A Multi-Model Assessment of Copper(II)-Humic Acid Binding: Insights from Ion-Selective Electrode Data and a Critical Comparison of Discrete and Continuous Binding, and Computational Chemical Equilibrium MINTEQA Models

A Multi-Model Assessment of Copper(II)-Humic Acid Binding

Abstract

Speciation of Cu²⁺ with Humic acid (HA) was studied by Cu-Ion Selective Electrode (ISE) potentiometry. Free Cu²⁺ concentration was measured directly from the Cu-ISE titration data, and the bound Cu²⁺ was calculated from the mass balance of total copper added [Cu²⁺]t. The titration data were analyzed and compared with six data reduction models: Scatchard, Ružić, Buffle Y function (BYF), Klotz and Hunston (KH), Discrete Binding Model (DBM), Continuous Binding Model (CBM), and the Computational Chemical Equilibrium Model (MINTEQA). All models yielded complexing capacity of HA and the binding constant, K. The models were primarily validated by residual plots. Only CBM gave statistically validated values for two binding sites. The effective molecular weight of HA and the binding constant of metal with protonated ligand HA were determined only by BYF. It is clear from this study that all models except KH and CBM are valid for 1:1 complex within a limited portion of the data, while CBM, a non-linear model, is valid for the entire data, therefore, yields a robust estimate of the parameters and their uncertainties for two binding sites of HA in CuHA complex. The model predictions are further validated by MINTEQA, a chemical reaction model firmly based on the reaction stoichiometry and thermodynamics.

Dhaka Univ. J. Sci. 74(1): 1-9, 2026 (January)