92168.pdf

Charged interfaces 191

Glass strengthening

rod attached to

C'joiO6 • view microscope stage

Top view Condenser

Long-working-

distance

objective

Calibrated

eyepiece scale

Figure 7.5 A vertically mounted flat particle microelectrophoresis cell^93 (By courtesy

of Academic Press Inc.)

for salt concentrations below about 0.001 mol dm~^3 to 0.01 mol dm"^3 ;
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