Theory Of The Polarographic Sensor; Introduction; Principle Of The Design Of An Oxygen Electrode - Mettler Toledo InPro 6850 i Bedienungsanleitung

32
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
generated 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 con-
stant.
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 measurements is the virtually
currentless determination of the electrode volt-
age. 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 con-
nected by an electrolyte. A suitable polariza-
tion voltage between the anode and the cathode
selectively reduces the oxygen at the cathode.
Cathode reaction:
O
These chemical reactions result in an electric
current 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
determined not only by the oxygen partial pres-
sure but by many other electrode parameters. The
electrode currents of different electrode types may
differ by several powers of ten. For this reason
oxygen electrodes and amplifiers cannot be freely
interchanged.

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)
InPro 6850 i / 6900 i / 6950 i
52 206 349
InPro 6850 i / 6900 i / 6950 i O
+ 2 H O + 4e
2 2
2
– 4 OH
$
). The oxygen electrode reduces
© 04 / 13 Mettler-Toledo AG
Sensor 12 / 25 mm
2
–
Printed in Switzerland