Hydrodynamic technique

There are a few electrochemical techniques in which the working electrode is moved with respect to the solution (i.e. either the solution is agitated or the electrode is vibrated or rotated). Under these conditions, the thickness of the diffusion layer decreases so that the concentration gradient increases. Since the rate of the mass transport to an electrode is proportional to the concentration gradient (Chapter 1, Section 4.2.2), the thinning of the diffusion layer leads to an increase of the mass transport, and hence to an increase of the faradaic currents. 

The ability of varying the rate of the mass transport by agitating the solution (or the working electrode) constitutes the basis of hydrodynamic methods (hydrodynamics = liquids in motion), which are a further support to the study of electrode kinetics. Nevertheless we wish to cite them here simply to cover a drawback of cyclic voltammetry. In fact, cyclic voltammetry is unable to discriminate between oxidation and reduction processes, and vice-versa. 

In order to understand the meaning of such a sentence one must consider that the starting potential for a cyclic voltammetric scan must be selected on the basis of the zero-current condition, or to start from a potential value at which no electron transfer occurs at the working electrode. Even if one was wrong in the choice of the starting potential, i.e. one could have selected a potential at which an electrode process is taking place, it is very unlikely that one can realize it, in that the 

To obviate possible erroneous interpretation of the cyclic voltammetric profiles (particularly in those cases in which an oxidation process occurs at negative potential values as well as a reduction process occuring at positive potential values), it is always wise to perform preliminarily hydrodynamic tests. 

The most well-known hydrodynamic technique is the Rotating Disk Electrode (RDE) voltammetry, which, however, needs proper equipment. For information on this technique the reader is referred to specialized treatments.2 4 We prefer here to mention a simpler technique which can be carried out on the same equipment used for cyclic voltammetry. This technique is referred to as voltammetry at an 

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Table Hydrodynamic techniques for characterising the biophysical properties of polysaccharides in solution ...

Enzyme linked electrochemical techniques can be carried out in two basic manners. In the first approach the enzyme is immobilized at the electrode. A second approach is to use a hydrodynamic technique, such as flow injection analysis (FIAEC) or liquid chromatography (LCEC), with the enzyme reaction being either off-line or on-line in a reactor prior to the amperometric detector. Hydrodynamic techniques provide a convenient and efficient method for transporting and mixing the substrate and enzyme, subsequent transport of product to the electrode, and rapid sample turnaround. The kinetics of the enzyme system can also be readily studied using hydrodynamic techniques. Immobilizing the enzyme at the electrode provides a simple system which is amenable to in vivo analysis. 

Hydrodynamic Techniques for Investigating Reaction Kinetics at Liquid-Liquid Interfaces Historical Overview and Recent Developments... 

The electrolyte dropping electrode [63] method, introduced in 1976, and subsequently used in conjunction with the four-electrode potentiostat [64], is a hydrodynamic technique, offering controlled convective transport. In essence, this approach is identical to the dropping mercury electrode [65] however, the drop consists of a flowing electrolyte liquid phase which forms a polarized ITIES with an immiscible continuous (receptor) phase. In... 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

Ambulgekar GV, Samant SD, Pandit AB (2005) Oxidation of alkylarenes using aqueous potassium permanganate under cavitation Comparison of acoustic and hydrodynamic techniques. Ultrason Sonochem 12 85-90... 

Experimentally, the form of 4>(z) has been recently established for adsorbed homopolymers and terminally anchored tails by the Bristol group (17,20). Knowing 4>(z) one may then calculate the r.m.s. thickness of the adsorbed layer. Previous measurements of the "thickness" (1-4) have usually involved ellipsometry (flat surfaces) or some hydrodynamic technique (particles). In neither case can the calculated thickness be unambiguously related to 4>(z), although recent theoretical work by Cohen Stuart et al. (21), to be discussed at this meeting, has made an attempt to relate the hydro-dynamic thickness, 6, to < >(z). 

Please note the use of the hydrodynamic technique to detect the zero-current potential value... 

Convection (of the electrolyte liquid phase as a whole) can be natural (due to thermal effects or density gradients) or forced (principal mass transport mode in hydrodynamic techniques). Still, however, close to the electrode surface a diffusion layer develops. 

In hydrodynamic techniques, convection is the principal mode of mass transport, and is brought about by the controlled movement of the electrode in the solution or by pumping the electrolyte through a pipe or channel. 

After perturbation, the surface concentrations of the reactants, intermediates, and products attain new values. In a genuine stationary technique, these new values are maintained by providing a constant rate of mass transport (hydrodynamic techniques). The surface concentration, c[ of a species i is related to its flux, J, by... 

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). 

MFEs are also useful in hydrodynamic techniques, such as rotating-disk voltammetry (Chap. 3) and electrochemical detection for liquid chromatogra-... 

Slevin, C.J., Unwin, P.R. and Zhang, J. (2001) Hydrodynamic techniques for investigating reaction kinetics at liquid-liquid interfaces historical overview and recent developments. In A.G. Volkov (Ed.) Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications. Dekker, New York, Chapter 13. 

Altering the convective rate of transport, e.g. by changing the rotation frequency of a rotating-disc electrode. Experiments in which the convective rate of transport can be altered are known as hydrodynamic techniques. 

The potential profile associated with hydrodynamic techniques usually takes the form of a linear sweep between two potentials in which the oxidation or reduction processes of interest occur. As for cyclic voltammetry, the gradient of the ramp represents the scan rate. However, for steady-state techniques, the scan rate used must be sufficiently slow to ensure that the steady state is attained at every potential during the course of the voltammetric scan. The upper value of the scan rate that may be used under the steady-state regime is therefore restricted by the rate of convective mass transport of material to the electrode surface. The faster the rate of convective mass transport the faster the scan rate that may be used consistent with the existence of steady-state conditions. 

Conversely for slow reactions, low rates of mass transport will be required to achieve significant deviations from N fs equalling one. Consequently, it can be appreciated that it is a study of the competition between the rates of mass transport and chemical kinetics that leads to the quantitative determination of electrode reaction mechanisms in hydrodynamic voltammetry. Importantly, for each hydrodynamic technique, there is one assessable convective transport parameter that directly relates to the kinetic time-scale. 

It should be realized that hydrodynamic techniques measure an effective thickness obtained by comparing the tangential flow along a polymer-covered surface with that along a bare surface. The effective thickness thus found is usually called the hydrodynamic layer thickness. The exact shape of the flow velocity profile is Important, and this depends on the shape of the surface in question, and on its orientation with respect to the flow field. Detailed data Interpretation is therefore only possible if simple geometries are chosen such as smooth cylindrical channels or spherical colloidal particles. 

Estimation of q from hydrodynamic techniques such as intrinsic viscosity or diffusion requires an additional parameter, the hydrodynamic diameter... 

It is clear from these studies that rotatory dispersion, especially if it is supplemented by hydrodynamic techniques, provides more accurate information on conformational changes than do monochromatic rotations. In any case, the changes in monochromatic rotation caused by disulfide cleavage can always be anticipated and perhaps in many cases eliminated by calculation, as Turner et al. (1958) and Wiirz and Haurowitz (1961) have been able to do. 

The critical flux or sustainable flux can be used as the design flux for the processes, which is to be enhanced by the hydrodynamic techniques applied. 

Most of hydrodynamic techniques will reduce the module/membrane cost due to enhancement of the design flux but increase the energy cost due to increased pressure drop through the module. The optimal process design should result in a minimum total cost or the sum of the energy and module cost. An example of this trade-off is described in Section 8.3. The important point to note is that the performance enhancing techniques involve a cost that must provide a benefit and return on investment. 

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