Aqueous sonochemistry

The effects of ultrasound on aqueous solutions of reagents has been exhaustively investigated, mostly to little end, from a synthetic point of view. Much of the early work was concerned with observations that a variety of inorganic substrates underwent not only oxidation, but also reduction reactions when exposed to ultrasound. For example  

sonolysis with an immersion horn under typical laboratory conditions will produce hydrogen peroxide at a rate of about 30 pM/minute. This appears to arise from combination of OH and H radicals whose existence have now been conclusively determined by spin trapping experiments [52]. In addition, Margulis has proposed the formation of solvated elec- 

The nature of the organozinc species has not been elucidated, but it seems likely that the reaction proceeds via a radical pathway, where the presence of water would be beneficial. It should also be stressed that these results are 

Solvents used EtOH/HaO (9 1) acetone/H20 (4 1) pyridine/H20 (1 2) pure H2O 

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The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Sostaric JZ (1999) Interfacial effects on aqueous sonochemistry and sonoluminescence. PhD thesis, University of Melbourne, Australia... 

Homogeneous non-aqueous sonochemistry is typified by the sonolysis of chloroform which has been studied using ultrasonic irradiation of frequency 300 kHz (I = 3.5 W cm ) to yield a large number of products amongst which are HCl, CCI4 and C2CI2 [42]. Decomposition was found to only occur in the presence of... 

Homogeneous non-aqueous sonochemistry is another unexplored area for analytical chemists that might provide very interesting results [99]. 

Experiments have shown that aqueous sonochemistry is unchanged over the frequency range in which cavitation occurs i.e. 10 Hz to 10 MHz [22]. Since there is no direct coupling of the sound field with species on a molecular level, changing the frequency of the sound input simply alters the resonant size of the cavitation bubble. The effect of this over the range of interest is negligible. It should, however, be noted that although there is both an upper and a lower limit to the frequencies at which cavitation will occur, the band of frequencies used for sonochemistry lies well within these limits. 

Aqueous sonochemistry is dominated by reactions of OH and H radicals as a consequence of the high vapour pressure of water relative to any organic or inorganic reagents present and despite the enormous amount of attention... 

Homogeneous Sonochemistry Bond Breaking and Radical Formation. The chemical effect of ultrasound on aqueous solutions have been studied for many years. The primary products are H2O2 there is strong evidence for various high-energy intermediates, including HO2,... 

Beckett M, Hua I (2001) Impact of Ultrasonic Frequency on Aqueous Sonoluminescence and Sonochemistry. J Phys Chem A 105 3796-3802... 

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