The stability constants of the complexes of Al, Cu, Fe, Ni with two sulphonated azo-ligands, namely Calcichrome and SPADNS, were determined with three different procedures. One procedure is based on ion exchange and involves the retention of the negatively charged complexes onto an anion exchange resin and the determination of free and complexed metal. The other two techniques are based on the different UV-visible absorbances of free ligand and complex: the first one provides a value for the conditional stability constants, which in turn allows the calculation of the apparent and thermodynamic constants; the second procedure is a graphic method of extrapolation and directly provides the value of the apparent stability constants
