Ultrasound power

The first requirement for research in sonochemistry is a source of ultrasound and whatever type of commercial instrument is used the energy will be generated via an ultrasonic transducer - a device by which mechanical or electrical energy can be converted to sound energy. There are three main types of ultrasonic transducer used in sonochemistry. 

2 Magnetostrictive transducers are electromechanical devices which use magnetostriction, an effect found in some ferromagnetic materials, e.g. nickel. Such materials reduce in size when placed in a magnetic field and 

Electrochemistry Almost any electrochemical process benefits from the presence of ultrasound with improved current efficiency and the prevention of electrode fouling. These advantages are particularly evident in electroplating but can be also beneficial in electrosynthesis and electroanalytical chemistry. 

Food technology Ultrasound provides a major new processing aid from the point of view of its applications in mixing, homogenisation, aeration, drying, freezing, sterilisation and even meat tenderisation. Several applications have also been reported for its use as an adjunct to extraction 

Environmental Traditionally power ultrasound is widely used protection for cleaning and has proved successful in the replacement of chlorinated solvents by aqueous based detergents and also for disinfection. A new area of interest is in water treatment for the destruction of chemical pollutants, for aeration and ozone treatment, for its bactericidal action and in filtration and dewatering processes. 

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Table 7.2 Some other uses of power ultrasound...

Chow R, Blindt R, Chivers R, Povey M (2005) A study on the primary and secondary nucleation of ice by power ultrasound. Ultrasonics 43 227-230... 

In a biphasic solid-liquid medium irradiated by power ultrasound, major mechanical effects are the reduction of particles size leading to an increased surface area and the formation of liquid jets at solid surfaces by the asymmetrical inrush of the fluid into the collapsing voids. These liquid jets not only provide surface cleaning but also induce pitting and surface activation effects and increase the rate of phase mixing, mass transfer and catalyst activation. 

Cravatto G, Cintas P (2006) Power ultrasound in organic synthesis Moving cavitational chemistry from academia to innovative and large-scale applications. Chem Soc Rev 35 180-196... 

Trotta F, Martina K, Robaldo B, Barge A, Cravotto G (2007) Recent advances in the synthesis of cyclodextrin derivatives under microwaves and power ultrasound. J Incl Phenom Macro Chem 57 3-7... 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Gonzalez-Garcia J, Banks CE, Sljukic B et al (2007) Electrosynthesis of hydrogen peroxide via the reduction of oxygen assisted by power ultrasound. Ultrason Sonochem 14 405 112... 

Amara N, Ratsimba B, Wilhelm A, Delmas H (2001) Crystallization of potash alum effect of power ultrasound. Ultrason Sonochem 8(3) 265-270... 

Keeping in mind the above work, experiments were carried out to examine the effects of ultrasound, on the dissolution of zinc metal in an alkaline medium and the decomposition of zinc-dithizone complex in the presence of an ultrasonic field. To examine the effect of power ultrasound on the dissolution of zinc metal in alkaline media, 0.0480 g zinc metal was treated with 10 ml of 5 M NaOH solution. Two samples of this solution were exposed to ultrasound for 15 and 30 min, while, control samples were also kept in the similar condition and for the same duration. To compare their spectra and concentration of dissolved zinc in sonicated and control conditions, zinc-dithizone complex was formed by adding 0.5 ml of 0.005% dithizone solution. The red coloured complex, thus obtained, was extracted in chloroform and made upto to the mark in 25 ml volumetric flask with chloroform. 

De Morais NLPA, Brett CMA (2002) Influence of power ultrasound on the corrosion of aluminium and high speed steel. J Appl Electrochem 32 653-660... 

Cravotto G, Binello A, Di Carlo S, Orio L, Zhi-Lin Wu, Ondmschka B (2010) Oxidative degradation of chlorophenol derivatives promoted by microwaves or power ultrasound a mechanism investigation. Environ Sci Poll Res 17(3) 674-687... 

Farmer AD, Codings AF, Jameson GJ (2000) The application of power ultrasound to the surface cleaning of silica and heavy mineral sands. Ultrason Sonochem 7(4) 243-247... 

Rehorek A, Tauber M, Gubitz G (2004) Application of power ultrasound for azo dye degradation. Ultrason Sonochem 11 177-182... 

Applied Sonochemistry Uses of Power Ultrasound in Chemistry and Processing. 

Since 1945 an increasing understanding of the phenomenon of cavitation has developed coupled with significant developments in electronic circuitry and transducer design (i. e. devices which convert electrical to mechanical signals and vice versa). As a result of this there has been a rapid expansion in the application of power ultrasound to chemical processes, a subject which has become known as Sonochemistry . 

Biology, Biochemistry Homogenisation and cell disruption Power ultrasound is used to rupture cell walls in order to release contents for further studies. 

Ultrasonic cleaning is another major application for power ultrasound. It is now such a well-established technique that laboratories without access to an ultrasonic cleaning bath are in a minority. It is important to recognise the historical significance of the development of ultrasonic cleaning technology on the growth of sonochemistry because, in the early years, the humble laboratory cleaner was almost certainly the first ultrasonic apparatus used by chemists. 

Maximum disruption is obtained in a zone close to the probe tip and the biological cells must be kept here for sufficient time to allow disruption to take place. A delicate balance must therefore be struck between the power of the probe and the disruption rate since power ultrasound, with its associated cavitational collapse energy and bulk heating effect, can denature the contents of the cell once released. Indeed for this type of usage it is important to keep the cell sample cool during sonication. The method is very effective and continues to be an important tool in microbiology and biochemistry research. 

A number of applications of power ultrasound are to be found in heavy industry both in metalworking and processing [17]. The machining of modem materials requires tools that can deal with unusual properties, complex shapes of work-pieces, and accuracy in working. Basic ultrasonic machining processes become of importance when dealing with carbides, stainless steels, ceramics and glass. Four main types of application are of industrial relevance ... 

The classical techniques for the solvent extraction of chemical compounds from vegetable material are based upon the correct choice of solvent and conditions e. g. heating or agitation. A range of commercially important pharmaceuticals, flavours and colourants are now derived from vegetable sources. It has been shown that the solvent extraction of organic compounds contained within the body of plants and seeds is significantly improved by the use of power ultrasound [25]. 

Power ultrasound also has an additional property which is particularly beneficial in crystallisation operations namely that the cleaning action of the cavitation effectively stops the encrustation of crystals on cooling elements in the crystallisation vat and thereby ensures continuous efficient heat transfer. 

From the above one might be tempted to attribute ultrasonically enhanced chemical reactivity mainly to the mechanical effects of sonication. However this cannot be the whole reason for the effect of ultrasound on reactivity because there are a variety of homogeneous reactions which are also affected by ultrasonic irradiation. How, for example, can we explain the way in which power ultrasound can cause the emission of light from sonicated water (sonoluminescence), the fragmentation of liquid alkanes, the liberation of iodine from aqueous potassium iodide or the acceleration of homogeneous solvolysis reactions ... 

Tab. 1.2. Possible benefits from the use of power ultrasound in chemistry.

J.-P. Xie Dyeing, J.F. Ding, T.J. Mason, and G.E. Attenburrow, Influence of power ultrasound on leather processing. Part 1, Journal of the American Leather Chemists Association, 1999, 94, 146-157. 

Most chemists working on sonochemistry in the laboratory will either use some form of ultrasonic bath or a commercial probe system. The latter instruments are often equipped with a pulse facility which was originally designed for biological cell disruption where temperature control is important. This pulse facility enables the power ultrasound to be delivered intermittently and thereby allow periods of cool-... 

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