Redox potential:

The oxidation of a molecule includes the loss of electrons.  The reduction of a molecule   includes the gain of electrons.   Because  electrons   are  not  formed  or destroyed  in a chemical  reaction if one molecule  is oxidized  other  must be decreased  instance example it is an oxidation-reduction reaction.  Therefore, through definition, oxidation-reduction reactions included the transfer of the electrons.  In the oxidation-reduction reaction:

NADH+H⁺  + ½ O₂ ↔ NAD⁺  +H₂O   

when the NADH  is oxidized  to NAD⁺ it will lose electrons. When the molecular oxygen is reduced to water and it gains electrons.

The E, oxidation-reduction potential or redox potential is a measure of the affinity of a substance for electrons and is measured relative to hydrogen. An optimistic redox potential means that the substance has a higher affinity for electrons than does hydrogen and so would accept electrons from hydrogen. A core with  a  negative  redox  potential  has  a  lower  affinity  for  electrons  than  does hydrogen and would donate electrons to H⁺, creating hydrogen. In the instance above, NADH is a strong reducing agent with a negative redox potential and has a tendency to donate the electrons. Oxygen is a strong oxidizing agent with a positive redox potential and has a tendency to accept electrons.

For biological  systems,  the typical  redox  potential  for a substance  (E₀') is measured  under standard  conditions,  at pH 7 and it is expressed  in volts. Within an oxidation-reduction reaction where electron transfer is occurring the total voltage modifies of the reaction (modification in electric potential and ΔE) it is the sum of the voltage changes of the individual oxidation-reduction steps. The ordinary free energy change of a reaction at pH 7, ΔG⁰′, can be readily calculated from the change in redox potential ΔE₀′ of the substrates and products:

?G⁰'= -nF ?E₀'

where n is the number of electrons transferred, ΔE₀ ′ is in volts (V), ΔG⁰′ is in kilocalories per mole (kcal mol^-1) and F is a constant called the Faraday  with 23.06 kcal V^-1 mol^-1. Note that a reaction with a positive ΔE₀′ has a negative ΔG⁰′ like is exergonic.

Thus for the reaction:

NADH+H⁺+ ½ O₂↔NAD⁺+H₂O

?E₀'=+1.14V

?G⁰'=-52.6 kcal mol^-1