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InPro 6850i / 6900i / 6950i O
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 measurement re-
sult.
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:
Anode:
The total equation thus is:
O
+ 2 e
2
→ H
2
Type D, 4-electrodes, InPro 6950(i)
The InPro 6950(i) is intended for measuring perma-
nent low oxygen level down to 0.1 ppb. In terms of
measurement principle it is a combination of the
InPro 6850i and InPro 6900(i). It consists of 4
electrodes. The anode (platinum) and reference (sil-
ver / silver chloride) are separated in two electrodes.
A guard ring is placed around the cathode. The cath-
ode has the highest diameter from all amperometric
oxygen sensors from METTLER TOLEDO
The electrodes show the following reactions:
Cathode:
Anode:
Reference: No current
Guard:
Due to the larger surface of the cathode a higher
current appears during measurement. This allows
for measurement of oxygen levels below 1 ppb.
© 02 / 09 Mettler-Toledo AG
Printed in Switzerland
O
2
O
+ 2 e
2
H
O
2
2
4 Ag → 4 Ag
+
4 Ag
–
O + 4 Ag → H
+ 2 H
2
–
O
+ 2 e
+ 4 Ag
2
O
+ 2 H
2
4 OH
O
+ 2 H
2
Sensor 12 / 25 mm
2
O
2
O
2
–
O → H
+ 2 H
2
–
→ 2 OH
+ 2 e
+
+ 4 e
–
→ 4 AgCl
+ 4 Cl
O
2
–
+
+ 4 Cl
O + 4 e
2
–
→ O
+ 2 H
2
O + 4 e
2
InPro 6850i, 6900i, 6950i
into OH
2
O
+ 2 OH
2
2
–
–
and
–
+ 2 OH
+ 4 Ag
2
→ 4 AgCl + 2 OH
–
–
→ 4 OH
–
O + 4 e
2
–
–
→ 4 OH
35
Polarization
voltage
Reference / Anode
Cathode
Guard
Electrolyte
Glass insulator
NTC 22 k
Membrane
–
at the
–
and
–
+
+ 4 e
–
52 206 349
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