each activity is replaced by the product of a concentration and an activity coefficient, the equilibrium constant is defined as where [S] denotes the concentration of S, etc. In practice equilibrium constants are determined in a medium such that the quotient of activity coefficient is constant and can be ignored, leading to the usual expression which applies under the conditions that the activity quotient has a particular (constant) value. References External links AIOMFAC online-model An interactive group-contribution model for the calculation of activity coefficients in organic–inorganic mixtures. Electrochimica Acta Single-ion activity coefficients Thermodynamic models Equilibrium chemistry Dimensionless numbers of chemistry