, each location, or each time, will have a different characteristic value.

There are two chemical potentials used by chemists.

The first is the potential for an oxidation or reduction system at "standard-state conditions". It is given by the symbol E^o.

The standard-state potential is given as either a standard oxidation potential or a standard reduction potential. The two are related only through a sign change and are specified in Volts.

The reduction potential is the IUPAC standard. A reduction potential is stated such that the species is being reduced. For example, the standard reduction potentials for the Fe²⁺/H⁺ half reactions are

The second potential used by chemists is the actual or measured potential. This potential varies with activity (concentration) of the species.

The equation that describes how the potential changes with activity is the Nernst equation

ln10 = 2.303. At room temperature

and

The Q factor in the Nernst equation is the standard activity of the products relative to reactants.

The chemical activities, A, are measures of chemically active concentration, relative to a standard state.

Important standard states are

1. for solutions, the activity of a solute, S, is A_S =[S] as the molar concentration, [S], approaches zero
2. for gases in solution, the activity of a dissolved gas, G, is A_G =1 as the partial pressure of the gas, P_G, approaches one Atmosphere pressure. At P_G = 1 Atm, we say we are at "unit fugacity".
3. for solids in contact with a solution, the activity of the solid, S, is unity, A_S = 1, as the mole fraction, X_S, approaches 1.

What this all means is

3) for solids, use the mole fraction of the solid, A_S = X_S

A "p" unit is a log₁₀-based quantity. We can report the activity of any chemical species in p units. The formula for this is

Thus the pH is

which, for low concentration solutions is

The pE is the -log₁₀ of the electron activity

The pE is related to the reduction potential by the FRT factor of the Nernst equation

In fact, the Nernst equation is typically used to determine pE.

The Nernst equation may be recast using pE formalism

which also defines pE⁰.