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

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 Fe2+ by using the stopped‐flow technique. The reaction followed is first and third order with respect to Fe2+ 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)2]2+ complex ion, leading to formation of [Fe(phen)3]2+ ion. The observed retardation of the reaction rate with increasing [H+]0 is due to the increased [phenH+]eq and low reactivity of phenH+ with [Fe(phen)2]2+ 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. © 2008 Wiley Periodicals, Inc. 40: 515–523, 2008