KINETIC STUDY OF THE REDUCTION OF FERRIC-1,10-ORTHOPHENANTHROLINE WITH URANIUM (IV) DTPA

The reduction of ferric 1,10-orthophenanthroline by Uranium (IV) complex of Diethylenetriaminepentaacetic acid was investigated in aqueous hydrochloric acid at 30°C, ionic strength 0.01 mole dm-3 and pH 3.5. The mechanism and rate law for the formation of [Fe(opt)3]2+ was established by isolation methodat constant and varying pH values. Spectroscopic method was employed for this investigation. The rate constant and order of reaction with respect to each of reactant the [U(IV) DTPA]^-1 and [Fe(opt)3]^3+ was established by plotting a graph ln (A∞ - At ) vs. time. The reaction was observed to be following first order with respect to each of the reactants.Overall reaction order was found to be two, having the value of the rate constant 571.59 M^-1 min^-1 at pH3.5. The rate law is suggested to be Rate = K k1 [H+][U(IV) DTPA]-1 [Fe (opt)3]3+,where K is equilibrium constant for the reaction[U(IV)DTPA] + [H+] « [U(IV)DTPAH+]and k1 is the specific rate constant for the reaction[U(IV) DTPAH+] + [Fe(opt)3]^3+ ¯ [U(VI) DTPA] + [Fe(opt)3]^2+d/dt=[Fe(opt)3 ]^ +2=k [U(IV)DTPAH[Fe(opt) ]^+3Thermodynamic parameters for the reaction were determined to be Ea = 26.47 kJ mol-1, ΔG# = 35.11 kJmol-1, DH# = 24.86 kJ mol-1 and ΔS# = 50.17 J mol-1. With the help of Arrhenius equation activationenergy for the reaction was calculated. Change in enthalpy and entropy for the reaction (ΔS#, ΔH#) were determined from the slope and the intercept of Eyring plot. Hydrogen ion dependence of the reaction was determined by varying the pH and the rate law was determined as Rate = K k1 [H+][U(IV) DTPA]-1 [Fe (opt)3]^3+,where K is equilibrium constant for the reaction[U(IV)DTPA] + [H+] ¯ [U(IV)DTPAH+] and k1 is the specific rate constant for the reaction[U(IV) DTPAH+] + [Fe(opt)3]^3+ ¯[U(VI) DTPA] + [Fe(opt)3]^2+