Sander and 4 co-authors reported in late 2020 that Henry’s law states that the abundance of a volatile solute dissolved in a liquid is proportional to its abundance in the gas phase. It applies at equilibrium and in the limit of infinite dilution of the solute. For historical reasons, numerous different definitions, names, and symbols are used in the literature to express the proportionality coefficient, denoted the “Henry’s law constant”. Here, a consistent set of recommendations is presented. An important distinction is made between two new recommended reciprocal quantities “Henry’s law solubility constant” (Hs)
and “Henry’s law volatility constant” (Hv).
Eight recommended variants of Hs and Hv are described and relations among them presented.
In his original publication from 1803, William Henry described his discovery: [1]
“[. . . ] water takes up, of gas condensed by one, two, or more additional atmospheres,
a quantity which, ordinarily compressed, would be equal to twice, thrice, &c. the
volume absorbed under the common pressure of the atmosphere,"
Today, this proportionality, at equilibrium, between the abundance of a species in the gas phase and its abundance in a liquid solution is known as Henry’s law. It is a limiting law, analogous to the ideal gas law, valid only in the limits of infinite dilution of the solute and ideality in the gas phase. Departure from ideality can be accounted for in Henry’s law by replacing abundances by activities (see Sect. 4.3).
Since the Henry’s law constant is an extrapolation to infinite dilution, it cannot be measured directly in a laboratory. To distinguish these, the authors suggested that the name “experimental Henry’s law constant” for values obtained at a non-zero abundance of the solute. Values of Henry’s law constants are known for many chemical species, mainly for aqueous solutions but also for other solvents. Numerous different and sometimes inconsistent definitions, names, and symbols for the Henry’s law constant are currently used in the scientific literature as illustrated in the detailed survey in the supplement.
Unfortunately, there is also a large variety of symbols used in IUPAC publications. In 1983, IUPAC recommended a symbol [see ref 2, p. 571]. In the glossary of atmospheric chemistry terms (IUPAC recommendations 1990) [3], a different definition was used, and the symbol was changed to H. The IUPAC book “Chemicals in the Atmosphere: Solubility, Sources and Reactivity” from 2003 [4] contains several different symbols (H˜x, K˜Hx , KWA, KAW, kH, and cw/cg) in different chapters.
The symbol KH (with an upper case K) was used in the introduction to the IUPAC-NIST Solubility Data Series [5]. The symbol was changed to kH (with a
lower case k) in the IUPAC recommendations from 2008 [6]. At the time of writing, the online version of the Gold Book [7] contains the recommendations from 1983.
The objective of this article is to present a consistent set of recommended quantities and symbols for Henry’s law constants based on IUPAC recommendations for expressing abundances in the two phases. Additionally, formulas are provided for converting between the variants of H. A complete treatment of the thermodynamics underlying Henry’s law is beyond the scope of this paper that focuses on quantities and symbols for the Henry’s law constant.
Keywords for the subject article: Henry’s law; solubility; volatility; gas-solution equilibrium
2021 Update: https://iupac.org/projects/project-details/?project_nr=2019-025-1-100
Original working URL: https://iupac.org/wp-content/uploads/2021/02/PAC-REC-2020-0302.R2_PR20210127.pdf
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