Kinetics, Mechanism, and Reactivity of Intermediates of the Cerium(IV)–Oxalate Reaction in a Sulfate Medium

The work presents an approach for studying the kinetics, mechanism, and reactivity of intermediates of a wide class of redox reactions for which the rate-limiting step is the redox decomposition of the intermediate complex. The approach was applied to the investigation of the oxidation of oxalic acid (H₂Ox) by cerium(IV) in a sulfuric acid medium as part of the Belousov–Zhabotinsky oscillating reaction (BZ reaction) catalyzed by cerium ions. Experimental, mathematical, and computational methods that are typically used to study metal complexes in a stable oxidation state were kinetically generalized to variable-valence metal complexes and were used to determine the characteristics of intermediate complexes of the cerium(IV)–oxalate reaction and derive a general equation for its rate based on a set of equations describing the rapid achievement of pre-equilibrium in the system and the subsequent nonequilibrium process. A quantitative model of the process was proposed; it included two parallel reaction pathways, for which two different cerium(IV)–oxalate intermediate complexes were identified and characterized. The complexes have similar reactivity, which may be due to the similarity of the structure of their inner coordination spheres and the inner-sphere mechanism of electron transfer in the complexes. Using the developed model, a diagram of the yields of all main species of cerium(IV) under the conditions of the BZ reaction was constructed, which indicates the need to take into account the formation of intermediate complexes of the composition CeOHOx\(_{n}^{{3 - 2n}}\) (n = 1, 2) in the oxidation of oxalic acid under these conditions. The main difference between the presented model of the cerium(IV)–oxalate reaction as part of the BZ reaction and the previous models is the explicit consideration of the participation of intermediate complexes of cerium(IV) with oxalic acid anions and sulfate background anions in the reaction.