Mettler Toledo InPro Serie Instructions D'utilisation page 34

34
Measurement
liquid or gas
Type C, 3-electrodes, InPro 6900(i)
InPro 6900 and InPro 6900 i. For oxygen mea-
surements in the lower ppb range. Here, the anode
and reference are united in a silver / silver chloride
electrode (as with type A). The sensors are equipped
with an additional guard ring around the cathode.
Like the cathode with the anode, this forms a closed
electric circuit, which prevents that oxygen diffuses
from the side to the cathode and falsifies the mea-
surement result.
Measurement
liquid or gas
In all sensors the function of the cathode is identical.
Between cathode and reference a constant voltage
of – 500 resp. – 675 mV (polarization voltage) is
applied. Thereby a reduction of O
cathode takes place.
Cathode: O
and
Anode:
and
The total equation thus is:
O + 2 e
2
H
$
2
InPro 6850 i / 6900 i / 6950 i
52 206 349
InPro 6850 i / 6900 i / 6950 i O
O
2
O
2
+ 2 e
2
H O + 2 e
2 2
4 Ag
$
+
4 Ag + 4 Cl
+ 2 H O + 4 Ag
–
2
O + 2 e + 4 Ag
–
2
O
2
O
2
O
2
O
2
+ 2 H O
–
$
2
2 OH
–
$
4 Ag + + 4 e
–
4 AgCl
$
H O + 2 OH
$
2 2
–
+
+ 4 Cl
© 04 / 13 Mettler-Toledo AG
Sensor 12 / 25 mm
2
Polarization
voltage
Reference
Cathode
Anode
Electrolyte
Glass insulator
NTC 22 k
Membrane
Polarization
voltage
Reference / Anode
Cathode
Guard
Electrolyte
Glass insulator
NTC 22 k
Membrane
–
into OH at the
2
H O + 2 OH
2 2
–
–
–
+ 4 Ag
4 AgCl + 2 OH
$
Printed in Switzerland
–
+
+ 4 e
–
–