The  and pH of a solution are related by the Nernst equation as commonly represented by a Pourbaix diagram . For a half cell equation, conventionally written as a reduction reaction (i.e., electrons accepted by an oxidant on the left side):
The half-cell standard reduction potential  is given by
where  is the standard Gibbs free energy change,  is the number of electrons involved, and  is Faraday's constant. The Nernst equation relates pH and :
where curly braces { } indicate activities, and exponents are shown in the conventional manner. as a function of pH with a slope of  volt (pH has no units).
This equation predicts lower  at higher pH values.  This is observed for the reduction of O into HO, or OH, and for reduction of H into H.