Theory Of The Polarographic Sensor; Introduction - Mettler Toledo InPro 6800 Serie Bedienungsanleitung

InPro 6800 O
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 gener-
ated 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 poten-
tial 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 electro-
lyte. A suitable polarization voltage between the
anode and the cathode selectively reduces the oxy-
gen at the cathode.
These chemical reactions result in an electric cur-
rent which is proportional to the oxygen partial
pressure (PO
oxygen which is continuously extracted from the
solution. The viscosity and flow rate of the solution
are therefore important parameters.
The electrode current of an oxygen electrode is deter-
mined not only by the oxygen partial pressure but
by many other electrode parameters. The electrode
currents of different electrode types may differ by
several powers of ten. For this reason oxygen elec-
trodes and amplifiers cannot be freely interchanged.
© 02 / 2023 METTLER TOLEDO
Printed in Switzerland
Sensor 12/25 mm
2
Cathode reaction:
O 2 H
2
Anode reaction:
4 Ag
2
O 4e
2
4 Cl
). The oxygen electrode consumes
4 OH
4 AgCl 4e
InPro 6800
52 200 953 C
35