Determination of Coordination Parameters of Cd²⁺ and Zn²⁺ with Polyvinyl Alcohol and Chitosan

Abstract

The apparent coordination numbers and coordination stability constants of polyvinyl alcohol-Cd²⁺ (PVA-Cd²⁺), polyvinyl alcohol-Zn²⁺ (PVA-Zn²⁺), and chitosan-Zn²⁺ (CTS-Zn²⁺) were determined in aqueous solution through the straightforward and effective measurement of the equilibrium metallic ion concentration. The difference of the parameters between these complexes was discussed in detail. The apparent coordination numbers of PVA-Cd²⁺, PVA-Zn²⁺, and CTS-Zn²⁺ complexes were 1.76, 1.36, and 1.44, respectively, which at the molecular level is due to multiple complexes with different coordination constants (1 or 2) being formed. The apparent coordination stability constants of PVA-Cd²⁺, PVA-Zn²⁺, and CTS-Zn²⁺ complexes were 4.0 × 10³, 1.2 × 10³, and 4.6 × 10⁴, respectively. In comparison with Zn²⁺, it was easier for Cd²⁺ to be coordinated with PVA and the apparent coordination constant was closer to 2. In addition, the larger apparent coordination stability constant of CTS-Zn²⁺ compared with that of PVA-Zn²⁺ indicated that the complex CTS-Zn²⁺ was more stable. This is because the amido groups of CTS are more active than hydroxyl groups in a coordination reaction and large numbers of hydroxyl groups and only a few carbonyl groups of CTS participate in coordination with the metallic ions. Both of the factors lead to the apparent coordination stability constant of CTS-Zn²⁺ being one order larger than that of PVA-Zn²⁺.

Keywords:

• apparent coordination number
• chitosan
• complexation
• coordination stability constant
• polyvinyl alcohol
• polymer metal complex

Acknowledgments

This project was supported by NSFC under grant No. 50572061.