 This immediately leads to the Nernst equation, which for an electrochemical half-cell is
For a complete electrochemical reaction (full cell), the equation can be written as

where

Similarly to equilibrium constants, activities are always measured with respect to the standard state (1 mol/L for solutes, 1 atm for gases).The activity of species X, , can be related to the physical concentrations via , where is the activity coefficient of species X.Because activity coefficients tend to unity at low concentrations, activities in the Nernst equation are frequently replaced by simple concentrations.