Study of Equilibria and Kinetics of the Interactions of Iron(III) with 8-Hydroxyquinoline and 8-Hydroxyquinoline-5-sulfonic Acid in HClO4–NaClO4 Media

Abstract

Abstract The equilibria and kinetics of the reaction of iron(III) with 8-hydroxyquinoline (oxine) and 8-hydroxyquinoline-5-sulfonic acid under the condition of 1 : 1 chelate formation at different temperatures (35–45 °C) and ionic strength I = 1.0 mol dm−3 (NaClO4 + HClO4) have been studied. A dual path mechanism involving both Feaq3+ and Fe(OH)aq2+ species and the undissociated ligand (LH2+, LH2±) is consistent with the experimental observations under the experimental condition. The different rate constants have been deternined from the attainment of the equilibrium from both directions i.e., from the study of complex formation at low acid concentration and from the study of decomposition of the complex in presence of higher acid concentration. The results obtained from the both studies are found comparable within the experimental error limit. The results conform to kobsd/B = k1[H+] + k2Kh where, ; Kh = hydrolysis constant of Feaq3+, k1 and k2 are the forward second order rate constants of Feaq3+ and Fe(OH)aq2+ respectively and Q is the equilibrium constant of the reaction, Fe3+ + LH2+ FeL2+ + 2H+ where LH2+ represents the protonated form of the ligand, 8-hydroxyquinoline and a similar reaction occurs for 8-hydroxyquinoline-5-sulfonic acid (LH2±). Thermodynamic parameters for each of the steps have been determined. The results are compared with those of recent findings in the interaction of cerium(IV) with these ligands in perchloric acid media. Fe(OH)aq2+ appears to react through a purely dissociative fashion (Eigen–Tamm mechanism) and its characteristic water exchange rate has been found, as a rough estimate. On the other hand Feaq3+ appears to react through the associative interchange (Ia) mechanism. The equilibrium constants (Q) obtained spectrophotometrically have been compared with those obtained from kinetic studies.