Using an internal standard method to determine Henry's law constants

An internal standard method is developed for the measurement of thermodynamic Henry's law constants (H). In this method, a mixture of the analytes and an internal standard is prepared and used to make a standard solution (organic solvent) and a dilute aqueous solution. Both the standard solution and the headspace samples above the aqueous solution at partitioning equilibrium in closed containers are subject to gas chromatographic (GC) analysis. Subsequently, the values of H for the analytes can be calculated from the known H of the internal standard and the GC peak-area ratios. Only approximate values of the concentrations of the compounds, the GC injection volumes, and the vapor-phase to liquid-phase volume ratios in the closed containers are needed in this approach. The method works particularly well for compounds that are highly soluble in water or that have low vapor pressures. Experimentally determined values of H are reported for some low-molecular-weight aldehydes, ketones, and nitriles, and their temperature dependencies are examined. The results also are compared with literature values. The applicability of this new approach is limited to compounds that have dimensionless H on the order of 10(-3) or less.