Kinetic Study of the Speciation of Copper (II) Bound to Humic Acid

The kinetically distinguishable species present in a solution of Cu(II) equilibrated with a soil humic acid have been identified by a kinetic method based on ligand displacement reactions with 3-propyl-5-hydroxy-5-(d-arabino-tetrahydroxybutyl) thiozolidine-2-thione (PHTTT). An approximate Laplace transform is used to assign the minimum number of components and approximate their associated concentrations and dissociation rate constants. The initial parameters are refined by a non-linear multi-exponential least squares regression routine. The two rate constants 0.093 and 0.0077 sec−1 consistently represent the middle and slow components of the copper-humic acid complex (CuHA). A fast component which reacts on mixing is the major component present. Species concentrations vary in a reasonable way with pH and HA/Cu(II) ratio supporting the conclusion that the speciation model has chemical significance.