a negative Gibbs free energy and a positive potential . The Gibbs free energy is related to the entropy by , where is the enthalpy and is the temperature of the system. Using these relations, we can now write the change in Gibbs free energy, and the cell potential, This is the more general form of the Nernst equation. For the redox reaction , and we have: The cell potential at standard temperature and pressure (STP) is often replaced by the formal potential , which includes the activity coefficients of the dissolved species under given experimental conditions (T, P, ionic strength, pH, and complexing agents) and is the potential that is actually measured in an electrochemical cell. Relation to the chemical equilibrium The standard Gibbs free energy is related to the equilibrium constant as follows: At the same time, is also equal to the product of the total charge () transferred during the reaction and the cell potential (): The sign is negative, because the considered system performs the work and thus releases energy. So, And therefore: Starting from the Nernst equation, one can also demonstrate the same relationship in the reverse way. At chemical equilibrium, or thermodynamic equilibrium, the electrochemical potential and therefore the reaction quotient () attains the special value known as the equilibrium constant (): Therefore, Or at standard state, We have thus related the standard electrode potential and the equilibrium constant of a redox reaction. Limitations In dilute solutions, the Nernst equation can be expressed directly in the terms of concentrations (since activity coefficients are close to unity). But at higher concentrations, the true activities of the ions must be used. This complicates the use of the Nernst equation, since estimation of non-ideal activities of ions generally requires experimental measurements. The Nernst equation also only applies when there is no net current flow through the electrode. The activity of ions at the electrode surface changes when there is current flow, and there are additional overpotential and resistive loss terms which contribute to the measured potential. At very low concentrations of the potential-determining ions, the potential predicted by Nernst equation approaches toward . This is physically meaningless because, under such conditions, the exchange current density becomes very low, and there may be no thermodynamic equilibrium necessary for Nernst equation to hold. The electrode is called unpoised in such case. Other effects tend to take control of the