When chemical equilibrium is reached, and according to Henry’s law, the solubility of O₂ gas depends on the partial pressure of O₂ above the solution.

where x_O2^L is the O₂ mol fraction in water, and K_H,O2 the Henry’s law constant of the gas.
Henry’s equilibrium is not verified in environments where O₂ gas is dissolved and used continuously (biological or chemical systems): this is the case for rivers and waste water. The dissolved dioxygen concentration is below the expected concentration, which is evidence of pollution.

To determine O₂ concentration in water, several methods are possible:

1. By colorimetric titration and the Winkler method.
2. With the use of a Clark cell.