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

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 degrees 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 degrees 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.