Sonochemical dosimetry: A comparative study of Weissler, Fricke and terephthalic acid methods

Acoustic cavitation and sonochemical reactions play a significant role in various applications of ultrasound. A number of dosimetry methods are in practice to quantify the amount of radicals generated by acoustic cavitation. In this study, hydroxyl radical (OH center dot) yields measured by Weissler, Fricke and terephthalic acid dosimetry methods have been compared to evaluate the validities of these methods using a 490 kHz high frequency sonochemical reactor. The OW yields obtained after 5 min sonication at 490 kHz from Weissler and Fricke dosimetries were 200 mu M and 289 mu M, respectively. Whereas, the OW yield was found to be very low (8 mu M) when terephthalic acid dosimetry was used under similar experimental conditions. While the results agree with those reported by Lida et M. (Microchem. J., 80 (2005) 159), further mechanistic details and interfering reactions have been discussed in this study. For example, the amount of OW determined by the Weissler and Fricke methods may have some uncertainty due to the formation of HO2 center dot in the presence of oxygen. In order to account for the major discrepancy observed with the terephthalic acid dosimetry method, high performance liquid chromatography (HPLC) analysis was performed, where two additional products other than 2-hydroxy terephthalic acid were observed. Electrospray ionization mass spectrometry (ESI-MS) analysis showed the formation of 2,5-dihydroxyterephthalic acid as one of the by-products along with other unidentified by-products. Despite the formation of additional products consuming OH center dot, the reason for a very low OH center dot yield obtained by this dosimetry could not be justified, questioning the applicability of this method, which has been used to quantify OW yields generated not only by acoustic cavitation, but also by other processes such as gamma-radiolysis. The authors are hoping that this Opinion Paper may initiate further discussion among researchers working in sonochemistry area that could help resolve the uncertainties around using these dosimetry methods.

Automated summary

This study compared the validity of different dosimetry methods in measuring the OH· yield generated by acoustic cavitation, and discussed possible mechanistic details and interfering reactions. Despite the formation of additional products consuming OH·, the reason for very low OH· yield obtained by terephthalic acid dosimetry method could not be justified, questioning its applicability in quantifying OH· yields.