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Estimated Association Constants for Some Complexes of Trace Metals with Inorganic Ligands¹

ABSTRACT

Chemical equilibrium studies of trace metals in soils by an ion-association model require reliable data on thermodynamic association constants for various metal-ligand complexes. Therefore, available experimental data at 25°C were compiled for complexes of the transition metal cations Mn²⁺, Fe²⁺, Nl²⁺, Cu²⁺, and Zn²⁺ with the ligands CO₃^2-, HCO₃^-, SO₄^2-, HSO₄^-, PO₄^3-, HPO₄^2-, H₂PO₄^-, Cl^-, and OH^-. However, experimental values of the thermodynamic association constants for many of the complexes were not available. This lack of information was alleviated by employing the theoretical model of Kester and Pytkowicz and the semiempirical models of Nieboer and McBryde, and Van Panthaleon Van Eck to make estimates of the as yet unmeasured association constants. These estimated constants were tabulated in a compact form along with the experimental values. This compilation of 100 association constants, although provisional, should prove helpful in a variety of studies on trace metal equilibria in soils until more reliable experimental data become available.

¹ Contribution from the Dep. of Soil and Environ. Sci., Univ. of California, Riverside, Ca 92521.

² Research Soil Scientist and Associate Professor of Soil Science, respectively.

Received for publication November 1, 1976. Accepted for publication June 24, 1977.