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 degrees C and fixed ionic strength (0.48 mol dm(-3)) under pseudo-first-order conditions with respect to Fe2+ by using the stopped-now 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 vertical bar Fe(phen)(2)vertical bar(2+) complex ion, leading to formation of vertical bar Fe(phen)(3)vertical bar(2+) ion. The observed retardation of the reaction rate with increasing vertical bar H-0(+vertical bar) is due to the increased vertical bar phenH(+)vertical bar(eq) and low reactivity of phenH(+) with vertical bar Fe(phen)(2)vertical bar(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 JK(-1) mol(-1))agree with the proposed mechanism and the formation of compact octahedral complex ion. (c) 2008 Wiley Periodicals, Inc.