A critical review of Henry's law constants for environmental applications

Abstract

Henry's law constants (HLCs or air‐water partition coefficients) for organic compounds of environmental concern are reviewed. Frequently, the most significant factor influencing HLC values for a particular compound is temperature. Conditions are delineated where other parameters (pH, compound hydration, compound concentration, complex mixtures, dissolved salts, suspended solids, dissolved natural organic material [DOM], surfactants, and natural water sample composition) may also significantly affect HLC values. HLC estimation techniques utilizing (1) thermodynamically based quantitative property‐property relationships (QPPRs), including the vapor pressure/aqueous solubility ratio (VP/AS) method, and (2) various quantitative structure‐property relationships (QSPRs), including use of UNIFAC, are summarized. Major limitations noted were: (1) the VP/AS approach — lack of reliable/accurate vapor pressure and aqueous solubility data, (2) UNIFAC — errors emanating from required extrapolation of vapor‐liquid equilibrium (VLE) data, and (3) other QSPRs — predictions limited to a single temperature (25°C). Following a review of HLC experimental determination techniques, 25 studies establishing directly measured HLC temperature‐dependent relationships (covering 130 compounds) are summarized and discussed. From these data, the average (and typical range) slope of the temperature‐dependent line was found to correspond to a 60% (30 to 100%), 140% (85 to 250%), and 90% (45 to 170%) increase in HLC per 10°C rise in temperature for hydrocarbons (omitting pesticides and polychlorinated biphenyls [PCBs]), pesticides and PCBs, and nonhydrocarbons, respectively. Finally, the directly measured values were compared with QPPR‐and QSPR‐predicted values.