Development of a Oxygen Sensor for Marine ... - DTU Nanotech

1.1. DISSOLVED OXYGEN LEVEL 3
K = aO2
fO2
(1.2)
Where aO2 is the activity of oxygen and fO2 the fugacity 1 of oxygen in
the gas phase.
However when the fugacity of oxygen is approximated by its partial pressure
PO2 and the activity of oxygen in water by its concentration CO2(aq),
Henry’s law 2 will be expressed as:
CH = CO2(aq)
PO2
(1.3)
Where CH is the Henry’s law constant, CH(O2) = 1.3 × 10 −3 Mol/atm.
PO2 can be calculated by multiplying the fraction volume of oxygen in the
air (21 %) with the pressure at sea level ( 760 mmHg, atmospheric pressure).
Also in dilute solutions and for perfect gases K = CH, hence the
concentration of oxygen in the ocean is
C = CHPO2n = 1.3 × 10 −3 · 0.21 · 760 · 32 = 6.6mg/L (1.4)
6.6 mg/L might not tell much on its own, however is the level of dissolved
oxygen water drops below 5.0 mg/L, fish and other aquatic life will start to
suffocate. The lower it drops the worse it gets, once the oxygen levels drop
below 1-2 mg/L it will only take a few hours before most or all the fish are
dead.[8]
Now as mentioned earlier there is also a connection between the concentration
of dissolved oxygen and temperature, or rather between water
temperature and the saturation of oxygen. This is the point where water
have the maximum amount of oxygen it can have, relative to pressure and
temperature. It is calculated from the following equation[9]:
Cp = C ∗
PW V (1 − )(1 − ΘP )
P P
(1 − PW V )(1 − Θ)
(1.5)
1 The measure of the tendency of a gas to escape or expand[7]
2 The mass of a gas that dissolves in a definite volume of liquid is directly proportional
to the pressure of the gas provided the gas does not react with the solvent.