Determination of stability constants of metal-azo dyes complexes by ion-exchange and spectrophotometry

The stability constants of the complexes of Al, Cu, Fe, Ni with two sulphon ated 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 re sin and the determination of free and complexed metal. The other two techni ques are based on the different UV-visible absorbances of free ligand and c omplex: the first one provides a value for the conditional stability consta nts, which in turn allows the calculation of the apparent and thermodynamic constants; the second procedure is a graphic method of extrapolation and d irectly provides the value of the apparent stability constants.