IUPAC

To provide scientists involved in chemical modelling of trace metals in environmental systems with access to the best possible (critically evaluated) equilibrium data for the reactions of these metals with major inorganic ligands.

The importance of considering chemical speciation (concentration of individual chemical entities) within a coherent framework is increasingly being recognised. Detailed understanding of the bioavailability and toxicity of heavy metals as well as their transport and sedimentation in natural aquifers requires knowledge of their speciation. The optimisation of many industrial processes, e.g. hydrometallurgy and pulp and paper processes, relies heavily on the understanding of chemical speciation in often-complicated multicomponent/multiphase systems.

Chemical speciation modelling based on the assumption that the system is at equilibrium is frequently utilised. The validity of an assumption like this should of course always be scrutinised. However, it should be born in mind that the analytical techniques required for measuring trace metal (and trace metal complex) concentrations are still to a great extent missing. Thus, quite often the only option that remains is the equilibrium approach to speciation.

The numerical modelling of equilibrium systems requires adequate, critically evaluated databases of numerical constants for equilibrium reactions. Access to the invaluable IUPAC Stability Constants Database has significantly improved the possibilities of doing literature searches for relevant thermodynamic data of metal complexes in solution. This database now contains more than 80000 records with over 18000 metal-ligand combinations. It has become obvious that the inexperienced user of a non-critical database like this runs into problems when trying to identify the most reliable data. Therefore, critical evaluation of data is necessary.

Within the Analytical Chemistry Division's Commission on Equilibrium Data one major responsibility has been to provide the scientific community with critically evaluated speciation schemes and corresponding stability constants for metal complexes in solution. Considering the great number of metals of the Periodic Table and the increasing vast number of ligands found in our environment, this task seems insurmountable. However, the need for reliable thermodynamic data describing metal complexation as a function of temperature, pressure and ionic strength is urgent. This is true in many but not all metal - ligand systems. To satisfy the most urgent needs, priorities on selecting metal - ligand systems must be done.

The present proposal puts forward a programme that identifies the most important metal ion - ligand systems from an environmental point of view. The pollutants Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II), together with the inorganic ligands Cl^-, OH^-, CO₃^2-, PO₄^3-and SO₄^2-, have not yet been critically evaluated and should have highest priority. This work is seen as Part I of a series that may subsequently consider (a) data applicable to body fluids, (b) data applicable to sea water, (c) data applicable to high component concentrations and high ionic strengths as may exist in industrial processes.

Besides an evaluation of soluble metal complexes in these systems, it is recommended that solubility products for pertinent solid phases be evaluated also.

The first paper dealing with the different Hg(II) systems is now completed. It has been evaluated by external referees and is now ready to be printed in PAC. A closely related paper entitled “Chemical Speciation of Hg(II) with Environmental Inorganic Ligands” has been published as a full paper in Australian Journal of Chemistry, 2004, 57,993-1000. [doi:10.1071/CH04063]

In addition, the Cu(II) and Pb(II) systems are close to completion with written reports (drafts). Main parts of the Zn(II) and Cd(II) systems remain to be done.

In March 2004, S. Sjöberg has given an invited lecture titled Chemical Speciation of Environmentally Significant Heavy metals and Inorganic Ligands at SOPRO 2004, Karlsruhe,
Germany.
A total of 6 articles are in preparation or will be prepared as output of this project. End of 2005 is the projected completion date.

> Nov 2004 report update (pdf file - 13KB)

> April 2005 - Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg²⁺– Cl^–, OH^–, CO₃^2–, SO₄^2–, and PO₄^3– aqueous systems (IUPAC Technical Report) published in Pure Appl. Chem. 77(4), 739-800, 2005

> May 2006 report update (pdf file - 20KB)

> May 2007 - Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 2: The Cu²⁺– OH^–, Cl^–, CO₃^2–, SO₄^2–, and PO₄^3– systems (IUPAC Technical Report) published in Pure Appl. Chem. 79(5), 895-950, 2007

> July 2007 report update (pdf file - 9KB)