Determination of the Henry's law constants of low-volatility compounds via the measured air-phase transfer coefficients

Accurate Henry's law constants (H) are unavailable for the majority of organic pollutants, especially those having a low volatility. A novel kinetics-based experimental method is introduced to determine H for a wide range of low-H compounds. The method consists of measuring independently the water-to-air transfer coefficient (K_L) and the associated air-phase transfer coefficient (k_G) of a low-H chemical (solute) in water when K_L ≅ k_GH prevails according to the two-film theory. The k_G for a solute is obtained via a developed gas-dynamic equation that relates k_G to the solute molecular weight and the solute-vapor escaping efficiency (β) through a boundary air layer. The value of β is only a function of the in situ air turbulence level, independent of the chemical species. Thus, the required β for solutes can be estimated from the evaporative rates of pure volatile liquids under the same ambient setting. By relating the estimated k_G with the measured K_L of a low-H solute, the solute H is established. The H values of 45 low-H chemicals, including many complex pesticides, in the range of ∼10⁻⁷ to ∼10⁻³ have thus been determined. The accountability of the method is underscored by the consistency of the measured and credible literature H values for a number of the low-H compounds studied.