92168.pdf

Kinetic properties 31

Porous plug method

The two liquids are separated by a sintered glass disc with a pore size

of 5-15 fjLm (the more concentrated solution on top) and kept stirred.

The liquid in the pores of the disc is effectively immobilised and freed

from the influence of external disturbances, so that the transport of

solute through the disc is due solely to diffusion. The extent of

diffusion can be determined by any analytical method.

By analogy with Pick's first law,

dm _ -AD(c{ -c 2 )

d/ /

(2.15)

where A is the cross-section of pores, and / is the effective length of
pores.
The ratio All is determined by calibrating the apparatus with a
substance of known diffusion coefficient.
This method has considerable advantages over the free boundary
methods with regard to experimental procedure. Possible objections
to the method are: (a) the calibration of the cell with material of
different relative molecular mass and/or shape from the material
under investigation is not necessarily valid; and (b) entrapment of air
bubbles in the pores or adsorption of the diffusing molecules on the
pore walls will invalidate the results.

Sintered

glass disc

Figure 2.5 Porous plug method