Simultaneous kinetic determination of zinc, magnesium, copper and nickel by a ligand substitution reaction

Abstract

The rate of the ligand substitution reaction between calmagite [3-hydroxy-4-(2-hydroxy-5-methylphenylazo)-naphthalene-1-sulphonic acid] complexes of Zn(II), Mg(II), Cu(II) and Ni(II) and EDTA was studied at 25.0 °C and pH 9.5 by a stopped-flow spectrophotometric technique. The reactions were first order in calmagite complexes and in incoming ligand for Zn, Cu and Ni, while a zero-order dependence on EDTA was observed for Mg. The change of absorbance at 500 nm, accompanying the substitution reaction, was used for the determination of the metal ion. For multi-component mixtures such as Zn, Mg and Cu or Ni, the absorbance change during the reaction was separated kinetically for each component, in the reactivity order Zn > Mg > Cu ≈ Ni, so that metal ions at concentrations down to 10^–6M could be determined simultaneously. The method was applied to the analysis of standard mixtures and samples of river water, tap water and mineral waters.