Very few papers are devoted to the study of the chemical effects of pulsed ultrasound in the low-frequency range. The present work consists of a systematic experimental study of the effects of pulsed ultrasound in the 20 kHz range using an immersed titanium horn. The light scattered by the bubble cloud, the acoustic pressure, and the sonochemical activity were measured. The sonochemical activity was studied by measuring the light emitted by a fast chemiluminescent reaction (oxidation of luminol). The chemiluminescence behavior observed at 20 kHz was compared with the behavior observed at 1.7 MHz. The chemiluminescence takes time to install when sonication starts and, at 20 kHz, the luminescence intensity decreases monoexponentially when sonication stops. Interestingly, at 1.7 MHz, the luminescence intensity decreases biexponentially with an important fast component. The interpretation of these various behaviors requires that the acoustical characteristics of the ultrasound generator and of the vessel, and also the properties of the bubble field, be considered.

In the first part of the work, it was observed, by a relative method, that the Weissler reaction and the Br2-catalysed isomerisation of maleic acid into fumaric acid are faster at 20 kHz than at 1.7 MHz. The difference between the relative reaction rates can be considered as small when the two order magnitude difference between the two frequencies is taking into account. In the second part of the work, the frequency effect associated to an isotope effect was studied. The Weissler reaction was performed in H2O and D2O at 20 kHz and 1.7 MHz. The isotope effect is not the same at the two frequencies.

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Some Physicochemical Aspects of So-Called "Homogeneous Sonochemistry" -[54 refs.]. -(REISSE, J.; CAULIER, T.; DEKERCKHEER, C.; KEGELAERS, Y.; SEGEBARTH, N.; BARTIK, K.; NATO ASI Ser., Ser. C 524 (1999) 205-224; Univ. Libre Bruxelles, B-1050 Brussels, Belg.; EN)