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EPA On-line Tools for Site Assessment Calculation

Estimated Henry's Law Constants

Background: Henry's Law Constants characterize the equilibrium distribution of dilute concentrations of volatile, soluble chemicals between gas and liquid. For this calculator, the liquid is water. Temperature-dependence is calculated by two methods: one developed by the EPA Office of Solid Waste and Emergency Response and the other published in the journal Ground Water and written by John Washington in 1996. Background information on each method is given on a separate page.

Special background information on methyl tert-butyl ether (MTBE) is available

1) Chemicals are only included if there is data for the temperature-dependence calculation. Henry's Constants for many petroleum hydrocarbons and oxygenated additives are available from a data set of estimated properties. For other chemicals see the chemical properties page.
2) The unit choices for Henry's constants include atm-m3/mol, atm, and two separate dimensionless values. These are listed below.
3) Previously the calculator contained some single-temperature values. These have been eliminated from the calculation but are available for reference.





Unit Choices for the Henry's Law Constant

H cc = Concentration/Concentration (dimensionless--volumetric basis) 1
H yx = Mole Fraction Y / Mole Fraction X (dimensionless)
H px = Partial Pressure / Mole Fraction X (atmospheres)
H pc = Partial Pressure / Solubility (atm m 3 /mol)

1The dimensionless form based on concentrations (volumetric basic) is the most commonly used of the dimensionless values. See Staudinger and Roberts, 1996, A Critical Review of Henry's Law Constants for Environmental Applications, in Critical Reviews in Environmental Science and Technology, 26(3):205-297 for more information on various units (specifically page 292).

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