New sonochemiluminescence involving solvated electron in Ce(III)/Ce(IV) solutions

Graphical abstract
Highlights • Luminescence of *Ce3+ at Ce4+ sonoreduction by es in H2O and C2H4(OH)2 was found. • This sonochemiluminescence is quenched by the H+ ion, acceptor of a solvated electron. • Sonochemiluminescence of *Ce3+ ion is also registered in aqueous CeCl3 solutions. • In this case Ce4+ is formed at Ce3+ oxidation by OH, then reduced to *Ce3+ by eaq. • In this case the sonochemiluminescence is quenched by both H+ and Br−, OH acceptor.
The moving single-bubble sonoluminescence of Ce3+ in water and ethylene glycol solutions of CeCl3 and (NH4)2Ce(NO3)6 was studied. As found, a significant part of intensity of the luminescence (100% with cerium concentration less than 10–4 M) is due to the sonochemiluminescence. A key reaction of sonochemiluminescence is the Ce4+ reduction by a solvated (or hydrated in water) electron: Ce4+ + es (eaq) → *Ce3+. Solvated electrons are formed in a solution via electrons ejection from a low-temperature plasma periodically generated in deformable moving bubble at acoustic vibrations. Reactions of heterolytic dissociation of solvents make up the source of electrons in the plasma. In aqueous CeCl3 solutions, the Ce4+ ion is formed at the oxidation of Ce3+ by OH radical. The latter species originates from homolytic dissociation of water in the plasma of the bubble, also penetrating from the moving bubble into the solution. The sonochemiluminescence in cerium trichloride solutions are quenched by the Br− (acceptor of OH) and H+ ions (acceptor of eaq). In water and ethylene glycol solutions of (NH4)2Ce(NO3)6, the sonochemiluminescence also quenched by the H+ ion. The sonochemiluminescence in CeCl3 solutions is registered at [Ce3+] ≥ 10–5 M. Then the sonochemiluminescence intensity increases with the cerium ion concentration and reaches the saturation plateau at 10–2 M. It was shown that sonophotoluminescence (re-emission of light of bubble plasma emitters by cerium ions) also contributes to the luminescence of Ce3+ in solutions with [Ce3+] ≥ 10–4 M. If the cerium concentration is more than 10–2 M, a third source contributes to luminescence, viz., the collisional excitation of Ce3+ ions penetrating into the moving bubble.