Ultrasonic baths selection

The extraction and cleanup of samples was dealt with earlier. As was indicated, this can be done very simply with excellent results. For example, the PCBs can be extracted by being vigorously shaken mechanically with dichloromethane or petroleum ether as solvent and then placed in an ultrasonic bath for a short while. After decantation, the organic phase is concentrated by selected evaporation. 

Both new compounds A and B gave satisfactory microanalyses and NMR spectra. Selected amino acids (R)- and (S)-alanine (Ala), (R)- and (S)-phenylglycine (PhGly) and (R)- and (S)- B-phenylalanine ( p-PhAla) as their sodium and potassium salts were dried in vacuo over phosphorus pentoxide prior to use and served as guest species. The host/guest ratio was 2 1 in deuteriochloroform solutions. Dissolution of solid salt was accomplished in an ultrasonic bath within 20 to 100 min. The host concentration was ca. 0.1 M. All the variable-temperature spectra were recorded with JEOL-JNM-4H-100 spectrometer at 100 MHz with TMS as internal standard and lock. The anomeric proton served as an NMR probe. The free energy of activation at the coalescence temperature was calculated from the expression 7 ... 

Generally, the monoacylation is 3-15 times faster than second acylation and in the batch procedure the diacylated product 54 was obtained as major product (Table 6.5, entries 6, 7, and 13). In contrast, the monoacylated diamine 53 was obtained as major product when the microfluidic droplet and capillary approaches were applied. Here, the droplet-based protocol leads to slightly better yields and selectivities than the capillary procedure (compare entries 3 and 5), which was attributed to the improved mixing efficacy with additional convection effects within the droplets. But the amount of the continuous phase is certainly a major drawback of the droplet protocol. The capillary procedure was therefore improved by the treatment with ultrasound in a commercially available ultrasonic bath (330 W). Sonication of the capillary during the reaction leads to comparable results as in the droplet procedure in a much shorter residence time of 0.5 min (entry 9, overall flow rate 200pl/min, that is, 0.005 mmol/min). Compared to the silent process, the yield was increased by a factor of 4. With an improved ultrasound-supported continuous capillary protocol, a number of symmetric diamines (e.g., piperazine) were successfidly monoacylated. 

Work by Ono et al. [66] has been specifically directed at ultrasonic control of product-selectivity in electroreductions. Using a lead cathode, in dilute methanolic sulphuric acid, at a constant current of 20 mA cm , Ono electroreduced benzaldehyde under stirred, unstirred and ultrasonic conditions (Fig. 6.17). In an unstirred system, benzyl alcohol (two-electron process) was the major product, while mechanical stirring reversed the position in favour of the hydrodimer (one-electron product). Ultrasonic irradiation from a cleaning bath (100 W, 36 kHz) so strongly favoured the hydrodimer that the alcohol was barely evident (Tab. 6.16). 

Drake [99] in which ultrasound is used to promote fast electron exchange at the electrode surface. The ultrasound agitation is provided by an ultrasonic transducer immersed in the bath. This method is particularly suitable for the selective plating of precious metals, e.g. Au on printed circuit boards. 

FIGURE 2.34 Experimental setup used for the two modes of continuous ultrasound-assisted leaching and the possibilities of their coupling to other steps of the anal)dical process. One, two or three steps can he used in a single method. Abbreviations used are LC, leaching carrier PP, peristaltic pump UP, ultrasonic probe EC, extraction chamber WB, water bath W, waste SV, selection valve EX, extract E, eluent IV, injection valve SPC, solid-phase column DR, derivatization reagent RC, reaction coil and D, detector. (Reprinted from Luque-Garcia, J. L. and Luque de Castro, M. D., Trends Anal. Chem., 22, 41-47, 2003. With permission from Elsevier.)... 

Selective oxidation of diuron in a MeOH-water mixture Potentiostatic control 35 kHz cleaning bath and 20 kHz ultrasonic probe Sonication improves current efficiencies, mass transport rates, and depassivates the probe was reported to be a much more reliable and effective method of insonation 65... 

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