Parameters Determining Current; Polarization Voltage - Mettler Toledo InPro 6850i Instructions D'utilisation

36

10.3 Parameters determining current

The quantity of oxygen diffused in and the magni-
tude of the electrode current are influenced by the
following parameters:
– Oxygen partial pressure of the solution
– Membrane material and thickness
– Size of cathode

– Polarization voltage

– Temperature
– Flow conditions in the solution
Fick's law gives the mathematical relationship be-
tween these parameters:
I = Electrode current
k = Constant
D = Coefficient of diffusion of O
a = Solubility of O
A = Cathode surface
pO = Oxygen partial pressure in the solution
2
x = Thickness of gas-permeable membrane
10.4 Polarization voltage
The voltage between the anode and the cathode is
so selected that the oxygen is fully (> A, see po-
larogram) reduced while other gases are unaffected
(< D). The ideal voltage for the Pt / Ag / AgCl or Pt / Pt /
Ag / AgCl system is between – 500 and – 750 mV.
100
90
80
70
60
50
40
30
20
10
0
The polarization voltage should remain as constant
as possible. Besides a constant source of voltage
the following prerequisites must be met: The electri-
cal resistance of electrolyte film must not exceed a
particular value so as to avoid a voltage drop.
The anode must have a large surface so as to
prevent polarization of the anode by the electrode
current.
InPro 6850 i / 6900 i / 6950 i
52 206 349
InPro 6850 i / 6900 i / 6950 i O
in the membrane material
2
Typical current voltage
curves in agitated
0.1 mol/l KCl
No. 1 Nitrogen
No. 2 21% Oxygen
No. 3 100% Oxygen
«A»
0.2 0.4
in the membrane
2
No. 3
No. 2
«D»
0.6 0.8 1.0
© 02 / 17 Mettler-Toledo GmbH
Sensor 12 / 25 mm
2
No. 1
– E (Volts)
1.2
Printed in Switzerland