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

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Abstract
The complex-formation equilibria between MnII, CoII, NiII, CuII, and ZnII 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 βpqr=[MpHqLr]/[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-MnII, log β101= 3.629(18); edma–CoII, log β101= 8.123(3), log β102= 13.050(9); edma–NiII, log β101= 10.106(5), log β102= 16.629(10); edma–CuII, log β101= 12.854(11), log β102= 20.373(19); edma–ZnII, 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.