Diffuse layer thickness, reducing

Coagulation takes place over several minutes in a rapid mix tank and encompasses several mechanisms such as reducing diffuse layer thickness and charge neutralization. 

Most successful is a rotating Pt wire microelectrode as illustrated in Fig. 3.75 as a consequence of the rotation, which should be of a constant speed, the steady state is quickly attained and the diffusion layer thickness appreciably reduced, thus raising the limiting current (proportional to the rotation speed to the 1/3 power above 200 rpm140 and 15-20-fold in comparison with a dme) and as a result considerably improving the sensitivity of the amperometric- titration. 

In the solution of mass transport problems, several dimensionless groups are used in order to reduce the number of variables. Diffusion layer thicknesses etc. are expressed in much of the literature in terms of these dimensionless variables. 

While experiments involving solution-phase reactants have provided deep insights into the dynamics of heterogeneous electron transfer, the magnitude of the diffusion-controlled currents over short timescales ultimately limits the maximum rate constant that can be measured. For diffusive species, the thickness of the diffusion layer, S, is defined as S = (nDt)1/2, where D is the solution-phase diffusion coefficient and t is the polarization time. Therefore, the depletion layer thickness is proportional to the square root of the polarization time. One can estimate that the diffusion layer thickness is approximately 50 A if the diffusion coefficient is 1 x 10-5 cm2 s-1 and the polarization time is 10 ns. Given a typical bulk concentration of the electroactive species of 1 mM, this analysis reveals that only 10 000 molecules or so would be oxidized or reduced at a 1 pm radius microdisk under these conditions The average current for this experiment is only 170 nA, which is too small to be detected with high temporal resolution. 

In flow systems that necessitate consideration of two-dimensional geometry, Flanagan and Marcoux did some early work [247]. They examined a variety of conditions, among them the importance of axial diffusion in a tube. They found that neglecting axial diffusion is justified for most flows except the slowest. This is because transport due to the flow dominates in the axial direction, and this holds for electrode lengths that are small compared with the tube radius. This is often called the Levich approximation. Levich [362] related the diffusion layer thickness to the tube radius. It is a function of distance x along the electrode and flow velocity,. The condition can then be reduced to the condition... 

The problem of estimating the diffusion layer thickness therefore reduces to that of estimating u and We have considered three possible mechanisms for the movement of phytoplankton through the water gravitational sinking, convective water movements and the swimming of flagellates. 

Influence of organic complexation. Ligands whose lead complexes dissociate only partially Within the diffusion layer during the ASV plating process (i.e., complexes partially labile to ASV) (29) have little effect on k-j- and kg. For example, the presence of 10 M humic acid (21), which is partially labile to ASV, reduces the TAF for lead by 31% and the EAF by 5-25% over the range of diffusion layer thicknesses considered here. The... 

Ultrasound is known to affect surface morphology through cavitation jets at the electrode-electrolyte interface it usually acts to increase the surface area. (3) Ultrasound reduces diffusion layer thickness and therefore ion depletion. A comprehensive review of the field has recently been given by Compton et al. [145]. 

In short, the accurate selection of experimental conditions can considerably reduce the incidence of these problems on the results of measurements. In particular, the type of electrode, the deposition potential and the hydrodynamic conditions at the electrode surface (which influences the diffusion layer thickness) are the most important. 

Diffusion layer thickness may be reduced by using smaller particles or by increasing the rate of stirring of the solution K m values will then approach the... 

Another study carried out by these authors [93] modeled the collapsing motion of a single bubble near an electrode surface, and equations for the motion of a spherical gas bubble were obtained. The jet speed and water hammer pressure during jet flow (liquid jet) were calculated, and when the jet speed was 120 m/s, the water hammer pressure was approximately 200 MPa upon the electrode surface. This pressure played an important part in the fineness of the crystal deposits. Mass transfer during the electrode reaction was by turbulent diffusion. The diffusion layer thickness was reduced to approximately 1/10th its size in the presence of the ultrasonic field. The baths contained the ions Cl-, SO -, and Zn2+. The ultrasonic frequency employed in the experiments was 40 kHz and it was seen that ultrasound considerably increased the deposition rate and current efficiency, as well as the smoothness and hardness of the deposit. Microscopy studies showed that the... 

Equation 4.8 indicates that, provided the electrode reaction is mass-transfer limited and uncomplicated by coupled chemical reactions, the electrolysis time may be reduced by making the electrode large and the volume small, and by stirring the solution as fast as possible to decrease the diffusion-layer thickness. In a well designed cell it is possible to carry an electrolysis to 99.9% completion in about 10 to 20 min, although times down to 1 min can be achieved by proper cell design. 

The quantity of oxygen diffusing to the cathode per unit time and area is directly proportional to the diffusion coefficient and the difference in oxygen concentration and inversely proportional to the thickness of the boundary layer. Therefore, an increase in temperature and a higher concentration of oxygen in the electrolyte increase the corrosion current density, whereas an increase in the boundary layer thickness reduces the current density. 

Most of the initial practical and theoretical work in cyclic voltammetry was based on the use of macroscopic-sized inlaid disc electrodes. For this type of electrode, planar diffusion dominates mass transport to the electrode surface (see Fig. II. 1.13a). However, reducing the radius of the disc electrode to produce a micro disc electrode leads to a situation in which the diffusion layer thickness is of the same dimension as the electrode diameter, and hence the diffusion layer becomes non-planar. This non-linear or radial effect is often referred to as the edge effect or edge diffusion . 

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