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Chemistry International
Vol. 24, No. 2
March 2002


Highlights from Pure and Applied Chemistry

Critical Evaluation of Stability Constants of Phosphonic Acids (IUPAC Technical Report)

by Konstantin Popov, Hannu Rönkkömäki, and Lauri H. J. Lajunen
Pure and Applied Chemistry Vol. 73, No. 10, pp. 1641-1677 (2001)

Organophosphonate chelating compounds are widely used in a broad variety of applications.Their ability to prevent precipitation of calcium salts at substoichiometric concentrations (threshold effect) finds wide application in water treatment for scale inhibition. Phosphonates, particularly EDTPH and DTPPH, are used extensively in laundry detergents. These materials are also used as corrosion inhibitors, in industrial cleaning and in peroxy bleach stabilization. Uses of organophosphonates span applications in flame-resistant polymers, photographic processing, ore flotation (aminophosphonic surfactants), actinide separation processes, and analytical chemistry. Recently, organophosphonates have been identified as promising reagents for the creation of so-called �structurally tailored� materials and microporous materials, in catalysis, and in the electrochemical treatment of polluted soils.

The high biological activity of carboxyalkylphosphonates, aminoalkylphosphonates, and alkylenediphosphonates makes them useful agents as components of microfertilizers and pesticides in agriculture, as well as drugs and diagnostic reagents in biology and medicine. Annual industrial output of organophosphonates is in the thousands of tons.

The broad and intensive applications of organophosphonates require reliable data on the stability constants of the corresponding complexes in order to permit equilibrium modeling and prediction of the important technological, environmental, and pharmacokinetic equilibria. As phosphonates resist biodegradation, chemical speciation calculations based on numerical equilibrium data are of extreme importance for applications such as environmental science, waste management, agriculture, scale inhibition, magnetic resonance imaging, behavior of radiopharmaceuticals in blood plasma, and many others.

The data of interest are partly accumulated in some monographs, reviews, and compilations of stability constants. Recently, two computer databases have become commercially available. The IUPAC Stability Constants Database (SC-Database) is the more comprehensive. For organophosphonates, some data in the �critically evaluated compilations� are not consistent with later, more precise results. These are therefore rejected in the present work.

This study critically evaluated available experimental data on stability constants of proton and metal complexes for 10 phosphonic acids [methylphosphonic acid, 1-hydroxyethane- 1,1-diylbisphosphonic acid, dichloromethylenebisphosphonic acid, aminomethanephosphonic acid, N-(phosphonomethyl) glycine, imino-N, N-bis(methylenephosphonic acid), N-methylamino-N, N-bis(methylenephosphonic acid), nitrilotris(methylenephosphonic acid), 1,2-diaminoethane- N,N,N�,N�-tetrakis-(methylenephosphonic acid), and diethylenetriamine-N,N,N�,N��,N��-pentakis-( methylenephosphonic acid)]. For the latter phosphonate, all the data are rejected, as well as protonation constants [HL]/[H][L] for three other ligands. Higher-quality data are selected and presented as �Recommended� and �Provisional.�

This review is generally based on the data published in the period 1965-1996, but some earlier and later publications are also included. The data collection embraced databases Chemical Abstracts, and the personal collections of the authors.






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