Is Henry's law constant? A review of its description in environmental engineering texts

Journal of SMET Education : Innovations and Research, Jul-Dec 2002 by Duggan, John W

Abstract

A survey of how Henry's Law is cited in a wide range of texts suggests that there is significant variation in the presentation of the definition. breadth of theory, applications and limitations of this relationship in technical resources used by students. This is of importance because students and practitioners must be able to evaluate and apply the theory of engineering principles in order to effectively design and model environmental systems. Critical design errors may resuit from incomplete or inaccurate explanations of a principle in a technical resource. Most topic-specific texts provided detailed descriptions of Henry's Law. The presentation of Henry's Law in more general environmental engineering texts and other technical resources provided less detailed descriptions. In many cases the theory and application of Henry's Law appear to have been oversimplified. This paper reviews the descriptions of Henry's Law in texts and other resources to demonstrate potential effects on student learning and professional practice that may result from oversimplifications of this engineering principle.

Introduction

One of the critical skills of an engineer is the ability to approximate how a system will perform in the absence of systemspecific data. Handbooks of thermodynamic data, texts, papers and other technical publications serve as resources to formulate such approximations. Engineers learn early in their academic and practicing careers that assumptions about an environmental system such as equilibrium, steady state and ideal solutions help in making approximations and estimations about a system. Engineers also know that theory guides and experiment decides and environmental systems don't generally behave as ideal, well-defined systems. Limitations of the approximation methods need to be understood. Henry's Law is one of the most fundamental principles of environmental engineering and use of it in characterizing a gas/liquid system demands an understanding of its limitations.

Credit for first characterizing concentration equilibria in gas/liquid systems has been given to William Henry. Henry was an early nineteenth century English physician turned chemist who focussed his energies on chemical manufacturing when poor health prohibited him from practicing medicine (Partington, 1960). His postulate that the weight of a gas dissolved in dilute solution by a liquid is proportional to the pressure of the gas upon the liquid was formulated in 1803. Although engineers and scientists identify Henry with this well known principle, perhaps Henry's most important contribution to chemistry was the influence of his experiments on his friend John Dalton's formulation of fundamental principles of modern atomic theory (Greenaway, 1966).

Environmental engineering practitioners use Henry's Law to investigate the measurement and transport of volatile organic compounds in and across air, soil and groundwater interfaces. "Henry's Law" is not a law but, rather, an empirical principle that applies for dilute solutions in ideal liquids when the gas phase behaves as an ideal gas. The following definition is a synthesis of definitions reviewed while researching this paper:

At a constant temperature the mass of substance dissolved in a fixed amount of a liquid at a stable dynamic equilibrium maintained by two-way diffusion across a planar interface is proportional to the partial pressure of the substance. The proportionality constant that holds for the substance in the gas/liquid system is called the Henry's Law Constant. This relationship holds only for dilute solutions that do not react, ionize or dissociate with the solvent liquid.

This relationship is used to predict equilibrium concentrations of a contaminant in a gas/liquid system. In environmental engineering, it is often used to describe the extent of transport across air/water systems such as:

the solubility of oxygen in natural waters, wastewater and drinking water supplies;

the indirect measurement of contaminants in groundwater when soil gas concentrations are known;

the design and performance of air stripping and other air-water mass transfer-based treatment/ remediation systems; and

the mobility potential of volatile organic compounds through soil gas, soil, groundwater and indoor air systems.

References

(1) Partington, J.R., 1960. A Short History of Chemistry, 31 Edition. Macmillan and Co. Ltd., London.

(2) Greenaway, F., 1966. John Dalton and the Atom, 1st Edition. Cornell University Press, New York.

(3) Perry, R.H., Ed., 1997. Perry's Chemical Engineers Handbook, 711 Edition.

(4) Lide, DR., Ed., 1999 . CRC Handbook of Chemistry and Physics, 801 Edition. CRC Press, Boca Raton.

(5) Lyman W., Reehl W., Rosenblatt D., 1990. Handbook of Chemical Properties Estimation Methods, American Chemical Society.

(6) MacKay and Shiu, 1981. A Critical Review of Henry's Law Constants for Chemicals of Environmental Interest, J. Phys. Chem. Ref. Data, 10, 1175-1199.

(7) Thibodeaux, L.J., 1979. Chemodynamics, 1 st Edition. Wiley, New York.