Principle Of The Design Of An Oxygen Electrode - Mettler Toledo InPro 6850i Bedienungsanleitung

InPro 6850i / 6900i / 6950i O
the electrode voltage. During measurement no
chemical reaction occurs and the solution remains
in equilibrium.
– In the case of amperometric electrodes, such as
the oxygen electrode, activity measurement is
based on a current measurement.
The conventional oxygen electrode consists of a
cathode and an anode conductively connected
by an electrolyte. A suitable polarization voltage
between the anode and the cathode selectively re-
duces the oxygen at the cathode.
Cathode reaction:
O
2
These chemical reactions result in an electric cur-
rent which is proportional to the oxygen partial
pressure (pO
constantly oxygen. Thereby, the concentration of
dissolved oxygen is reduced. By diffusion this
oxygen is replaced. The viscosity and flow rate of
the solution are therefore important parameters.
The electrode current of an oxygen electrode is de-
termined not only by the oxygen partial pressure
but by many other electrode parameters. The elec-
trode currents of different electrode types may differ
by several powers of ten. For this reason oxygen
electrodes and amplifiers cannot be freely inter-
changed.
10.2 Principle of the design of an
oxygen electrode
There are two main types of oxygen electrodes:
– Electrodes without membrane
– Electrodes with gas-permeable membrane
(Clark Principle)
The membrane electrode according to Clark is today
most widely used. As compared to the electrode with-
out membrane it possesses the following advan-
tages:
– Oxygen measurement in gases and solutions
– No mutual contamination of electrode and solu-
tion
– No or little dependency on flow
In the case of the Clark electrode, geometrical config-
uration is very important. In particular, the thickness
of the electrolyte film between the cathode and the
membrane must be within narrow tolerances so as to
ensure good linearity and a low zero current (current
in a nitrogen atmosphere).
METTLER TOLEDO oxygen sensors are available in
different designs:
© 02 / 09 Mettler-Toledo AG
Printed in Switzerland
+ 2 H O + 4e
2
). The oxygen electrode reduces
2
Sensor 12 / 25 mm
2
– –
→ 4 OH
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