Ca²⁺-Induced Oxygen Generation by FeO4²⁻ at pH 9–10

Although FeO₄^2- (ferrate(IV)) is a very strong oxidant that readily oxidizes water in acidic medium, at pH9-10 it is relatively stable (<2 % decomposition after 1 h at 298 K). However, FeO4^2- is readily activated by Ca²⁺ at pH9-10 to generate O₂. The reaction has the following rate law: d[O₂]/dt=k_Ca[Ca²⁺][FeO₄^2-]². ¹⁸O-labeling experiments show that both O atoms in O₂ come from FeO₄^2-. These results together with DFT calculations suggest that the function of Ca²⁺ is to facilitate O-O coupling between two FeO₄^2-ions by bridging them together. Similar activating effects are also observed with Mg²⁺ and Sr²⁺.