In acid solution (pH 2) the nickel(II)-EDTA complex exists in a form in which one carboxylate arm is protonated and replaced in the co-ordination sphere by a water molecule, while in neutral solution (pH 6–7) a temperature-dependent equilibrium exists between two forms both probably six-co-ordinate, one containing hexadentate EDTA, the other pentadentate EDTA and a co-ordinate water molecule.