Ultrasonic treatment effects on hydroxyl radical generation in various solution systems: an iodometric analysis

Hydroxyl radicals (•OH) are key reactive species in sonochemical processes; however, their generation is strongly dependent on the operating parameters and solution composition. In this study, probe-ultrasonication-induced oxidizing response was evaluated using an iodometric method in a 0.4mol L^-1 KI background, where the measured response reflects the accumulated titratable iodine (I₃^-) signal and is therefore discussed as an apparent •OH yield rather than an instantaneous steady-state [•OH]. The effects of sonication time (5-35 min), ultrasonic power (60-300 W), duty cycle (20%-100%), and probe position (top/middle/bottom) were systematically investigated. The apparent •OH yield increased with time and power, reached a maximum at a duty cycle of 60%, and was highest when the probe was positioned at the center of the liquid column. The influence of solution composition was further evaluated by adding ethanol, acetic acid, NaOH, NaHCO₃, and NaCl to the KI background. Increasing the ethanol and NaOH concentrations generally decreased the apparent •OH yield, whereas acetic acid showed a biphasic response. NaHCO₃ decreased the apparent •OH yield in a concentration-dependent manner, and NaCl showed a non-monotonic concentration dependence. In phosphotungstic acid systems, the iodometric response increased with concentration at 10 min but the concentration dependence diminished at 30 min, and a no-ultrasound control showed a measurable baseline signal while ultrasound produced a stronger response. Hydrogen peroxide markedly enhanced the response at moderate concentration but reduced the apparent •OH yield at higher levels due to competing reactions, while a t-BuOH scavenger control reduced the signal to about 10% of that without t-BuOH, indicating predominantly •OH-driven response with minor non-specific contributions. These results provide practical guidance for regulating the accumulated oxidative response in probe-ultrasonic systems.