Polarization Voltage; Temperature; Dependence On Flow - Mettler Toledo InPro 6850i Bedienungsanleitung

InPro 6850i / 6900i / 6950i O
l = k
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 polarogram)
reduced while other gases are unaffected (< D). The
ideal voltage for the Pt/Ag/AgCl or Pt/Pt/Ag/AgCl sys-
tem 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 electrical re-
sistance of electrolyte film must not exceed a particu-
lar 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.

10.5 Temperature

The temperature dependence of the current passing
through an oxygen electrode when referred to a con-
stant O
properties of the gas-permeable membrane.

10.6 Dependence on flow

With most oxygen electrodes the electrode current is
smaller in stagnant solutions than in agitated ones.
In consequence of the oxygen consumption of the
electrode, a reduction of oxygen takes place outside
the membrane in close proximity to the cathode. The
missing oxygen is replaced by diffusion. If the elec-
trode current is strong, the solution cannot fully re-
store the reduced oxygen by diffusion. This results in
an electrode current weaker than would correspond
to conditions in the solution. In agitated solutions the
© 02 / 09 Mettler-Toledo AG
Printed in Switzerland
D a A
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
partial pressure is determined mainly by the
2
Sensor 12 / 25 mm
2
pO
2
x
in the membrane
2
No. 3
No. 2
«D»
0.6 0.8 1.0
InPro 6850i, 6900i, 6950i
No. 1
1.2
– E (Volts)
52 206 349
37