Energy-normalized ascorbic acid sono-dosimetry for assessing cavitation-derived oxidative activity

A UV-visible sonochemical dosimetry method based on ascorbic acid (AscA) sono-oxidation was developed to assess cavitation-derived oxidative activity in aerated aqueous solution. AscA was used as a fast-reacting probe for hydroxyl-radical-initiated oxidation and was stabilized against non-sonochemical oxidation by adding 0.2 M NaCl. Its consumption was monitored directly at 262 nm. Experiments were performed in a jacketed sonoreactor at 585, 860, and 1140 kHz using 300 mL of 100 µM AscA solution at 25 ± 1 °C. AscA depletion followed apparent zero-order kinetics under all investigated conditions, indicating that the measured rate was governed mainly by the quasi-steady generation and transfer of cavitation-derived oxidants rather than by the instantaneous AscA concentration. At 585 kHz, the apparent zero-order rate constant increased from 1.58 to 4.23 µM/min when the calorimetric intensity increased from 0.83 to 4.30 W/cm². Energy-density normalization showed that the oxidative response decreased with increasing frequency, with G_AscA values of 2.23 × 10^-10, 1.28 × 10^-10, and 1.01 × 10^-10 mol/J at 585, 860, and 1140 kHz, respectively. The sono-oxidation rate was pH-dependent, with an approximately invariant response between pH 4.8 and 8. Non-sonicated aerated controls showed no measurable AscA loss over the experimental time window, whereas under ultrasound the gas-atmosphere effect followed the order air > Ar ≫ N₂. This distinction indicates that dissolved O₂ enhanced oxygen-mediated sonochemical radical pathways rather than causing appreciable background oxidation of AscA. Comparison with H₂O₂ formation confirmed that AscA sono-dosimetry provides a complementary measure of the oxidative dose accessible to a dissolved radical scavenger. This method offers a simple, reproducible, and energy-normalized platform for comparing sonochemical activity in acoustic cavitation systems.