temperatures exist. Expression of the Nernst equation as a function of pH The and pH of a solution are related by the Nernst equation as commonly represented by a Pourbaix diagram . explicitly denotes expressed versus the standard hydrogen electrode (SHE). For a half cell equation, conventionally written as a reduction reaction (i.e., electrons accepted by an oxidant on the left side): The equilibrium constant of this reduction reaction is: where curly braces { } indicate activities (), rectangle braces [ ] denote molar or molal concentrations (), represent the activity coefficients, and the stoichiometric coefficients are shown as exponents. Activities correspond to thermodynamic concentrations and take into account the electrostatic interactions between ions present in solution. When the concentrations are not too high, the activity () can be related to the measurable concentration () by a linear relationship with the activity coefficient (): 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 the Faraday's constant. The Nernst equation relates pH and as follows: In the following, the Nernst slope (or thermal voltage) is used, which has a value of 0.02569... V at STP. When base-10 logarithms are used, VT λ = 0.05916... V at STP where λ = ln[10] = 2.3026. This equation is the equation of a straight line for 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 O2 into H2O, or OH−, and for reduction of H+ into H2. is then often noted as to indicate that it refers to the standard hydrogen electrode (SHE) whose = 0 by convention under standard conditions (T = 298.15 K = 25 °C = 77 F, Pgas = 1 atm (1.013 bar), concentrations = 1 M and thus pH = 0). Calculation of a Pourbaix diagram When the activities () can be considered as equal to the molar, or the molal, concentrations () at sufficiently diluted concentrations when the activity coefficients () tend to one, the term regrouping all the activity coefficients is equal to one, and the Nernst equation can be written simply with the concentrations () denoted here with square braces [ ]: There are three types of line boundaries in a Pourbaix diagram: Vertical, horizontal, and sloped. Vertical boundary line When