When a mixture of two or more reducing agents is titrated, a curve with multiple inflection points results provided the reduction potentials of the species are sufficiently different (1). This is comparable to the titration of two acids with different dissociation constants or of two ions forming precipitates of different solubilities with same reagent (2). The effect of complexation on redox potentials is well documented (3-19) and some articles to this effect are published in this Journal (3, 4). Our experiment is a simple potentiometric titration aimed to invoke the underlying concept of the complexation effect on redox potential of a redox couple. Potentiometric titrations have been widely discussed in this Journal (20-24); however, no method is reported for potentiometric titration of metal ions on the basis of the ligand effect. We choose iron(III)-iron(II) couple because it occurs in many natural redox systems. In this experiment, three iron(II) octahedral complexes, iron(II) aqua, [Fe(OH 2 ) 6 ] 2þ ; iron(II) EDTA, [Fe EDTA] 2-; and iron(II) phenanthroline [Fe(o-phen) 3 ] 2þ , are combined into mixtures of reducing agents and are oxidized to the corresponding iron(III) complexes. Multiple inflections in the titration curve depict the different redox potential of iron(III)-iron(II) couple in these complexes that can be attributed to the complexation effect. This experiment illustrates the effect of the stabilities of iron(III)-iron(II) complexes with EDTA, water, and 1-10 phenanthroline ligands on redox potential.

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The formation constants of the binary 1:1 and 1:2 complexes of Cu(II), Zn(II), Cd(II), Hg(II), and Pb(II) withN-acetylcysteine (NAC) and 1:1:1 ternary complexes of the said metal ions with NAC as a primary ligand and some biologically important amino acids as secondary ligands have been determined potentiometrically in aqueous medium. Acid dissociation constants of the ligands used and the formation constants of the binary and the ternary complexes were determined at 25 °C and in ionic strength I=0.1 mol dm-3(KNO3).The formation constants of the 1:1 complexes were found to be higher than 1:2 complexes and the metal ions follow the order Hg(II) >Cu(II) >Cd(II) >Zn(II). In addition UV-spectral studies of the NAC-Metal (II) complexes have also been conducted at appropriate pH values to give further information about the structural nature of NAC- Metal (II) complexes in aqueous medium.

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DIDE and rhodanine prevent the copper induced oxidative damage to proteins by binding copper into a redox inactive state.