Theory Of The Polarographic Sensor; Introduction - Mettler Toledo InPro 6950 Serie Bedienungsanleitung

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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.
– P otentiometric electrodes develop a voltage gener-
ated by the activity of a particular ion. Examples of
such electrodes are glass electrodes (like pH elec-
trodes) 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 mea-
surements is the virtually currentless determination
of the electrode voltage. During measurement no
chemical reaction occurs and the solution remains
in equilibrium.
– I n 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
reduces the oxygen at the cathode.
Cathode reaction:
O
2
Anode reaction:
4 Ag + 4 Cl
In the case of the InPro 6950 sensor, the mea-
surement system has been expanded by reference
and guard ring electrodes. This design modifica-
tion allows the precise concentration of oxygen
to be measured down to trace level. In contrast to
conventional sensors, this also changes the elec-
trochemical reaction at the anode..
InPro 6950 anode reaction:
4 OH → O
These chemical reactions result in an electric
current 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.
InPro 6950
52 206 137 F
+ 2 H O + 4e
2
–
→ 4 AgCl + 4e
+ 2 H
2
). The oxygen electrode consumes
2
InPro 6950 Series O
– –
→ 4 OH
–
–
O + 4e
2
© 02 / 2023 METTLER TOLEDO
Sensor 12 mm
2
Printed in Switzerland