. This particular concentration quotient, , has the dimension of concentration, but the thermodynamic equilibrium constant, , is always dimensionless. Unknown activity coefficient values It is very rare for activity coefficient values to have been determined experimentally for a system at equilibrium. There are three options for dealing with the situation where activity coefficient values are not known from experimental measurements. Use calculated activity coefficients, together with concentrations of reactants. For equilibria in solution estimates of the activity coefficients of charged species can be obtained using Debye–Hückel theory, an extended version, or SIT theory. For uncharged species, the activity coefficient γ0 mostly follows a "salting-out" model: log10 γ0 = bI where I stands for ionic strength. Assume that the activity coefficients are all equal to 1. This is acceptable when all concentrations are very low. For equilibria in solution use a medium of high ionic strength. In effect this redefines the standard state as referring to the medium. Activity coefficients in the standard state are, by definition, equal to 1. The value of an equilibrium constant determined in this manner is dependent on the ionic strength. When published constants refer to an ionic strength other than the one required for a particular application, they may be adjusted by means of specific ion theory (SIT) and other theories. Dimensionality An equilibrium constant is related to the standard Gibbs free energy change, , for the reaction by the expression Therefore, K, must be a number from which a logarithm can be derived. In the case of a simple equilibrium A + B <=> AB the thermodynamic equilibrium constant is defined in terms of the activities, {AB}, {A} and {B}, of the species in equilibrium with each other. Now, each activity term can be expressed as a product of a concentration and a corresponding activity coefficient, . Therefore, When , the quotient of activity coefficients, is set equal to 1, we get . K then appears to have the dimension of 1/concentration. This is what usually happens in practice when an equilibrium constant is calculated as a quotient of concentration values. This can be avoided by dividing each concentration by its standard-state value (usually mol/L or bar), which is standard practice in chemistry. The assumption underlying this practice is that the quotient of activities is constant under the conditions in which the equilibrium constant value is determined. These conditions are usually achieved by