Kinetics of cerium (IV) oxidation of mandelic acid. Ionic strength and specific cation effects

Abstract

The kinetics of the redox reaction between mandelic acid (MA) and ceric sulfate have been studied in aqueous sulfuric acid solutions and in HzSO4-MC104 (M+ = H+, Li+, Na+) and HzS04-MHS04 (M+ = Li+, Na+, K+) mixtures under various experimental conditions of total electrolyte concentration (that is, ionic strength) and temperature. The oxidation reaction has been found to occur via two paths according to the following rate law:constant. The cations considered exhibit negative specific effects upon the overall oxidation rate following the order H+ < Li+ < Na+ < K+. The observed negative cation effects on the rate constant k l are in the order Na+ < Li+ < H+, whereas the order is in reverse for kz, namely, H+ 6 Li+ < Na+. Lithium and hydrogen ions exhibit similar medium effects only when relatively small amounts of electrolytes are replaced. The type of the cation used does not affect significantly the activation parameters.

Cerium (IV) oxidation rate of p‐chloromandelic acid in perchlorate and sulfate media

“…The same reaction stoichiometry was previously found for cerium (IV) oxidation of mandelic acid [2,6] and its p-methyl [3,4] and p-chloro [l] derivatives. The wavelength of maximal absorption of p -chlorobenzaldehyde in the H2SO4-HC104 mixture used is 260 nm (log e = 4.21), whereas that observed in 0.200 mol/dm3 HC104 is slightly smaller ( A , , , = 258 nm and log E = 4.22; see Fig.…”

“…When Li+ is used to replace the hydrogen ion, the oxidation rate remains independent of the sulfuric acid concentration (i.e., [H+]), whereas in the presence of NaHS04 and KHSO4 the rate con- stant k decreases markedly with decreasing acidity, the decrease being more significant for potassium ions. As also pointed out in previous works [2][3][4], in the latter cases a medium effect is operative, the negative specific alkali cation effect following the order Na+ < K+.…”

“…Therefore the oxidation rate is first order in both cerium (IV) and organic substrate, and formation of an intermediate complex between the reacting compounds is not indicated by the kinetic data. Analogous results were found previously for mandelic acid [2] and its p-methyl derivative [3,4]. The second-order rate constants k given in Tables IV and V were thus estimated by dividing the observed pseudofirst-order rate constants by the organic substrate concentrations.…”