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Henry's Law

Henry's law is used in gas-transfer calculations. At equilibrium the concentration of a gas dissolved in water (or any solvent) is proportional to its partial pressure. For example, the N2 concentration is given by

where square brackets indicate concentration (mol/L), is the partial pressure (atm), and kH has the dimensions mol/(L-atm) (Moore 1972). For example, the dissolved N2 concentration in equilibrium with atmospheric N2 at 25ºC is

Henry's law is only valid for low dissolved gas concentrations. However, the atmospheric partial pressures of N2, NH3, N2O, NO, NO2, and HNO2 are all low enough that Henry's law applies.

Most Henry's law constants have been determined at 25ºC, but they decrease with increasing temperature (Figure 1). However, for the temperatures of most natural systems, the value of kH at 25ºC is off by no more than a factor of two.

Figure 1. Henry's law constants for NO and N2 from 0 to 50ºC (Data from Dean 1979)


References Cited:

Dean, J. A. 1979. Lange's Handbook of Chemistry, 12th ed. New York:McGraw-Hill.

Moore, W. J. 1972. Physical Chemistry, 4th Edition. Englewood Cliffs, NJ:Prentice-Hall.