On the Complexation of Cu(II) and Cd(II) With Polycarboxyl Ligands. Potentiometric Studies With ISE-H+, ISE-Cu2+, and ISE-Cd2+

The interaction of Cu2+ and Cd2+ ions with polyacrylates (PAA, 2 kDa and 100 kDa), polymetacrylate(PMA, 5.4 kDa), and alginate (AA, 70 kDa to 100 kDa) was studied by potentiometry, using ISE-Cu2+, ISE-Cd2+, and ISE-H+ electrodes. The investigations were performed in NaNO3 aqueous solutions, in the ionic strength range 0.10 e I (mol ·L-1) e 0.75, at T ) 298.15 K. The “diprotic-like model” was used to explain the acid-base behavior of the polycarboxylates under investigation (for this model, the monomeric unit of the polyelectrolyte is considered as a dicarboxylate). The results give evidence for the formation of the ML species in all the systems investigated. In addition, the MLH species was found in the M-PAA, (2 and 100) kDa, and M-PMA, 5.4 kDa (M ) Cu2+ and Cd2+). The values of stability constants of complex species obtained by ISE-H+ and by ISE-Cu2+ and ISE-Cd2+ electrodes are in excellent agreement. Measurements carried out at different ionic strengths allowed us to evaluate the dependence of formation constants on ionic strength by using an extended Debye-Hu¨ckel type equation and to calculate the formation constants at infinite dilution. At I ) 0.1 mol ·L-1, log K(M+L) ranges from 5.67 (PMA, 5.4 kDa) to 3.63 (AA, 70 kDa to 100 kDa) for copper(II) and from 4.60 (PAA, 100 kDa) to 3.07 (AA, 70 kDa to 100 kDa) for cadmium(II) complexes. This stability is quite similar to that shown by low molecular weight dicarboxylates. The specific interaction coefficients for all the species involved in the complexation model were also calculated by using the specific interaction ion theory (SIT). The stability data of species formed were used to quantitatively define the sequestering capacity of the synthetic and naturally occurring polycarboxylates considered. The value of pL50 (i.e., the total ligand concentration necessary to bind 50 % of the metal ion) is reported for each metal-ligand system.