Theory Of The Polarographic Sensor - Mettler Toledo InPro 6800 Serie Instructions D'utilisation

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InPro 6800 Series O Sensor 12/25 mm
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10

Theory of the polarographic sensor

10.1 Introduction
Two types of electrodes are employed in analytical
work: potentiometric and amperometric electrodes.
– Potentiometric electrodes develop a voltage gene -
rated by the activity of a particular ion. Examples
of such electrodes are glass electrodes (like pH
electrodes) and most ion-selective electrodes. Their
individual potentials cannot be determined. The
measurable quantity is the difference of potential
between the measuring electrode and an inert
reference electrode. The potential of the reference
electrode must be constant.
All potentiometric electrodes are subject to Nernst's
law and for this reason electrodes and measuring
instruments are in most cases interchangeable. An
important requirement of potentiometric measure-
ments is the virtually currentless determination of
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 oxygen electrode consists of a cathode and
an anode conductively connected by an elec-
trolyte. A suitable polarization voltage between the
anode and the cathode selectively reduces the
oxygen at the cathode.
These chemical reactions result in an electric
current which is proportional to the oxygen partial
pressure (pO ). The oxygen electrode consumes
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oxygen which is continuously extracted from the
solution. The viscosity and flow rate of the solution
are therefore imortant parameters.
The electrode current of an oxygen electrode is
determined 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
interchanged.
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InPro 6800
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