Mettler Toledo InPro 6850 i Bedienungsanleitung Seite 38

38
the measurement medium. However, since this is not
measured by the sensor current, the concentration
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 fac-
tors "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
© 04 / 13 Mettler-Toledo AG
Sensor 12 / 25 mm
2
/ l
2
/ l
2
/ l
2
Printed in Switzerland