Potentiometric study of the complex-formation equilibria of manganese(II), cobalt(II), nickel(II), copper(II), and zinc(II) with ethylenediamine-N-acetic acid

Abstract

The complex-formation equilibria between Mn^II, Co^II, Ni^II, Cu^II, and Zn^II and ethylenediamine-N-acetic acid (Hedma) were investigated by potentiometric titrations in 0.5 mol dm^–3 KCl solution at 25 °C. It was found that no protonated or hydrolysed complexes were formed in the pH range 2.0–11.5. The following overall formation constants β_pqr=[M_pH_qL_r]/[M]^p[H]^q[L]^r were obtained: edma, log β₀₁₁= 9.888(2), log β₀₂₁= 16.860(3), log β₀₃₁= 18.350(4); edma-Mn^II, log β₁₀₁= 3.629(18); edma–Co^II, log β₁₀₁= 8.123(3), log β₁₀₂= 13.050(9); edma–Ni^II, log β₁₀₁= 10.106(5), log β₁₀₂= 16.629(10); edma–Cu^II, log β₁₀₁= 12.854(11), log β₁₀₂= 20.373(19); edma–Zn^II, log β₁₀₁= 8.064(4), log β₁₀₂= 13.655(7). The probable structures of the chelated compounds formed in aqueous solution were deduced and their stability constants compared with those of analogous chelated compounds. The equilibrium constants follow the Irving–Williams series.