Henry's Law (Concentration Convention)

Use the explicitly stated concentration convention for Henry’s law: C_g = k_H·p_g. C_g is the equilibrium concentration of dissolved gas, and p_g its partial pressure above the liquid. At the same temperature, doubling the partial pressure also doubles C_g within the linear range. For example, if 1.0 mmol/L is dissolved at 100 kPa, about 0.20 mmol/L is expected at 20 kPa. This assumes the gas, solvent and temperature stay unchanged. Also check how k_H is defined: some sources use the inverse convention p/C.

In the form used here, C_g = k_H·p_g, k_H is the proportionality factor between gas pressure and dissolved concentration. It depends on the gas, the solvent and temperature; it is not universal. If C_g is in mol/L and p_g in kPa, its unit is mol/(L·kPa). A larger k_H in this convention means a higher dissolved concentration at the same pressure. Under the alternative convention p_g = k_H·C_g, the interpretation is reversed and the unit becomes pressure/concentration. The symbol alone is not enough to compare constants between sources.

A liquid often contacts a gas mixture, such as water in air. For oxygen solubility, the oxygen partial pressure matters, not total air pressure. In dry air near sea level, p_O₂ is about 0.21·101 kPa ≈ 21 kPa. Substituting 101 kPa instead would overestimate dissolved oxygen by roughly a factor of five. Humidity and other conditions can change partial pressures. The law describes equilibrium at the interface; it does not calculate how fast a gas dissolves, nor account for losses through reaction or flow.

Henry’s law gives the proportionality between partial pressure and equilibrium concentration at a fixed temperature. Temperature also affects the value of k_H. For many gases in water, solubility decreases as temperature rises, so k_H in this concentration convention becomes smaller. A warmer stream can therefore hold less oxygen at the same oxygen partial pressure. The direction depends on the substance and should be checked against relevant data; do not reuse a 20 °C constant unchanged at 30 °C. Biological consequences also depend on mixing, photosynthesis and organismal consumption.

The simple linear relation is intended for a specific gas–solvent pair at fixed temperature and within an appropriate solubility range. It can fail when the solution is highly concentrated, the gas and solvent react or interact strongly, temperature changes, or pressure is high enough that linearity no longer holds. An open vessel away from equilibrium or a flowing system also cannot be described by the equation alone. Those cases require measured solubility data or a more complete thermodynamic model. Always check the constant’s units and validity conditions first.