Page 387 - Instrumentation Reference Book 3E

370 Chemical analysis: electrochemical techniques

assembly is in the measured solution. Immersion Electrode assemblies should be designed so
depths up to 3 m are available. that the electrodes can be kept wet when not in
use. It is often possible to arrange for the easy
removal of the assembly from the process vessel
so that it can be immersed in a bucket filled with
process liquid, water, or buffer solution during
shutdown.
The design of the assembly is often modified to
suit the use. For example, in measuring the pH of
pulp in a paper beater the electrodes and resist-
ance bulb are mounted side by side in a straight
line and then inclined downstream at about 45 "C
from the vertical so that they present no pockets
to collect pulp and are self-cleaning.
When the assembly is immersed in a tank, care
must be taken in the siting to ensure the instru-
ment is measuring the properties of a representa-
tive sample; adequate mixing of the process
material is essential. Sometimes it is more con-
rference element
venient to circulate the contents of a tank through
a flow type of assembly and then return the liquid
to the tank.
The main cause of trouble in electrode assem-
blies is the fouling of the electrodes. In order to
reduce this, two forms of self-cleaning are avail-
able, and the choice of method is dependent on
the application. Where the main cause of trouble
IC is deposits on the glass, electrode and mechanical
cleaning is required; this may be achieved by the
uY cleaning attachment shown on a dip system in
Direction of flow Figure 17.28. The pneumatically driven rubber
membrane wipes the electrode. providing a sim-
(C)
ple, reliable cleaning action. It is driven by com-
Figure 17.27 Flow-type of electrode system (courtesy pressed air at preset intervals from a controller
Kent Industrial Measurements Ltd. Analytical Instruments):
(a) external view, (b) upper section detaches for easy which incorporates a programmed timer mechan-
buffering, (c) exploded view showing the components. ism that governs the frequency of the wiping