Concentration polarization agitation

From this equation it can be seen that the concentration polarization will be smaller with an increased concentration (activity) of ions in the solution at a given current density. Its value can also be decreased by increasing the term k = nDF/T8. This can be achieved either by the application of higher temperature (the diffusion coefficient increases in proportion to the temperature), or by agitating the solution which would reduce the thickness of the diffusion layer. Concentration polarization can be reduced by agitating, but not comple-... 

It is seen from this result that the extent of concentration polarization for a given current density is smaller the greater the activity, or concentration, of the ions in the bulk of the solution further, AE can be decreased by increasing the factor k and this can be achieved eitlier by decreasing the thickness of the diffusion layer, e.g., by agitating the solution, or by increasing the diffusion coefficient D of the ions, e.g., by raising the temperature. 

The experimental variables that influence the degree of concentration polarization are (1) reactant concentration, (2) total electrolyte concentration, (3) mechanical agitation, and (4) electrode size. 

Convection Reactants can also be transferred to or from an electrode by mechanical means. Forced convection, such as stirring or agitation, tends to decrease the thickness of the diffusion layer at the surface of an electrode and thus decrease concentration polarization. Natural convection resulting from temperature or density differences also contributes to the transport of molecules and ions to and from an electrode. 

It is well known that pumping of the fluid has a major effect on flux in the mass tTcinsfer controlled region for UF/MF process. Indeed agitation and mixing of the fluid near the membrane surface sweep away the accumulated solutes, thus reducing the thickness of boundary layers. This is the simplest and most effective method of controlling the effect of concentration polarization. 

The first set of experiments, carried out to compare the performance of membranes with MWCO of 10, 20, and 50 kDa, were carried out using a pressure of 2 bar and 200 rpm agitation, to expose the systems to the same concentration polarization and fouling effects. Before and after the UF experiments, the membrane hydraulic permeability coefficient (Lp) was measured to evaluate the permeability loss. 

Several factors can affect the retention properties of the membrane and some of these will be discussed here. During ultrafiltration the transport of solute to the surface of the filter Is faster than the rate at which permeation through the membrane occurs. This is further complicated, as ultrafiltration progresses, by an increase in the concentration of retained molecules at the membrane. Both events contribute to the phenomenon called concentration polarization. This effectively introduces a second layer of membrane , and as a consequence the retention characteristics of the system are altered. The build-up of solute can be reduced by introducing some form of agitation at the filter surface. However. this procedure does not seem to be effective against the gel-type layers formed by proteins. Various procedures have been suggested to slow down this build-up of solute the solution can be diluted with an appropriate solvent the ultrafiltration process can be interrupted and the flow reversed momentarily a low operation pressure could be used. 

Concentration polarization is important in several electroanalytical methods. In some applications, steps are taken to eliminate it in others, however, it is essential to the method, and every effort is made to promote it. The degree of concentration polarization is influenced experimentally by (1) the reactant concentration, with polarization becoming more pronounced at low concentrations (2) the total electrolyte concentration, with polarization becoming more pronounced at high concentrations (3) mechanical agitation, with polarization decreasing in well-stirred solutions and (4) electrode size, with polarization effects decreasing as the electrode surface area increases. 

Agitation. By agitation, the thickness of the diffusion layer is decreased, and the rate of diffusion of ions increases. There is no build up of any concentration gradient between the corroding surface and bulk electrolyte. The end result is a decrease in concentration polarization and an increase in the rate of corrosion. As the rate of agitation is increased. 

It ought to be stressed that, in fact, a large number of variables in the plating process have a bearing on structure. These include metal-ion concentration, additives (see Chapter 10), current density (see Chapter 12), temperature, agitation, and polarization. It is outside the scope of this book to discuss these in much more detail. To visualize the effects that these parameters can have on grain size, see Figure 16.3. 

Solid phase micro-extraction (SPME) allows isolation and concentration of volatile components rapidly and easily without the use of a solvent. These techniques are independent of the form of the matrix liquids, solids and gases can be sampled quite readily. SPME is an equilibrium technique and accurate quantification requires that the extraction conditions be controlled carefully. Each chemical component will behave differently depending on its polarity, volatility, organic/water partition coefficient, volume of the sample and headspace, speed of agitation, pH of the solution and temperature of the sample (Harmon, 2002). The techniques involve the use of an inert fiber coated with an absorbant, which govern its properties. Volatile components are adsorbed onto a suitable SPME fiber (which are usually discriminative for a range of volatile components), desorbed in the injection chamber and separated by a suitable GC column. To use this method effectively, it is important to be familiar with the factors that influence recovery of the volatiles (Reineccius, 2002). 

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