A CRITICAL-REVIEW OF HENRYS LAW CONSTANTS FOR ENVIRONMENTAL APPLICATIONS

Henry's law constants (HLCs or air-water partition coefficients) for o rganic compounds of environmental concern are reviewed. Frequently, th e most significant factor influencing HLC values for a particular comp ound is temperature. Conditions are delineated where other parameters (pH, compound hydration, compound concentration, complex mixtures, dis solved salts, suspended solids, dissolved natural organic material [DO M], surfactants, and natural water sample composition) may also signif icantly affect HLC values. HLC estimation techniques utilizing (1) the rmodynamically based quantitative property-property relationships (QPP Rs), including the vapor pressure/aqueous solubility ratio (VP/AS) met hod, and (2) various quantitative structure-property relationships (QS PRs), including use of UNIFAC, are summarized. Major limitations noted were: (1) the VP/AS approach - lack of reliable/accurate vapor pressu re and aqueous solubility data, (2) UNIFAC - errors emanating from req uired extrapolation of vapor-liquid equilibrium (VLE) data, and (3) ot her 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 relat ionships (covering 130 compounds) are summarized and discussed. From t hese data, the average (and typical range) slope of the temperature-de pendent line was found to correspond to a 60% (30 to 100%), 140% (85 t o 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, respecti vely. Finally, the directly measured values were compared with QPPR- a nd QSPR-predicted values.