Outer-Sphere and Inner-Sphere Ligand Protonation in Metal Complexation Kinetics: The Lability of EDTA Complexes

H.P. van Leeuwen, R.M. Town

Research output: Contribution to journalArticleAcademicpeer-review

12 Citations (Scopus)


A generic framework, based on the Eigen mechanism, is formulated to describe the formation/dissociation kinetics of inner-sphere metal complexes that may undergo protonation. In principle, all protonated forms of the ligand contribute to the formation of the precursor outer-sphere complexes, but only the sufficiently stable ones effectively contribute to the overall rate of inner-sphere complex formation. The concepts are illustrated by experimental data for Cd(II)-EDTA complexes. Up to pH 8 the dissociation flux in this system is dominated by the protonated inner-sphere complex, even though it is a very minor component of the equilibrium speciation in bulk solution. The results highlight the importance of distinguishing between the thermodynamically predominant species versus the kinetically relevant ones in considerations of dynamic speciation analysis and bioavailability in natural and engineered systems
Original languageEnglish
Pages (from-to)88-93
JournalEnvironmental Science and Technology
Issue number1
Publication statusPublished - 2009



  • anodic-stripping voltammetry
  • preceding chemical-reaction
  • nuclear magnetic-resonance
  • fundamental features
  • speciation analysis
  • electrode process
  • planar diffusion
  • aqueous systems
  • acid complexes
  • ion

Cite this