ESR Studies of Uncomplexed and Complexed HO₂ Radical Formed in the Reaction of H₂O₂ with Ce⁴⁺, Fe²⁺ and Ti³⁺ Ions

Abstract

Excess of H2O2 reacting in 0.5N HClO4 with Ce4+, Ti3+ and Fe2+ yields the uncomplexed HO2 radical. This radical gives a broad ESR signal with a line width of 27±1 gauss and a g‐value of 2.0140±0.0015. The HO2 radical forms a complex with excess of Ce3+ which gives an ESR spectrum with a linewidth of 1.3±0.1 gauss and a g‐value of 2.0180±0.0005. The equilibrium between complexed [Ce3+‥HO2] and uncomplexed HO2 was studied. Two complexes are formed between HO2 and [Ti4+…H2O2]. These complexes give two ESR signals with a linewidth of 0.7 ± 0.1 and 0.5± 0.1 gauss and g‐value of 2.0140± ± 0.0005 and 2.0120± 0.0005 respectively.

Oxidation of Ce3+ by HO2 Radical and Ce3+‐HO2 Complex Formation

“…When Ce4+ reacts with H 202 in either H 2S04 or HN0 4 we find that the same H0 2 ESR signal is observed as in HCl0 4 [9] (Fig. 4, upper trace).…”

“…The existence of the H0 2 radical in this system was shown spectrophotometrically [4] by quenching methods [4,5) and by ESR spectroscopy [6][7][8][9]. Czapski et al (10) investigated the kinetics of the Ce4+ -H 202 reaction and confirmed the mechanism proposed by Sigler and Masters [2].…”

“…The ESR spectrum of the H0 2 radical formed in the reaction of Ce4+ with H 202 and the effect of added Ce 3 + ions in HCl0 4 (9), H 2S04 [6] and HN0 3 (8), were studied. The results of the experiments carried out in HCl0 4 show the formation of two ESR signals [9]: at low [Ce3+] only a signal due to H0 2 is observed, whereas at higher [Ce 3 + ] another signal attributed to a Ce 3 + -H0 2 complex appears.…”

“…The stopped flow apparatus used has been described in detail elsewhere [4]. In the case where the spectrophotometric detection was replaced by ESR spectroscopy, the reactants have been passed through the mixing cell under gas pressure as previously explained [9].…”

“…(IV) Earlier studies did not consider the possibility of a complex formation between cerous ion and H0 2 radical although this possibility has been suggested before [6,11). Since the existence of Ce 3+ -H0 2 complex was recently proven [4,9), and evidence for the back reaction (III) is indirect, we considered it advantageous to further investigate the reaction of Ce H with H 202 .…”