Rotating disk electrode thickness

Volt mmetiy. Diffusional effects, as embodied in equation 1, can be avoided by simply stirring the solution or rotating the electrode, eg, using the rotating disk electrode (RDE) at high rpm (3,7). The resultant concentration profiles then appear as shown in Figure 5. A time-independent Nernst diffusion layer having a thickness dictated by the laws of hydrodynamics is estabUshed. For the RDE,... 

The rqjroducibility of polymer film formation is greatly improved by the spin coating technique where the polymer solution is applied by a microsyringe onto the center of a rapidly rotated disk electrode Rather thick films can be produced by repeated application of small volumes of stock solution. A thorough discussion and detailed experimental description of a reliable spin coating procedure was given recently... 

The mass transfer boundary layer thickness, d, on a rotating disk electrode can be estimated by d = 1.6/J V a) where D is the substrate diffusion coefficient, v is the solution viscosity, and CO is the disk rotation speed. 

The basic assumption is that the rotating filter creates a laminar flow field that can be completely described mathematically. The thickness of the diffusion boundary layer (5) is calculated as a function of the rotational speed (to), viscosity, density, and diffusion coefficient (D). The thickness is expressed by the Levich equation, originally derived for electrochemical reactions occurring at a rotating disk electrode ... 

As can be expected, with bath agitation the effective thickness 8 will diminish hence the diffusion rate will increase. Typical values of 8 are about 0.2 mm without agitation, and with a rotating-disk electrode it can be made as small as 0.02 mm. Of course, this thickness will vary from species to species. 

Diffusion layer (cont.) polarography. 12-46 thickness, 1335 turbulent flow and, 1234 Diffusion coefficient, and rotating disk electrode. 1141... 

Influence of Rotating Disk Electrode Condition (Stationary or Rotating) on the Diffusion-Layer Thickness and the Limiting Current Density for the Reaction... 

For a rotating disk electrode operating at sufficiently great potential, the redox reaction rate is governed by the rate at which analyte diffuses through the diffusion layer to the electrode (Figure 17-12b). The thickness of the diffusion layer is... 

Hydrodynamic boundary layer — is the region of fluid flow at or near a solid surface where the shear stresses are significantly different to those observed in bulk. The interaction between fluid and solid results in a retardation of the fluid flow which gives rise to a boundary layer of slower moving material. As the distance from the surface increases the fluid becomes less affected by these forces and the fluid velocity approaches the freestream velocity. The thickness of the boundary layer is commonly defined as the distance from the surface where the velocity is 99% of the freestream velocity. The hydrodynamic boundary layer is significant in electrochemical measurements whether the convection is forced or natural the effect of the size of the boundary layer has been studied using hydrodynamic measurements such as the rotating disk electrode [i] and - flow-cells [ii]. 

The thickness of the Nernst layer increases with the square root of time until natural - convection sets in, after which it remains constant. In the presence of forced convection (stirring, electrode rotation) (see also Prandtl boundary layer), the Nernst-layer thickness depends on the degree of convection that can be controlled e.g., by controlling the rotation speed of a -> rotating disk electrode. See also - diffusion layer. See also Fick s law. 

Figure 11.1 Concentration distribution for a coated rotating disk electrode a) results obtained for 6i /6f — 2 with coating porosity as a parameter b) results obtained for a porosity e = 0.3 and relative coating thickness Sf /Sf as a parameter.

Transfer of electroactive species is possible at x = /, and C(/) = 0, but dC l)/dx 0. This is the conducting or transmissive boundary. It is observed, for example, in a rotating disk electrode, where the diffusion layer thickness is determined by the rotation rate. 

In hydrodynamic systems Planar diffusion to a uniformly accessible electrode, e.g. for rotating disk electrodes (hypothetical Nernst model with S = diffusion layer thickness)... 

To give an even more precise description, the Levich law can be applied to express the thickness of the Nernst layer in experiments using a rotating disk electrode ... 

In Eqn (2.82), Cm is normally treated as the solubility of the oxidant in the ionomer membrane. The diffusion layer thickness 5o can be obtained using a rotating disk electrode technique, which will be given in a very detailed discussion in Chapter 5. The equivalent thickness of the ionomer membrane can be calculated according to the amount of ionomer applied in the electrode layer using the following equation ... 

In practical applications, very often diffusion is not semi-infinite. Such finite-length linear diffusion is observed, for example, for internal diffusion into mercury film deposited on a planar electrode, in deposited conducting polymers, for hydrogen diffusion into thin films or membranes of Pd or other hydrogen absorbing materials, or for a rotating disk electrode where the diffusion layer corresponds to the layer thickness. There are two cases of finite-length diffusion displayed Fig. 4.11 ... 

To estimate ko or Xo quantitatively we must resort to an electrochemical technique that operates under steady-state conditions and for which the diffusion layer thickness is well-defined and quantifiable. The technique of choice is the rotating disk electrode (RDE) method. " Both of the aforementioned criteria are valid for the RDE. The solution hydrodynamics are well-defined, and the diffusion layer thickness can be estimated quantitatively in terms of the rotation speed... 

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