Redox Couple

Redox Couple

Always every oxidation is accompanied by a reduction process. All such reactions are termed as oxidation-reduction reactions and shortly referred as redox. These redox reactions are associated with movements of electron. The electron donor is called as reductant or reducing agent and the electron acceptor, the oxidant or oxidizing agent. The system which transfer its electron is changed into oxidant form while the system which accepts electrons gets converted to the reductant form. Oxidation reduction system is simplified and shown below (fig 8.1)

A specific example is oxidation of ferrous iron to ferric iron indicates the removal of electron (e-)from ferrous iron.

Since, the electron is not stable in the free form it gets attached to another molecule and thus every oxidation is followed by a reduction. Always these redox reactions are energy yielding. A direct transfer of electrons from substrate to the oxidant would liberate a sudden burst of energy and most of it will be wasted by dissipation. Normally when the electrons of hydrogen combine with oxygen results in explosion. In biological system this oxidation reduction process takes place smoothly without increasing the temperature because the transfer of hydrogen pairs occurs in a step by step process till it reacts with oxygen. This permits the liberation of energy in small amounts so that it can be captured and saved.

Redox Potential

In oxidation and reduction reactions the free energy exchange is proportionate to the tendency of reactants to donate or to accept electrons. The affinity for the electron by the oxidant is called the electron affinity or redox potential. In biochemistry, the oxygen has the highest redox potential or electron affinity (E₀) and therefore the electron pass from the systems of hydrogen donors which have lower potentials. It is usual to compare the redox potential of a system (E₀) against the potential of the hydrogen electrode, which is at pH 0 designated as 0.0 volts. However, in a biological system it is normal to express the redox potential (E₀) at pH 7.0 at which the electrode potential of hydrogen electrode is - 0.42volts. In the biological system, the enzymes concerned with this oxidation reduction processes are designated as oxidoreductases. Some of the redox potentials are given in the table 8.1.

A positive value for the standard reduction potential means that a compound in question preferentially is reduced when involved in a redox reaction with hydrogen. A negative value means that a compound in question is preferentially oxidized. However, listings of standard reduction potentials are always given in the form of a reduction reaction.