Electrode surfaces electrolyte solution flow rate

Figure 5.1 (A) Schematic of the electrolyte solution flow along the electrode surface (B) the flow rate distribution near the electrode surface along the direction parallel to the electrode surface.

Figure 5.3 shows a schematic structure of the RDE. The disk electrode s planner surface is made to contact with the electrolyte solution. An electronic isolator made from isolating material such as Teflon is used to cover the remaining part of the disk with only the planner surface exposed. An electrical brush is used to make the electrical connection between the electrode shaft and the wire during the electrode rotating. When the electrode is rotating, the solution will run from the hulk to the surface, and then be flushed out along the direction parallel to the disk surface, as shown in Figure 5.3(B). Using these three coordinates r, x, and 0, shown in Figure 5.3 through a complicated mathematic operation, the solution flow rates near the electrode surface at x and r directions can be expressed as ...

As we discussed above, the RDE theory is based on the convection kinetics of the electrolyte solution. If the electrode rotating rate is too small, meaning that the solution flow rate is too slow, it will be difficult to establish the meaningful diffusion-convection layer near the electrode surface. In order to make meaningful measurement, there is a rough formula that can be used to obtain the limit of electrode rotating rate (wiow) ... 

Janssen and Hoogland (J3, J4a) made an extensive study of mass transfer during gas evolution at vertical and horizontal electrodes. Hydrogen, oxygen, and chlorine evolution were visually recorded and mass-transfer rates measured. The mass-transfer rate and its dependence on the current density, that is, the gas evolution rate, were found to depend strongly on the nature of the gas evolved and the pH of the electrolytic solution, and only slightly on the position of the electrode. It was concluded that the rate of flow of solution in a thin layer near the electrode, much smaller than the bubble diameter, determines the mass-transfer rate. This flow is affected in turn by the incidence and frequency of bubble formation and detachment. However, in this study the mass-transfer rates could not be correlated with the square root of the free-bubble diameter as in the surface renewal theory proposed by Ibl (18). 

The deposition of metals has also been studied by a large number of electrochemical techniques. For the deposition of Cu2+, for example, it is reasonable to ask whether both electrons are transported essentially simultaneously or whether an intermediate such as Cu+ is formed in solution. Such questions, like those of the ECE problem discussed above, have usually been investigated by forced convection techniques, since the rate of flow of reactant to and away from the electrode surface gives us an important additional kinetic handle. In addition, by using a second separate electrode placed downstream from the main working electrode, reasonably long-lived intermediates can be transported by the convection flow of the electrolyte to this second electrode and detected electrochemically. 

In Section 2.3.6 we considered ion transfer at the interface between two immiscible electrolyte solutions (ITIES), where we found that a potential difference can arise because of differential transfer of ions. Ion movement across the interface can also be driven by the application of an external potential, and the rate of ion transfer can be detected as a current flow. This response allows one to examine the ITIES via voltammetric methods in the same way that electron transfer can be monitored at electrode surfaces (29-32). 

The percent acetonitrile and the maximum flow rate that can be used are limited by the electrochemical detector. For example, buffer solutions that contain more than 50% acetonitrile do not provide enough electrolyte for proper performance of the detector. Use of quaternary ammonium salts allows higher concentrations of acetonitrile in the mobile phase. Flow rates greater than 2.5 ml/min can erode the surface of the carbon paste electrode. The limits of detection for the seven anilines (based on a quantity... 

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