Ultrasonic devices baths

Two different types of ultrasonic devices are used in laboratories ultrasonic bath and ultrasonic probe. However, as a result of inhomogeneity of ultrasonic energy distribution in the whole solution and a decrease in power with time, the repeatability and reproducibility of experimental conditions for ultrasonic baths is often unsatisfactory. With ultrasonic probes the energy is focused on a small sample area, which significantly improves cavitation efficiency and, thereby, extraction effectiveness [56]. 

Of the four types of laboratory ultrasonic apparatus commercially available for practising chemists in general (namely, whistle reactors, ultrasonic cleaning baths, probes and cup-horn devices) analytical chemists, except for a few specialists working in (or with) ultrasound detectors, use mainly cleaning baths and probes both of which are usually operated at a fixed frequency dependent on the particular type of transducer, that is usually 20 kHz for common probe systems and 40 kHz for baths. Both types of devices are described below. 

Except in some special cases where the users themselves have designed and produced their own ultrasonic devices, US equipment for leaching consists of commercial ultrasonic baths or probes. 

The irradiation time is highly dependent on the power of the ultrasonic device. A color change of the reaction mixture from orange-brown to deep purple indicates reduction of the diphosphene to the corresponding anion radial. If this occurs, the flask should be removed from the ultrasonic bath. Addition of few drops of MeOH restores the orange color. 

Concerning the laboratory devices used for sonochemistry, common cleaning baths are constructed aroimd one or more ceramics fitted to the external face of a tank (p. 304). Such devices work at a single frequency, generally between 20-50 kHz, fixed by the manufacturer with an acoustic power of ca, 1 W. Immersion horns are used when more acoustic power is required. Emitters are composed of a "pancake" of PZT ceramics compressed between a titanium rod and a steel countermass (p. 305). Usually horn devices work from 20 to 100 kHz, and the acoustic power emitted can reach several tens of W. For higher frequencies, piezoceramics are simply fixed to the reactor. The reader interested in the construction of ultrasonic devices should consult Ref. 21. 

As noted above this type of mechanical transducer is predominantly used for homo-genisation/emulsification. These devices differ markedly from the more usual bath and probe types in that they derive their power from the medium (by mechanical flow across the blade) rather than by the transfer of energy from an external source to the medium. The majority of the chemical effects observed on using whistle type transducers for the sonication of homogeneous reactions can be attributed mainly to the generation of very fine emulsions rather than the ultrasonic irradiation itself. 

Industry uses special devices similar to ultrasonic baths and probes but appropriately scaled up in size and ultrasound irradiation power. The UIP16000 model from Hielscher Ultrasound Technology is by far the most powerful ultrasonic processor available worldwide the apparatus is capable of delivering a continuous power of 16000 W at efficiency above 80%. Such powerful systems have been developed in response to the demand for the ultrasonic treatment of liquids on a large scale in fact, the ultrasound power required usually increases in proportion to the amount of liquid to be treated within a certain time. 

An immersible transducer is a radiating device accommodated in a housing which can be submerged in a liquid bath to energize the liquid and produce cavitation. An immersible transducer placed in a still tank therefore turns that tank into an ultrasonic cleaner. The immersible transducer is, in effect, a standard tank (turned inside out) with the radiating surface on the outside and the transducers on the inside. 

In a disorete approach, the analytical system is confined in a vessel or container through the walls of which US energy is transmitted if an ultrasonic bath is used. The use of a US probe in this case can involve either to dip it into the vessel or into the transmitting liquid where the vessel is located. The complexity of the analytical system determines the type of vessel or container to be used, namely an open or closed, atmospheric pressure or pressurized device, a jacket-tailored device for maintaining the optimum temperature, etc. 

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