Mettler Toledo InPro 6850i Instructions D'utilisation page 38

38
in the measurement medium. However, since this is
not measured by the sensor current, the concentra-
tion of oxygen must be calculated in the transmitter.
To do this, Henry's law is applied which states that
the concentration of oxygen is proportional to the
partial pressure of oxygen (pO
a = Solubility factor
If "a" is constant, the oxygen concentration can be
determined by means of the electrode. This applies
at constant temperature and with dilute aqueous
solutions such as drinking-water.
The solubility factor is strongly influenced not only
by the temperature but also by the composition of
the solution:
Medium,
saturated with air
Water
4 mol / l KCI
50 % Methanol-water
Although the solubilities vary widely, the oxygen
electrode gives the same reading in all three solu-
tions.
Thus, determination of the oxygen concentration is
o nly possible with constant and known solubility
factors "a".
Solubility may be determined by a Winkler titration
or the method developed by Käppeli and Fiechter.
Measurement in gases
The concentration of oxygen when measuring in
gases is always given as a proportion by volume of
the gas composition. Common units are % (by vol.)
ppm (by vol.).
They can simply be converted from one unit of mea-
surement to the other.
Example:
The percent by volume of the composition of air is
generally known. For example, air contains 20.95 %
oxygen. This corresponds to 209,500 ppm (by vol.).
(Conversion: ppm value = 10,000 3 value in %)
References
– W. M. Krebs, I. A. Haddad Develp. Ind. Microbio.,
13, 113 (1972)
– H. Bühler, W. Ingold GIT 20, 977 (1976)
– W.M. Krebs, MBAA Techn. Quart. 16, 176 (1975)
– D.P. Lucero, Ana. Chem. 40, 707 (1968)
InPro 6850 i / 6900 i / 6950 i
52 206 349
InPro 6850 i / 6900 i / 6950 i O
).
2
Solubility at 20 °C
(68 °F) and 760 mm Hg
9.2 mg O
2 mg O
21.9 mg O
© 02 / 17 Mettler-Toledo GmbH
Printed in Switzerland
Sensor 12 / 25 mm
2
/ l
2
/ l
2
/ l
2