EFFECT OF IONIC INTERACTIONS ON THE OXIDATION OF FE(II) WITH H2O2 IN AQUEOUS-SOLUTIONS

The oxidation of Fe(II) with H2O2 has been measured in NaCl and NaClO4 solutions as a function of pH, temperature T (K) and ionic strength (M, mol-L-1). The rate constants, k (M-1-sec-1), d[Fe(II)]/dt = -k[Fe(II)][H2O2] at pH = 6.5 have been fitted to equations of the form, log k = log k0 + AI1/2 + BI + CI1/2/T where log k0 = 15.53 - 3425/T in water; A = -2.3, -1.35; B = 0.334, 0.180; and C = 391, 235, respectively, for NaCl (sigma = 0.09) and NaClO4 (sigma = 0.08). Measurements made in NaCl solutions with added anions yield rates in the order B(OH)4- > HCO3- > ClO4- > Cl- > NO3- > SO4(2-) and are attributed to the relative strength of the interactions of Fe2+ or FeOH+ with these anions. The FeB(OH)4+ species is more reactive while the FeCO3-degrees, FeCl+, FeNO3+ and FeSO4-degrees species are less reactive than the FeOH+ ion pair. The general trend is similar to our earlier studies of the oxidation of Fe(II) with O2 except for B(OH)4-. The effect of pH on the log k was found to be a quadratic function of the concentration of H+ or OH- from pH = 4 to 8. These results have been attributed to the different rate constants for Fe2+ (k0) and FeOH+ (k1) which are related to the measured k by, k = k0 alpha-Fe + k1 alpha-FeOH, where alpha-i is the molar fraction of species i. The rates increase due to the greater reactivity of FeOH+ compared to Fe2+. k0 is independent of composition and ionic strength but k1 is a function of ionic strength and composition due to the interactions of FeOH+ with various anions.