Rotated diffusion cell

This was the earliest device to permit control of film thickness in the study of two-phase liquid/liquid reactions, Fig. 5.19, and the reaction takes place at the lower side of the porous 

The channel flow cell has been used to study a wide variety of liquid-solid reactions. It consists of a tube of rectangular cross section, typically 4.5 cm long by 0.6 cm wide by 0.1 cm deep (Fig. 5.20). The reactive solid is embedded in the base of the cell and a detector downstream of the reactive solid is used to monitor reactant and product concentrations. 

The mass transfer by convection and diffusion within the channel is precisely calculable. Numerical methods are used to fit measured concentrations at the detector electrode to the fluid flow in the cell, and to the reaction kinetics at the surface and in solution [24]. The technique was extensively developed during a study of the dissolution of calcite in dilute aqueous acid [25], and has latterly been applied to a number of organic reactions. 

A recent example is the hydrolysis of chloranil with aqueous sodium hydroxide, which is a multistep process leading to the chloranilate dianion, Fig. 5.21 [26]. In a two-phase reaction system consisting of solid chloranil and aqueous sodium hydroxide, the rate-limiting step is the initial attack of hydroxide ion on chloranil. Hydrolysis rates at the 010, 100 and 001 cleavage planes of chloranil were separately measured [27], and different reactivities were found this was rationalised by considering the exposure of the initially reactive functionality (assumed to be the carbonyl group) at the surface, as shown in Fig. 5.22. Hydrolysis at the 010 and 100 planes was shown to be a surface reaction driven by the hydroxide concentration adjacent to the interface the 001 plane was shown to react more slowly by prior dissolution of chloranil before reaction. 

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FIG. 6 Schematic of the rotating diffusion cell. The reaction is usually followed by sampling the bulk solution of the outer phase using a suitable analytical technique. 

Rotating diffusion cells These techniques bring the two phases in contact at the surface of a rotating membrane filter having well characterized hydrodynamics. 

In this cell, the aqueous and the organic phase are brought in contact at the snrface of a microporous membrane filter, the pores of which are filled with the organic phase [20]. The microporous filter is attached to a hollow cylinder filled with the same organic phase. The cylinder is dipped into the aqueons phase and is rotated by a motor. A scheme of the rotating diffusion cell is presented in Fig. 5.12. The cell has a filter with well-defined hydrodynamics on both... 

Fig. 5.12 Cross section of the rotating diffusion cell B, bearing BA, internal baffle FM, filter mount I, inner compartment L, lid with holes M, membrane MA, mounting rod O, outer compartment P, pulley block S, hollow rod T, thermostated beaker. (From Ref. 20.)...

The concept of measuring such rates is not new, particularly in the pharmaceutical field. Van de Waterbeemd [14] measured rates of transfer of various drugs from octanol to water and empirically related these rates to the partition coefficient. Similarly Brodin [15], using a different experimental method, obtained rates of transfer for another series of compounds between cyclohexane and water. The rotating diffusion cell has been introduced for similar purposes [16-18]. It is necessary to look into the broader background of liquid-liquid interfacial kinetics, in order to illustrate aspects of the issues under consideration. The subject has been reviewed in part by Noble [19]. 

Rotating diffusion cell Set up hydrodynamics of a rotating disc on both sides of a supported interface Reproducible between laboratories. Known interfacial area. Mathematically straightforward but with a limit on measurement of the fastest rates... 

Fig. 6.2. Cross section of standard rotating diffusion cell.

Fig. 6.3. Cross section of clamp rotating diffusion cell.

To category 2 belong the rotating diffusion cell (RDC) [17], the short-time phasecontacting method [18], the rotating stabilized cell (RSC) [21] and the rotating membrane cell (RMC) [22, 23]. 

As in the case of the short-time phase-contacting method described above, this technique operates in a transient, non-stationary, regime that highlights the role of the chemical reaction. Moreover, this technique shares with the rotating diffusion cell (RDC) the capability of control of the hydrodynamics. 

H.V. Patel The Kinetics of Liquid-Liquid Extraction of Metal in a Rotating Diffusion Cell, Ph.D. Thesis, Department of Chemical Engineering, University of Bradford, UK, 1988. 

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