The solubility of oxygen is
affected by temperature and
by the partial pressure of
oxygen over the water.
How
does temperature affect
dissolved oxygen levels?
The solubility of oxygen
is greater in colder water
than in warm water. Oxygen
slips into "pockets" that
exist in the loose hydrogen-bonded
network of water molecules
without forcing them apart.
The oxygen is then caged
by water molecules, which
weakly pin it in place.
The dissolution is exothermic
overall, so cooling shifts
the equilibrium towards
the dissolved form [1].
How
does oxygen partial pressure
affect dissolved oxygen
levels?
Oxygen in water
obeys Henry's law rather
well; the solubility is
roughly proportional to
the partial pressure of
oxygen in the air:
pO2
= KO2 xO2
where pO2
is the partial pressure of
oxygen in Torr, xO2
is the mole fraction of oxygen
in oxygen-saturated water,
and KO2 is the
Henry's law constant for oxygen
in water (about 3.30 ×
107 K/Torr for
at 298 K [2]).
Higher air pressure means
higher partial pressure of
oxygen, so waters at sea level
can contain dissolve slightly
more oxygen than mountain
streams at the same temperature.
High humidity very slightly
lowers the fraction of oxygen
in the air, and so lowers
saturated dissolved oxygen
levels slightly.
Many empirical
equations are available
to accurately estimate oxygen
solubility as a function
of temperature, pressure,
and humidity. The more accuracy
you require, the more complex
the equations are.
Here
are some very simple empirical
equations that give the
saturated dissolved O2
concentration (DO) in mg
O2/L water. They
apply to oxygen in distilled
water at a barometric pressure
of P (in torr), at a temperature
of t (°C), with a water
vapor pressure of p (in
torr) [3]:
| 0°C
< t < 30°C
|
| DO
= |
(P-p)
× 0.678 |
|
35
+ t
|
|
| 30°C
< t < 50°C
|
| DO
= |
(P-p)
× 0.827 |
|
49
+ t
|
|
Here's
a quick DO calculator based
on these functions. Just
enter the barometric pressure
in torr, and the water temperature
in °C. Hit the Calculate
button to see the saturated
water vapor pressure and
the predicted DO concentration.
The saturated water vapor
pressure is estimated by linear
interpolation from a table
of experimental values, and
is reliable to 3-4 figures.
The DO concentration is reliable
to 2-3 figures.
Oxygen
Solubility 
