Complexation kinetics of Fe²⁺ with 1,10‐phenanthroline forming ferroin in acidic solutions

Abstract

The formation kinetics of ferroin is studied under varied acid conditions at 25 • C and fixed ionic strength (0.48 mol dm −3 ) under pseudo-first-order conditions with respect to Fe 2+ by using the stopped-flow technique. The reaction followed is first and third order with respect to Fe 2+ and 1,10-phenanthroline (phen) T , respectively. Increasing the acid concentration retarded the reaction, and the reaction rate showed a positive salt effect. The rate-limiting step involved the complexation of the phen or protonated phen with [Fe(phen) complex ion. Simulated curves for the acid variation experiments agreed well with the corresponding experimental curves and the estimated rate coefficients supporting the proposed mechanism. Relatively low energy of activation (26 kJ mol −1 ) and high negative entropy of activation (−159.8 J K −1 mol −1 ) agree with the proposed mechanism and the formation of compact octahedral complex ion. C 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 515-523, 2008

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