Equilibrium in the Solutions of Complex Compounds

Solvated ions, which form when dissolving substances in some solvent W, in essence are coordination compounds with a saturated inner coordination sphere. If the solvent is water, the so-called aqua complexes form. In this case, the number of immediately bound monodentate ligands (H2O molecules) is equal to the coordination number, N, of the metal ion. 

Upon addition of ligands X and Y to the solution containing solvate complexes MW , the former can displace part of W molecules in the inner coordination sphere forming extra complexes. Usually, this process takes place until equilibrium is established  

X should be read as the activity of X and likewise for the other terms in curly brackets. The reciprocal quantity is called instability constant. As activity is the product of concentration and activity coefficient (y), this definition could also be written as 

To avoid the complications involved in using activities, stability constants are determined, where possible, in a medium consisting of a solution of a background electrolyte at high ionic strength, that is, under conditions in which y can be assumed to be always constant. Any chemical interactions between the species in equilibrium and the background electrolyte are unwanted, but such interactions 

stability constants are determined for experimental conditions that deviate from the standard conditions used by convention in thermodynamics and refer to concentrations, but not to activities. Such conditional (apparent) stability constants are used whenever the activity coefficients and concentrations of the species are not known or not accessible, or when these simplifications are sufficient to treat certain equilibrium. As outlined next, 0 values expressed in concentration terms, that is. 

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