Determination of Conditional Stability Constants of Copper(ii) Complexes With Naturally Occurring Macromolecules
A critical evaluation was made of models currently in use for determining stability constants of metal complexes with humic and fulvic acids. Most models are shown to reduce to forms based on Adair's equation, in which the macromolecule is assumed to be the central group and where binding is evaluated graphically as a reciprocal plot, a saturation plot, a Scatchard plot, or a Hill plot. Application of a single set of experimental data (ISE and H('+) release measurements were made) for the reactions of Cu(II) with humic acid showed that values of log K* (apparent K) were lowest when calculated by either the Hill plot method (linear form) or by the Bjerrum plot method (where the metal is assumed to be the central group), as compared to the Scatchard plot method. Values for B(,1) (Bjerrum plot method) were 2 or 3 orders of magnitude lower than reported values for k(,1)* (intrinsic apparent k values obtained with the Scatchard plot method). This difference in B(,1)* and k(,1)* could not be attributed to differences between research groups because similar differences were found when B(,1)* and k(,1)* were calculated from the same experimental data. Apparent intrinsic stability constants obtained by Scatchard plot analysis for binding at the strongest (k(,1)*) and weakest (k(,j)*) sites differed by several orders of magnitude. In addition, the Scatchard plot technique yielded log K(,1)* values larger than overall log K* or B* values of other methods. Average log apparent stability constants for 0.25 mg humic acid/ml in 0.01 KClO(,4) at pH 4 for the same set of ISE data points above 10('-6) Cu('2+) were 6.13 (k(,1)*) and 4.97 (k(,j)*), 4.99 (B(,1)*), 4.99 (B(,1)*) and 9.13 (B(,2)*), and 3.66 (k*) for the Scatchard, Bjerrum, and Hill plot methods, respectively. When data below 10('-6) Cu('2+) were used, log k(,1)* was 7.02. Results are also reported for pH values 4, 5, and 6, ionic strengths of 0.01, 0.05, 0.1, and humic acid concentrations of 0.06, 0.09, 0.12, 0.25 mg/ml. Apparent stability constants were also determined for the Cu(II) complexes of some naturally occurring polymeric phenols from "wood hemicellulose extracts". An evaluation of the copper ion selective electrode showed the following: (1) the lower limit of detection can be extended from pM 6 to 8 in the presence of humic acid; (2) humic acids do not adsorb at the electrode surface or affect subsequent use of the electrode; and (3) a plot of electrode responses to log (K('+) ligand concentration) for known ligands can be utilized to estimate the magnitude of the stability constant for naturally occurring macromolecules provided molecular weight is known.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign