A redox (reduction-oxidation) reaction is any of a large class of chemical reactions, including many familiar processes such as combustion, corrosion, and respiration. Oxidation was originally defined simply as the combination of an element or compound with oxygen, or the removal of hydrogen from a compound; and reduction as combination with hydrogen or removal of oxygen. In the modern theory has been generalized: oxidation is defined as loss of electrons, and reduction as gain of electrons. The two always go together: there is an oxidizing agent which is reduced, and a reducing agent which is oxidized. Thus in the reaction

 

    Fe³⁺ + I ^– → Fe²⁺ + ½I₂

 

the iron (III) gains ion gains an electron and is reduced to iron (II), and the iodide ion loses ion loses an electron and is oxidized to iodine.

 

The strength of a redox reagent, expressing its tendency to react, is measured by the electrode potential of the half-reaction, and so redox reagents may be ranked in an extended electrochemical series. In a covalent compound or complex ion, each atom is assigned an oxidation number, which is the charge it would have if all the bonds were ionic – the electrons in a covalent bond between two atoms are assigned to the atom with the higher electronegativity. Thus the sulfur atom in sulfur dioxide has an oxidation number of +4, and each of the two oxygen atoms has an oxidation number of -2. When sulfur is oxidized by oxygen, S + O₂ → SO₂, its oxidation number increases from 0 (for elements, by definition) to +4, corresponding to a virtual loss of electrons.

 

Disproportionation

Disproportionation is the simultaneous oxidation-reduction of the same chemical substance. An example is the disproportionation of copper (I) chloride, involving oxidation to copper (II) chloride and reduction to metallic copper:

 

    2CuCl → CuCl₂ + Cl.

 

Redox potential

The redox potential is the voltage difference at an inert electrode immersed in a reversible oxidation-reduction system. Redox potential measures the affinity of a substance for electrons compared with that of hydrogen, whose redox potential is set at 0. Strong oxidizing agents have a positive redox potential; strong reducing agents have a negative redox potential. Electrons move spontaneously towards those compounds with the more positive redox potentials.

 

The standard for the whole scale is pure hydrogen gas at 1 atmosphere pressure in contact with a platinum electrode immersed in 1 molar acid. This combination is called a standard hydrogen electrode (SHE) and is defined to have a redox potential of zero.

 

Redox sensor

A redox sensor is an instrument capable of detecting the kind of non-equilibrium concentrations of reduced iron and manganese, along with the oxides of nitrogen and sulfur, which are strongly indicative of life processes on Earth. Miniature redox sensors have recently been developed that are capable of withstanding high pressure liquid environments. It is thought that arrays of such sensors could form part of the instrumentation on a probe to search for life in Lake Vostok and, eventually, in oceans that might exist on Jupiter's moons Europa and Callisto.