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 |
|
| 30°C < t < 50°C |
|
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.