Comparison of hydroxyl radical formation in aqueous solutions at different ultrasound frequencies and powers using the salicylic acid dosimeter.

Ultrasonic frequencies of 20kHz, 382kHz, 584kHz, 862kHz (and 998kHz) have been compared with regard to energy output and hydroxyl radical formation utilising the salicylic acid dosimeter. The 862kHz frequency inputs 6 times the number of Watts into water, as measured by calorimetry, with the other frequencies having roughly the same value under very similar conditions. A plausible explanation involving acoustic fountain formation is proposed although enhanced coupling between this frequency and water cannot be discounted. Using the salicylic acid dosimeter and inputting virtually the same Wattages it is established that 862kHz is around 10% more efficient at generating hydroxyl radicals than the 382kHz but both of these are far more effective than the other frequencies. Also, it is found that as temperature increases to 42°C then the total dihydroxybenzoic acid (Total DHBA) produced is virtually identical for 382kHz and 862kHz, though 582kHz is substantially lower, when the power levels are set at approximately 9W for all systems. An equivalent power level of 9W could not be obtained for the 998kHz transducer so a direct comparison could not be made in this instance. These results have implications for the optimum frequencies chosen for both Advanced Oxidation Processes (AOPs) and organic synthesis augmented by ultrasound.