Standard Article

Stability Constants & Their Determination

  1. Alan T. Hutton,
  2. Peter W. Linder

Published Online: 15 MAR 2006

DOI: 10.1002/0470862106.ia227

Encyclopedia of Inorganic Chemistry

Encyclopedia of Inorganic Chemistry

How to Cite

Hutton, A. T. and Linder, P. W. 2006. Stability Constants & Their Determination . Encyclopedia of Inorganic Chemistry. .

Author Information

  1. University of Cape Town, Rondebosch, South Africa

Publication History

  1. Published Online: 15 MAR 2006

Abstract

This article is concerned with quantitative aspects of reactions in solution between metal ions and ligands to form complexes. The equilibrium constants of these reactions are usually called stability constants or formation constants. Formal definitions are presented of stability constants and a distinction is made between thermodynamic constants (activity quotients) and stoichiometric constants (concentration quotients). The practical difficulties of determining thermodynamic constants are discussed and the general resort to measuring stoichiometric constants and its implications are rationalized. The important idea and usefulness of formation curves for a given metal–ligand system are described. Mention is made of old-fashioned curve-fitting procedures for processing the relevant experimental data and references are given to the currently more important computer programs that have been developed for the purpose. Brief descriptions, especially of the underlying principles, are given of a selection of the experimental methods that have been developed for determining stability constants, that is, potentiometry, voltammetric methods (polarography and anodic stripping voltammetry), pH-metry, competitive equilibria, use of cation exchange resins, liquid–liquid partition, spectophotometric methods, and nuclear magnetic resonance spectroscopy. Mention is made of a few other methods. pH-metry is by far the most widely used and arguably the most sensitive and accurate of all the methods that have been developed. Justification for the considerable effort that has been, and continues to be, put into determining stability constants is presented in a section that summarizes the uses of stability constants. In particular, applications considered are to extraction metallurgy, the nuclear energy industry, analytical methods, and to medical, environmental, and industrial research. These applications all require stability constant values of high reliability and sources of critically evaluated published constants are referenced. Finally, a summary is presented of contributions that have been made to some fundamental ideas of chemistry arising from consideration of stability constant values.

Keywords:

  • stability constants;
  • formation constants;
  • metal–ligand equilibria;
  • formation curves;
  • speciation;
  • potentiometry;
  • computer programs;
  • modeling;
  • databases;
  • biological fluids;
  • environmental chemistry