Redox Reaction Between Synthesized Amino Heterocyclic Azo Dye and Heptaoxodichromate(VI) Ion in Aqueous Acidic Medium

Abstract

 The kinetics and mechanism of the redox reaction between a synthesized amino heterocyclic azo dye (DYE) and heptaoxodichromate(VI) ion were investigated spectrophotometrically in aqueous hydrochloric acid medium at 30 ± 1 °C and ionic strength of 0.50 mol dm⁻³ (NaCl). The stoichiometry of the reaction, determined by the mole-ratio method, revealed a 1:1 relationship between DYE and Cr₂O₇²⁻. Under pseudo-first-order conditions, the reaction exhibited first-order dependence on both the dye and dichromate ion concentrations, with a slope of 1.02 obtained from the plot of log k_obs versus log[Cr₂O₇²⁻], indicating an overall second-order reaction. The second-order rate constant (k₂) remained relatively constant within the range of 3.05–3.14 dm³ mol⁻¹ s⁻¹. The reaction rate increased with increasing hydrogen ion concentration, giving a slope of 0.8645 from the plot of log k_obs versus log[H⁺], while the plot of k₂ versus [H⁺] was linear and passed through the origin, suggesting proton involvement in the rate-determining pathway. Increasing ionic strength from 0.1 to 1.3 mol dm⁻³ resulted in a decrease in reaction rate, and the plot of log k₂ versus √I yielded a negative slope of –2.7464, indicating a negative salt effect and interaction of oppositely charged species in the activated complex. The reaction rate was unaffected by Ca²⁺ and Mg²⁺ ions but was inhibited by SO₄²⁻ and CO₃²⁻ ions. Spectroscopic scans showed no detectable intermediate species, while the Lineweaver–Burk plot passed through the origin, indicating the absence of a kinetically significant intermediate complex. Furthermore, acrylamide polymerization tests revealed no evidence of free-radical formation. The collective kinetic evidence supports an outer-sphere electron-transfer mechanism for the oxidation of the amino heterocyclic azo dye by heptaoxodichromate(VI) ion in aqueous acidic medium.