7
Interferences:
All substances which can diffuse through the membrane and for which
800 mV potential suffices for polarographic reduction, will be reduced in
the electrode. This will give a corresponding current contribution, if they
are present. Interference can be caused by ions entering the electrode
through porous or mechanically damaged membranes and by diffusion of
other reactive gases apart from oxygen, e.g. CO
2
, Cl
2
, SO
2
, and H
2
S.
These substances react in undesired manner with the electrode.
Acidic or basic gases change the pH value of the electrolyte solution and
thus disturb the reading, particularly when measuring small oxygen
concentrations. High salt concentrations in the sample solution can falsify
readings too.
Always store the electrode in distilled water after use! The membrane may
cease to function correctly after prolonged use or mechanical damage.
This will falsify the readings and the electrolyte may drain out. To replace
the membrane (see page 49), unscrew the measuring head and dismantle
it further to remove the old membrane. Take a new membrane and mount
it carefully, fill the measuring head with electrolyte solution, and then
slowly screw it onto the electrode. The electrode is now ready for use
again.
The electrode body consisting of the electrode shaft and the plug head
has almost unlimited life and requires no maintenance. After prolonged
use of the electrode in solutions containing high concentrations of oxygen,
the silver anode may become deactivated by undesired deposits. An
indication is that the electrode no longer responds correctly to calibration.
In this case, screw off the measuring head and cautiously remove the
precipitated deposits from the silver electrode surface with the aid of filter
paper. The electrode is then ready again for operation after refilling the
measurement head with new electrolyte solution.
Maintenance
Measurements
with 1-point
calibration
Start this method only with a well polarised electrode!
1.
Before making measurements, check that the membrane is
undamaged and that there are no gas bubbles present in the
electrolyte space.
2.
Connect the electrode and switch the instrument on. After a few
minutes, the electrode is polarised and ready for making
measurements.
3.
Select the ppm (mg/l) or % range by pressing
MODE
. The display will
immediately show the measured oxygen according to the previous
calibration. Should you want to restandardise, press
CAL
.
4.
The display shows the salinity correction, e.g. [SAL=2.1%]. Leave this
value to zero unless you are going to measure in heavily salted
solutions such as e.g. sea-water (3.5%). Pre-set the proper value with
¿
or
À
and press
CAL
or
MODE
to cancel.
5.
The display shows the pressure correction, e.g. [P=1023hP]. Pre-set
the actual air pressure (hP) with
¿
or
À
and press
CAL
or
MODE
to
cancel.
6.
Press
¿
or
À
until the display shows [1.Point]. Press
CAL
to start the
calibration procedure.
