Page 219 - Introduction to Colloid and Surface Chemistry

208 Charged interfaces

epA£
/ s =-^ (7.29)
7)/
where A is the cross-sectional area of the capillary.
The streaming current and the streaming potential are related by
Ohm's law,
JJ_
E =
S
M
where fc () is the conductivity of the electrolyte solution. Therefore,
ep£
E s =-^ (7.30)
rjk 0
k 0 can be corrected to include a surface conductivity term & s and
equation (7.30) becomes

A more general derivation 90 for porous plugs also leads to
equations (7.29) and (7.30). However, for porous plugs, there is no
satisfactory method of correcting streaming potential data for surface
conductance. If equation (7.31) is used with a equal to the average
pore radius, the calculated zeta potentials are too low. The
importance of surface conductance can be investigated qualitatively
by comparing the conductivity ratio of two relevant electrolyte
concentrations in bulk and in the plug. A knowledge of the surface
conductance is not required when relating streaming current to zeta
potential. The situation in porous plugs may also be more complicated
than accounted for above if (a) the effective area of the plug for the
streaming current differs from that for the leak current as a result of
the different mechanisms involved, and (b) the plug is compressible
and the applied pressure affects the average pore size. The validity of
all zeta potentials calculated from streaming (and electro-osmotic)
measurements on porous plugs is somewhat dubious.