Oxygen Concentration in Aqueous Solutions

If oxygen-free water comes in contact with atmospheric air, the water tries to draw oxygen molecules (as well as molecules of other gases) from the air until it reaches saturation. By doing so, the water dissolves the oxygen gas like any other solid or liquid. The solubility depends on temperature. For solids and liquids, solubility increases with rising temperature, whereas for gases, solubility decreases with rising temperature.

The increasing temperature provides the kinetic energy for the acceleration of all gas molecules. Some gas molecules escape from the water back into the air. This is the reason why a glass of beer goes flat when it gets warm and why fish seek deeper water in summer. They cannot survive in the oxygen-starved warm water at the surface.

Oxygen exerts the same partial pressure when dissolved in water as it does in air. The in-and-out movement of oxygen molecules at the air/water phase boundary stops when the partial pressure of oxygen in the water equals the partial pressure of oxygen in air above the water. The water is now saturated with oxygen, and a state of equilibrium is reached at the air/water phase boundary.

Therefore, only the partial pressure of oxygen in the air above the water determines the maximum partial pressure of oxygen in the water.

In addition to oxygen molecules, water molecules also leave the water, as well as those of nitrogen, argon, and other gases. The total partial pressure is therefore spread amongst all gases and vapors in the air; the volume concentration of oxygen (20.95%) is not affected. Therefore, the air above the water/air phase boundary is always moist to the extent that the air is saturated with water vapor.