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A Chelating-Resin Method for Characterizing Soluble Metal Complexes¹

ABSTRACT

A method is presented to differentiate between free and complexed trace metals in solution at concentrations at or below the sensitivity of flameless atomic absorption spectrophotometry using a pH and solution composition consistent with natural systems. The method utilizes a pH-adjusted chelating resin saturated with Ca and selected trace metals that maintains pH and solution metal activities essentially constant when equilibrated with a Ca(NO₃)₂ solution. Equilibration of this solid-phase buffer with a Ca(NO₃)₂ solution containing a soluble complexing agent results in a soluble metal concentration in excess of that obtained when equilibrated with Ca(NO₃)₂ alone. The increase in the soluble metal concentration is attributed to the formation of soluble metal complexes. The resin method was verified by evaluating Cd, Zn, and Ca complexation by EDTA (ethylenediaminetetraacetic acid). Relative selectivity coefficients were consistent with published values, and the concentration of EDTA added to the system was accurately determined. Solution Cd concentrations were also increased by Cl^- addition to an extent consistent with published formation constants. The ability of the method to evaluate complexation of several metals simultaneously may make it more attractive than existing methods for characterizing simple monofunctional complexing agents. The method was also used to characterize soluble complexing agents extracted from a sludge-amended soil. The results indicated that a portion of the ligands had approximately 10^5.5 times greater affinity for Cd and Zn than for Ca. This marked selectivity for Cd and Zn is much greater than existing selectivity data for similar systems obtained under conditions of low pH and/or unrealistically high metal levels and is likely attributable to the much greater Ca to metal ratios maintained by the resin system.

¹ Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, and by EPA Grant CR807270.

² Project Associate and Professor, respectively, Dep. of Soil Science, University of Wisconsin-Madison. Senior Author is presently employed as a Research Agriculturist for Allied Chemical, Syracuse Research Lab., Solvay, NY 13209.

Received for publication December 5, 1981. Accepted for publication January 19, 1983.