Summary

Henry's Law describes the relationship between the pressure of a gas above a liquid and the amount of the gas that will dissolve in the liquid.

Definitions

Dh:Hydraulic Diameter
:Concentration (moles per volume)
:Henry's Constant
:Partial Pressure
:Pressure
:Mole Fraction in Liquid

Subscripts:

:Aqueous Phase
:Gaseous Phase


Henry's law

Henry's law tells us that the amount of gas that will dissolve in a liquid is proportional to the pressure of the gas. Unfortunately Henry's Law may be defined in a number of ways, and the constant used must be selected to match the particular formulation of the law.

Formulations of Henry's Law

The various formulations of the Henry's Law constant are shown below.

Form Equation Notes
Concentration over Pressure

Often used by Atmospheric Chemists
Mole Fraction over Pressure

Often taught to Chemical Engineers
Dimensionless

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Molality per Pressure

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Henry's Solubility and Henry's Volatility

The above relationships are often called Henry's solubility. When these relationships are inverted (e.g. ) they are referred to as Henry's Volatility and the inverse constant may be presented.

Assumptions

Henry's law was developed for real gases and liquids, but typically for low concentrations of solutes in water. Thus, the constants are generally only accurate while there is a low concentration of the component in question. Vapour Pressure of a Mixture

Henry's law is typically accurate along the A-B line for component 'a' in the above graph.



Henry's Constant

The table below gives some typical values for Henry's Constant for a species dissolved in water at low concentration and 298.15 K.

Species
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Nitrogen 6.40E-6 6.48E-4 1.59E-2 6.50E-4 1.17E-5
Oxygen 1.30E-5 1.32E-3 3.22E-2 1.32E-3 2.38E-5
Argon 1.40E-5 1.42E-3 3.47E-2 1.42E-3 2.56E-5
Carbon Dioxide 3.30E-4 3.34E-2 8.18E-1 3.35E-2 6.04E-4
Neon 4.50E-6 4.56E-4 1.12E-2 4.57E-4 8.24E-6
Helium 3.90E-6 3.95E-4 9.67E-3 3.96E-4 7.14E-6
Methane 1.40E-5 1.42E-3 3.47E-2 1.42E-3 2.56E-5

Refer to this compilation of values for a much more extensive list.

Conversion Factors

The factors below can be used to convert between different forms of Henry's Constant.

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Further Reading

  1. Chemical Engineering Volume 2, Fifth Edition
  2. Perry’s Chemical Engineers’ Handbook, Eighth Edition
  3. Compilation of Henry's Law Constants (Version 4.0) for Water as Solvent