Stability Constants and Their Measurement

Abstract

The equilibrium constant involving the formation of a metal complex from the aquo metal ion and the most basic form of the ligand is a standard measure of the effectiveness of the ligand in coordinating metal ions. The constants involved are called stability constants or formation constants. Most complex formation reactions are measured in aqueous medium under controlled conditions, and the formation constants generally apply to that medium. However, ligands that are not soluble in water but are soluble in organic solvents are frequently employed and their formation constants with metal ions are often determined in mixed solvents such as dioxane-water (up to 70% dioxane by volume); ethanol- and methanol-water systems are also quite common. For completely organic systems such as acetonitrile or tetrahydrofuran, metal complexes can be formed quite readily but their formation constants are generally not known. Approximate values have been used occasionally for such systems but there is no mathematical expression that can relate the equilibrium constants in such systems to the formation constants or stability constants in water or water/organic mixtures.