Rotating diffusion cell

The RDC used in the majority of this work is shown in Fig. 6.2, and is based on the design of Albery et al. [16]. The baffle in the interior is 

Two methods have been used, both using the ultra-violet (UV) absorption of the compound under test. In order to interpret absorbance vs. time data, extinction coefficients were obtained by conventional methods. 

The C lj vs. W 1/2 data were collected and analysed by linear regression. 

This was occasionally necessary if it was shown that the compound was absorbed by the pump tubing. The total experimental duration is similar to (a) but the rate was measured by sampling the outer compartment. 

The filters used were from Millipore (various pore sizes, typically 0.22 p.m, with filters 0.15 mm in thickness). In order to render the outer part of the filter impermeable, the literature procedure [16] was used, except that no surfactants or glycerol were used in the process. Mutually saturated octanol and pH 7 buffer were prepared all subsequent references to octanol or buffered water pertain to these mutually saturated phases. 

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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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