RADIOMETER ABL800 FLEX Reference Manual

2. Amperometric measuring principles ABL800 FLEX Reference Manual
pO
2
electrode
The pO
2
electrode is an amperometric electrode which consists of a silver anode,
platinum cathode and Ag/AgCl reference band, all protected by an electrode jacket
which is filled with electrolyte solution. At the tip of the electrode jacket an
oxygen-permeable membrane protects the Pt cathode from protein contamination
and is covered on the inner side with Pt-black.
Description
The electrode chain is polarized
with constant voltage of -630 mV.
Oxygen from the sample diffuses
across the membrane into the
electrolyte and is reduced on the
cathode (electrons are consumed)
according to the following
equation:
O
2
+ 4H
+
+ 4e
−
→ 2H
2
O
The H
+
ions come from the
electrolyte solution.
This represents the complete
reduction of O
2
. Some of the O
2
however is only partially reduced
according to the following
equation:
O
2
+ 2H
+
+ 2e
−
→ H
2
O
2
Electrode jacket
A
gCl reference band
Membrane
Electrolyte
Electrode contact
In the presence of Pt- black, H
2
O
2
produced by the incomplete reduction of O
2
at
the cathode is immediately decomposed:
2H
2
O
2
→ 2H
2
O + O
2
This oxygen is then also reduced at the cathode. The reduction of oxygen produces
a flow of electrons (an electrical current) the size of this current, I, proportional to
the amount of oxygen and measured by the amperemeter:
I = Sens(pO
2
) × pO
2
+ I
o
pA
where:
Sens(pO
2
) = Sensitivity of the pO
2
electrode
pO
2
= Partial pressure of O
2
in the sample
I
o
= Zero current i.e. the current flowing through the circuit when
pO
2
= 0 kPa (mmHg)
To complete the electrical circuit, an oxidation reaction where electrons are
released is necessary. The reaction at the silver anode is the conversion of Ag to
Ag
+
:
Ag → Ag
+
+ e
−
In order to maintain a charge balance between the anode and cathode, 4 atoms of
Ag need to be oxidized for one molecule of O
2
to be reduced.
Continued on next page
2-4

RADIOMETER ABL800 FLEX Specifications

General

Brand	RADIOMETER
Model	ABL800 FLEX
Category	Measuring Instruments
Language	English