Diffusion cell design

It is assumed that the convective flow of water across the ABL, cell mono-layer, and filter owing to pressure gradients is negligible and that the cell mono-layer is uniformly confluent. When these conditions are not met, Katz and Schaeffer (1991) and Schaeffer et al. (1992) point out that mass transfer resistances of the ABL and filter [as described in Eq. (21)] cannot be used simply without exaggerating the permeability of the cell monolayer, particularly the paracellular route. An additional diffusion cell design was described by Imanidis et al. (1996). 

Imanidis G, C Waldner, C Mettler, H Leuenberger. (1996). An improved diffusion cell design for determining drug transport parameters across cultured cell monolayers. J Pharm Sci 85 1196-1203. 

Figure 14.8 Basic diffusion cell designs. Static horizontal cells may be jacketed [as in the Franz-type) or unjacketed [and temperature controlled using water bath or heating block). Flow-through cells usually have a small receptor chamber to maximize mixing. Side-by-side cells are used mainly for solution vehicles.

Diffnsion finxes develop as a result of these concentration gradients. The layer of electrolyte where the concentration changes occur and within which the substances are transported by diffnsion is called the diffusion layer. Its thickness, 5 (the diffusion path length), depends on cell design features and on the intensity of convechve... 

Figure 3.13 Cell designs for the thenael conductivity detector. A, flow-through cell B, semi-diffusion cell C, diffusion cell. (Reproduced with permission from ref. 163. Copyright Preston Publications, Inc.)...

G Flynn, E Smith. Membrane diffusion I Design and testing of a new multifeatured diffusion cell. J Pharm Sci 11 1713, 1971. 

Although this may look very unpleasant, techniques for solving equations of this sort both analytically and numerically are very well established in fact it is usually possible to solve the equation analytically, at least for the simpler geometries often encountered in cell design. Moreover, the structure of the electrode-electrolyte interface allows us to make a substantial simple fication under normal circumstances in the region beyond the diffuse-layer boundary. We can take E as zero in this region, as indicated above, since the potential in the electrolyte bulk must be constant, and so ... 

Contrary to traditional fuel cells, biocatalytic fuel cells are in principle very simple in design [1], Fuel cells are usually made of two half-cell electrodes, the anode and cathode, separated by an electrolyte and a membrane that should avoid mixing of the fuel and oxidant at both electrodes, while allowing the diffusion of ions to/from the electrodes. The electrodes and membrane assembly needs to be sealed and mounted in a case from which plumbing allows the fuel and oxidant delivery to the anode and cathode, respectively, and exhaustion of the reaction products. In contrast, the simplicity of the biocatalytic fuel cell design rests on the specificity of the catalyst brought upon by the use of enzymes. 

Thin-layer cell design is based on reduction of diffusion path length the mobile phase is directed along the working electrode surface as a thin film of liquid (see Figure 3-1). 

For typical fuel-cell designs, mass transport through the fuel-cell sandwich occurs mainly by diffusion. The simplest way to describe diffusion is by Pick s law ... 

Two particular aspects of the transport of degradable contaminants were considered in laboratory experiments that used soil originating from the field experiments described in the previous sections. Studies on diffnsion of degradable insecticides were performed in diffusion cells, while the spatial redistribntion of pesticides from a point source was measured in specially designed pans (60 cm high, 40 cm diameter). Periodic sampling and contaminant analysis enabled visnaUzation of the contaminant transport pathway. 

Alegret et al. devised a pH ISFET based on a flow-through cell designed by themselves and an FI manifold including a gas-diffusion module for the on-line separation of gaseous analytes with acid-base properties. In this way, they obtained a linear determination range of 1 x 10 -1 x 10 M for ammonia and 7 x 10" -4 x 10 M for sulphur dioxide, with an RSD of 1% and 0.5%, respectively [153]. 

Figure 1 shows several types of mass transfer or diffusion cells, which are of the simplest design for performing bulk liquid membrane (BLM) processes. Each of the devices is divided into two parts a common part containing the membrane liquid, M and a second part in which the donor solution F and acceptor solution R are separated by a solid impermeable barrier. The liquid, M contacts with the two other liquids and affects the transfer between them. All three liquids are stirred with an appropriate intensity avoiding mixing of the donor and acceptor solutions. For a liquid-ion exchange in a BLM system. Fig. 2 shows the transfer mechanism of cephalosporin anions, P , from donor (F) to acceptor (R) solution... 

Another cell design that aids to minimize the effects of thermal diffusion is the capillary cell. Fig. 15.10c. By using capillary tubing on the vent side of the cell, a sufficiently high linear flow velocity is maintained to prevent... 

The automatic flow-through diffusion cell system described is designed to allow the rapid analysis of drug flux through human skin in vitro with minimum variability. 

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