Conditional Equilibrium Constants Reviewed.

A free energy-based conceptual theoretical framework from which the conditional equilibrium constant can be comprehensibly understood is presented. This constant is found to be a weighted geometric mean of the equilibrium constants of the reactions of all forms of the conditioned species under buffering conditions, where the weight is given by a function of their predominance in terms of their mole fractions. Once it is shown that this type of equilibrium constant can be easily deduced form free energy functions, it is shown how corrections for activity coefficient can be incorporated as well. The framework additionally permits to interpret side-reactions coefficients as free energy terms related to the chemical speciation of the system, allowing the use of the generalization of Hess' law to obtain conditional constants and a straightforward deduction of multiconditional equilibrium constants. Furthermore, different uses of the conditional constants along the actual literature are reviewed as well allowing to have a complete and updated panorama of the employment of this important concept in chemical and speciation analysis in many areas of research.