92168.pdf

198 Charged interfaces

Compressed

air

Figure 7.10 A streaming potential apparatus^93 (By courtesy of Academic Press Inc.)

be generated by forcing liquid through the plug by means of a

reciprocating pump. The main advantage of studying alternating

rather than direct streaming currents is that electrode polarisation is

far less likely.

Electro-osmosis

Figure 7.11 illustrates a suitable apparatus for studying electro-
osmotic flow through a porous plug. Reversible working electrodes
are used to avoid gas evolution. A closed system is employed, the
electro-osmotic flow rate being determined by measuring the velocity
of an air bubble in a capillary tube (c. 1 mm diameter) which provides
a return path for the electrolyte solution.
It may be necessary to correct the experimental data for effects
such as electro-osmosis in the measuring capillary tube and electro-
osmotic leak back through" the plug.