The Kinetics of the Autoxidation of Ferrous Ions in Aqueous Tripolyphosphate Solutions

The kinetics of the autoxidation of ferrous ions in tripolyphosphate solutions (pH 2–8) at an ionic strength of 0.20 m NaClO4 was studied using a spectrophotometric technique. The rate expression for the oxidation reaction is given by: d[Fe(III)]⁄dt=4kI[FeII(Htpp4−)][O2]+4kII[FeII(Htpp4−)2][O2], and 4kII found is to be 1.0×102 m−1 sec−1 at 15°C. The simultaneous reaction paths of FeII(Htpp4−)+O2(Htpp4−)+O2- and FeII(Htpp4−)2+O2(Htpp4−)2+O2- are consistent with the results obtained. When there is an excess of the triphosphate solution in the pH range from 6 to 8, the latter may be the rate-determining step. The acceleration of the oxidation of the ferrous ions in the triphosphate solution may be due to the fact that the stabilization of the oxidation products, FeIII(Htpp4−) and FeIII(Htpp4−)2, is greater than that of the corresponding ferrous triphosphate complexes. As the activation parameters for the latter reaction, the experiments gave the values of ΔH∼10 kcal mol−1 and ΔS∼−25 cal mol−1. deg−1. The negative and relatively small value of the entropy of activation suggests that the activated complex is more extensively hydrated than is the ferrous complex of FeII(Htpp4−)2.