Hydrodynamic pulse techniques

The technique of hydrodynamic modulation voltammetry (HMV), in which the rate of stirring is pulsed between high and low values, is demonstrated in this experiment. The application of HMV for the quantitative analysis of ascorbic acid in vitamin C tablets using the method of standard additions also is outlined. 

MRI to characterize hydrodynamics within reactors is already established. The extent to which the potential of MR to study both hydrodynamics and chemical conversion is fully realized will depend on our ability to integrate the well-established MR spectroscopy techniques in liquid- and solid-state NMR into imaging pulse sequences, and still provide quantitative data in the magnetically heterogeneous environments typical of catalysts and reactors. 

Chapter 1 serves as an introduction to both volumes and is a survey of the fundamental principles of electrode kinetics. Chapter 2 deals with mass transport — how material gets to and from an electrode. Chapter 3 provides a review of linear sweep and cyclic voltammetry which constitutes an extensively used experimental technique in the field. Chapter 4 discusses a.c. and pulse methods which are a rich source of electrochemical information. Finally, Chapter 5 discusses the use of electrodes in which there is forced convection, the so-called hydrodynamic electrodes . 

CV has become a standard technique in all fields of chemistry as a means of studying redox states. The method enables a wide potential range to be rapidly scanned for reducible or oxidizable species. This capability, together with its variable time scale and good sensitivity, makes CV the most versatile electroanalytical technique thus far developed. It must, however, be emphasized that its merits are largely in the realm of qualitative or diagnostic experiments. Quantitative measurements (of rates or concentrations) are best obtained via other means (e.g., step, pulse, or hydrodynamic techniques). Because of the kinetic control of many CV experiments, some caution is advisable when evaluating the results in terms of thermodynamic parameters (e.g., measurement of E° for irreversible couples). 

Recent developments in ultrashort, high-peak-power laser systems, based on the chirped pulse amplification (CPA) technique, have opened up a new regime of laser-matter interactions [1,2]. The application of such laser pulses can currently yield laser peak intensities well above 1020 W cm 2 at high repetition rates [3]. One of the important features of such interactions is that the duration of the laser pulse is much shorter than the typical time scale of hydrodynamic plasma expansion, which allows isochoric heating of matter, i.e., the generation of hot plasmas at near-solid density [4], The heated region remains in this dense state for 1-2 ps before significant expansion occurs. 

The second part of the book discusses ways in which information concerning electrode processes can be obtained experimentally, and the analysis of these results. Chapter 7 presents some of the important requirements in setting up electrochemical experiments. In Chapters 8—11, the theory and practice of different types of technique are presented hydrodynamic electrodes, using forced convection to increase mass transport and increase reproducibility linear sweep, step and pulse, and impedance methods respectively. Finally in Chapter 12, we give an idea of the vast range of surface analysis techniques that can be employed to aid in investigating electrode processes, some of which can be used in situ, together with photochemical effects on electrode reactions— photoelectrochemistry. 

The form of the response is a succession of points following the same profile as a conventional voltammogram. However, since a pulse causes greater mass transport than a steady-state technique (hydrodynamic electrode), a reaction that appears reversible in the steady state can appear quasi-reversible with this technique. On the other hand, given the short timescale, effects due to coupled homogeneous reactions may not be observed. 

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

Most of hydrodynamic methods have focused on increasing the particle back transport from the membrane-liquid interface by increasing the shear rate and the flow instability in the boundary layer. These techniques include secondary flows, spacers and inserts, pulsed flow, high shear rate devices, vibrations, and two-phase flow. The physical methods that are currently been tested to enhance filtration performance of membranes include the application of electric fields and ultrasound. 

The dendrimer size in solution can he obtained employing diffusion NMR spectroscopy [pulsed gradient spin-echo (PGSE), diffusion ordered spectroscopy (DOSY)] that allows the determination of the self-diffusion coefficient of a molecule, which is related to the hydrodynamic radius through the Stokes-Einstein equation. Using this technique, the flexible structure of dendrimers in solution was demonstrated in PAMAM derivatives, which swell or shrink with pH modification. This size variation in solution should be taken into account if interactions with nucleic acids are being studied. 

Ellis, A.T. Techniques for Pressure Pulse Measurements and High-Speed Photography in Ultrasonic Cavitation. Proc. N.P.L. Symposium on Cavitation in Hydrodynamics 1955. Paper 8, 1—32, Her Majesty s Stationery Office, London. 

Other types of column are also being investigated in addition to the spray column. In a pulsed sieve plate column the same measuring techniques can be used as in a spray column. Concentration profiles and hydrodynamic parameters have been measured on columns of two sizes, namely 50 and 100 mm diameter, and the mass transfer coefficients evaluated. The results were similar to... 

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