Coordination, Equilibria, and Stability of Zn(II), Cu(II), Ni(II), Co(II), Pb(II), and Hg(II) Metal Complexes with 4-(2-Pyridylazo)resorcinol in Aqueous/Ethanol Solutions

A potentiometric investigation has been undertaken to illustrate the coordination competence of 4-(2-pyridylazo) resorcinol through its reaction with particular metal ions Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Pb²⁺, and Hg²⁺, where metal complexes are formed in the solution. The dissociation constants of the ligand PAR are calculated together with the stability constants of its complexes with the above metal ions. The experimental conditions are controlled to achieve all measurements required in the solution (ethyl alcohol (10%) and water (90%)) at (25 ± 0.1)°C and an ionic background of 0.1 mol dm^–3 NaCl. The ligand acts as a tri-dentate, coordinating through the pyridine nitrogen, one of the azo group nitrogen and the ortho-hydroxy of the resorcinol moiety. Four dissociation constants were identified experimentally with values 2.40, 3.34, 5.63, and 10.84. It has been observed that several metal complexes originated in the solution, under the mentioned experimental conditions for each metal ion used. The values of the overall stability constants log ß_n for all species and the dissociation constants of the PAR ligand are determined using the HYPERQUAD-2008 computer program. All species present in the solution are displayed with the use of speciation diagrams.