Page 202 - Introduction to Colloid and Surface Chemistry
P. 202
Charged interfaces 191
Glass strengthening
rod attached to
C'j • microscope stage
oiO6 view
Top view Condenser
Long-working-
distance
objective
Calibrated
eyepiece scale
93
Figure 7.5 A vertically mounted flat particle microelectrophoresis cell (By courtesy
of Academic Press Inc.)
3
3
for salt concentrations below about 0.001 mol dm~ to 0.01 mol dm" ;
otherwise appropriate reversible electrodes, such as Cu|CuSO 4 or
AgjAgCl, must be used to avoid gas evolution.
Electrophoretic measurements by the microscope method are
complicated by the simultaneous occurrence of electro-osmosis. The
internal glass surfaces of the cell are usually charged, which causes an
electro-osmotic flow of liquid near to the tube walls together with
(since the cell is closed) a compensating return flow of liquid with
maximum velocity at the centre of the tube. This results in a parabolic
distribution of liquid speeds with depth, and the true electrophoretic
velocity is only observed at locations in the tube where the electro-
osmotic flow and return flow of the liquid cancel. For a cylindrical cell
the 'stationary level' is located at 0.146 of the internal diameter from
