Convection, in electrolytes

Usual conditions for LSV or CV experiments require a quiet solution in order to allow undisturbed development of the diffusion layer at the electrode. Some groups, however, have purposely used the interplay between diffusion and convection in electrolytes flowing in a channel or similar devices [23]. In these experiments (see also Chapter 2.4), mass transport to the electrode surface is dramatically enhanced. A steady state develops [54] with a diffusion layer of constant thickness. Thus, such conditions are in some way similar to the use of ultramicroelectrodes. Hydro-dynamic voltammetry is advantageous in studying processes (heterogeneous electron transfer, homogeneous kinetics) that are faster than mass transport under usual CV or LSV conditions. A recent review provides several examples [22]. 

Forced convection may be effected by stirring the solution, rotating or vibrating electrodes (or both), or working with a flowing solution. Theoretical treatment of forced convection in electrolytic cells is difficult, but is possible under certain assumed boundary conditions. A quantity of interest is the diffusion layer away from the electrode surface, and empirical relationships have been developed for some electrode geometries. Two relationships are summarized below ... 

In cases where the values of 5,.(, and thns of f., are large enough (without significant convection in the electrolyte solntion), another limiting state is attained which is typical for galvanostatic conditions and where the reactant concentration at the surface falls to zero (Fig. 3b). For the time required to attain this state, Eq. (11.6) yields... 

As demonstrated in the preceding section, an electric potential gradient is formed in electrolyte solutions as a result of diffusion alone. Let us assume that no electric current passes through the solution and convection is absent. The Nernst-Planck equation (2.5.24) then has the form ... 

Fig. 7.91. The change in electrolyte concentration produces a change in the density of the electrolyte a flow of electrolyte results from natural convection and the hydrostatic equilibrium is thus upset.

Transport Processes. The velocity of electrode reactions is controlled by the charge-transfer rate of the electrode process, or by the velocity of the approach of the reactants, to the reaction site. The movement or trausport of reactants to and from the reaction site at the electrode interface is a common feature of all electrode reactions. Transport of reactants and products occurs by diffusion, by migration under a potential field, and by convection. The complete description of transport requires a solution to the transport equations. A full account is given in texts and discussions on hydrodynamic flow. Molecular diffusion in electrolytes is relatively slow. Although the process can be accelerated by stirring, enhanced mass transfer... 

A special feature of this reaction is that the first order kinetics is not related to the propylene concentration fed by convection and gas-liquid mass transfer (a saturated propylene concentration assumed in electrolyte), but related to the OH ion concentration, because the OH ion pass through the membrane from the cathode side ... 

To conclude the discussion on quasi-reversible reactions, we now direct our attention to sonoelectrochemical reactions on diamond electrodes [107-109], In sonoelectrochemistry, power ultrasound is applied to electrochemical cell, causing forced convection in the electrode-electrolyte system. As a result of the enhanced mass transfer, non-steady-state potentiodynamic curves with current peak turn to steady-state curves with a limiting current plateau (Fig. 21). Notice a significant increase in the current. It must be emphasized that in sonoelectrochemistry electrode materials are exposed to extreme conditions with mechanical strains induced by pressure waves and cavitation-induced liquid jets strong enough to cause severe erosion. Diamond withstands the sonoelectrochemical conditions perfectly. This opens up fresh possibilities for efficient electrolyses and electroanalyses with diamond electrodes. 

Several steps can be taken to combat convection in a free electrolyte. Hjerten, for example, used a horizontal tube rotating around its own axis [7]. Any small convective displacement due to gravity is exactly reversed as the tube rotates 180°. Thus the gravitational convective displacements— while not eliminated—exactly cancel one another in the course of rotation. More recent experiments aboard earth satellites are carried out under near-zero gravity (nonconvective) conditions [8,9]. It has also been found useful to work at 4°C where water has its maximum density and where, consequently, density is least sensitive to temperature. Stabilizing density gradients have been introduced in some cases to counteract convection. 

For a vesicle of thickness 6, Immersed in a stagnant (l.e. no convection) dilute electrolyte solution the transport of species across the bllayer can be formulated by the following set of equations (29) ... 

The third form of mass transport is convection driven by pressure. When forced circulation exists in electrolyte, convection may be the dominant form of mass transport. Thus, in general, a flux Jj (mol/s cm) of species j may occur due to the above three types of mass transport mechanisms. The flux can be described by the Nernst-Planck equation [5]... 

This treatment is based on the assumption that there is no convection in the electrolyte. This is far from the situation during industrial electrolysis. However, the major contribution from electronic conduction originates in the diffusion layer near the cathode, which can be assumed to be stagnant. 

In order to undergo a redox process, the reactant must be present within the electrode-reaction layer, in an amount limited by the rate of mass transport of Yg, to the electrode surface. In electrolyte media, four types of mass-transport control, namely convection, diffusion, adsorption and chemical-reaction kinetics, must be considered. The details of the voltammetric procedure, e.g., whether the solution is stirred or quiet, tell whether convection is possible. In a quiet solution, the maximum currents of simple electrode processes may be governed by diffusion. Adsorption of either reactant or product on the electrode may complicate the electrode process and, unless adsorption, crystallization or related surface effects are being studied, it is to be avoided, typically... 

Obretenov, W., Petrov, L, Nachev, L, and Staikov, G. 1994. The kinetics of structural changes in Cu adlayers on Au(lll). Journal of Electroanalytical Chemistry 371, 101-109. O Brien, R.N., and Santhanam, K.S.V. 1997. Magnetic field assisted convection in an electrolyte... 

Instant coagulation also excludes methods that require a stable dispersion, such as electrophoresis. The manufacturers of certain types of zetameters claim that their instruments are suitable to perform measurements in electrolyte solutions up to about 1 M. However, in order to use a zetameter, one has to prepare a stable dispersion first, and this may be problematic. Electro-osmosis does not require stability against sedimentation, but other problems, such as low absolute values of the potential (which may be smaller than the scatter of results) and the production of heat, convective currents, or electrolysis products (acids, bases, and gases), severely limit the application of classical electrokinetic methods (including electrophoresis) in measurements at ionic strengths greater than 0.1 M. Very few publications report potentials obtained by classical electrokinetic methods at higher electrolyte concentrations, and the results are controversial. 

To increase the electrolyte conductivity, an additional ionic component that does not participate in the electrochemical reactions is often added to a solution. This nom-eactive component is called a supporting electrolyte or indifferent electrolyte. In the presence of a supporting electrolyte, there is a lowering of the electric field in solution, due to the electrolyte s high conductivity. Transport of the minor ionic species in solution is due primarily to diffusion and convection, in accordance with Equation (26.54) with VO = 0. Also, in the presence of a supporting electrolyte, the convective diffusion equation for a minor component in solution is written as... 

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