is the cell potential (electromotive force) at the temperature of interest, is the standard cell potential, is the universal gas constant: , is the temperature in kelvins, is the number of electrons transferred in the cell reaction or half-reaction, is the Faraday constant, the magnitude of charge (in coulombs) per mole of electrons: , is the reaction quotient of the cell reaction, and is the chemical activity for the relevant species, where is the activity of the reduced form and is the activity of the oxidized form. Thermal voltage At room temperature (25 °C), the thermal voltage is approximately 25.693 mV. The Nernst equation is frequently expressed in terms of base-10 logarithms (i.e., common logarithms) rather than natural logarithms, in which case it is written: where λ = ln(10) ≈ 2.3026 and λVT ≈ 0.05916 Volt. Form with activity coefficients and concentrations Similarly to equilibrium constants, activities are always measured with respect to the standard state (1 mol/L for solutes, 1 atm for gases, and T = 298.15 K, i.e., 25 °C or 77 °F). The chemical activity of a species , , is related to the measured concentration via the relationship , where is the activity coefficient of the species . 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 used. Taking into account the activity coefficients () the Nernst equation becomes: Where the first term including the activity coefficients () is denoted and called the formal standard reduction potential, so that can be directly expressed as a function of and the concentrations in the simplest form of the Nernst equation: Formal standard reduction potential When wishing to use simple concentrations in place of activities, but that the activity coefficients are far from unity and can no longer be neglected and are unknown or too difficult to determine, it can be convenient to introduce the notion of the "so-called" standard formal reduction potential () which is related to the standard reduction potential as follows: So that the Nernst equation for the half-cell reaction can be correctly formally written in terms of concentrations as: and likewise for the full cell expression. According to Wenzel (2020), a formal reduction potential is the reduction potential that applies to a half reaction under a