Editing Nernst equation (section)

===Form with activity coefficients and concentrations===

Similarly to equilibrium constants, activities are always measured with respect to the [[standard state]] (1&nbsp;mol/L for solutes, 1&nbsp;atm for gases, and T = 298.15 K, ''i.e.'', 25 °C or 77 °F). The chemical activity of a species {{math|i}}, {{math|''a''<sub>i</sub>}}, is related to the measured concentration {{math|''C''<sub>i</sub>}} via the relationship {{math|1=''a''<sub>i</sub> = ''γ''<sub>i</sub> ''C''<sub>i</sub>}}, where {{math|''γ''<sub>i</sub>}} is the [[activity coefficient]] of the species {{math|i}}. Because activity coefficients tend to unity at low concentrations, or are unknown or difficult to determine at medium and high concentrations, activities in the Nernst equation are frequently replaced by simple concentrations and then, formal standard reduction potentials <math>E^{\ominus'}_\text{red}</math> used.  

Taking into account the activity coefficients (<math>\gamma</math>) the Nernst equation becomes: 

<math display="block">E_\text{red} = E^\ominus_\text{red} - \frac{RT}{zF} \ln\left(\frac{\gamma_\text{Red}}{\gamma_\text{Ox}}\frac{C_\text{Red}}{C_\text{Ox}}\right)</math>

<math display="block">E_\text{red} = E^\ominus_\text{red} - \frac{RT}{zF} \left(\ln\frac{\gamma_\text{Red}}{\gamma_\text{Ox}} + \ln\frac{C_\text{Red}}{C_\text{Ox}}\right)</math>

<math display="block">E_\text{red} = \underbrace{\left(E^\ominus_\text{red} - \frac{RT}{zF} \ln\frac{\gamma_\text{Red}}{\gamma_\text{Ox}}\right)}_{E^{\ominus '}_\text{red}} - \frac{RT}{zF} \ln\frac{C_\text{Red}}{C_\text{Ox}}</math>

Where the first term including the [[activity coefficient]]s (<math>\gamma</math>) is denoted <math>E^{\ominus '}_\text{red}</math> and called the formal standard reduction potential, so that <math>E_\text{red}</math> can be directly expressed as a function of <math>E^{\ominus '}_\text{red}</math> and the concentrations in the simplest form of the Nernst equation:
 
<math display="block">E_\text{red}=E^{\ominus '}_\text{red} - \frac{RT}{zF} \ln\frac{C_\text{Red}}{C_\text{Ox}}</math>