Temperature dependence of Henry's law constant in an extended temperature range

The Henry's law constants H for chloroform, 1,1-dichloroethane, 1,2-dichloropropane, trichloroethene, chlorobenzene, benzene and toluene were determined by the EPICS-SPME technique (equilibrium partitioning in closed systems––solid phase microextraction) in the temperature range 275–343 K. The curvature observed in the ln H vs. 1/ T plot was due to the temperature dependence of the change in enthalpy ΔH 0 during the transfer of 1 mol solute from the aqueous solution to the gas phase. The nonlinearity of the plot was explained by means of a thermodynamic model which involves the temperature dependence of ΔH 0 of the compounds and the thermal expansion of water in the three-parameter equation ln ( Hρ T T)= A 2/ T+ BT B+ C 2, where ρ T is the density of water at temperature T, T B=ln( T/298)+(298− T)/ T, A 2=− ΔH 0 298/ R, ΔH 0 298 is the ΔH 0 value at 298 K, B= ΔC p 0/ R, and C 2 is a constant. ΔC p 0 is the molar heat capacity change in volatilization from the aqueous solution. A statistical comparison of the two models demonstrates the superiority of the three-parameter equation over the two-parameter one ln H vs. 1/ T). The new, three-parameter equation allows a more accurate description of the temperature dependence of H, and of the solubility of volatile organic compounds in water at higher temperatures.