The measurement of fugacity and the Henryʹs law constant for volatile organic compounds containing chromophores

A new method for the measurement of the fugacity and the Henryʹs law constant for volatile organic compounds containing chromophores is described; this was tested with benzene, toluene and ethylbenzene distributed between air and water. The steadily flowing vapor phase of fixed composition is prepared with a mass-flow controller and syringe pump, a method that has been used to calibrate the adsorption by fibres in solid phase micro extraction. The vapor phase flows in and out of the headspace above pure water in a 1-cm spectrophotometric cell housed in a temperature-controlled holder in a UV–vis spectrophotometer. The approach to equilibrium, as the chemical is transferred from the vapor to the stirred liquid phase, is monitored by following the absorbance of the liquid phase at a selected wavelength. Calibration leads to the following molar absorption coefficients (±standard errors, units of L mol−1 cm−1) in water at 25 °C; for benzene at 260 nm, 101 ± 1; for toluene at 261 nm, 284 ± 1; and, for ethylbenzene at 257 nm, 253 ± 3. The fugacities are linear with respect to the mole fractions in the water. The resulting Henryʹs law constants (±standard errors, units of 104 kPa) are comparable to published values; at 25 °C, these are for benzene, 2.22 ± 0.05; for toluene, 3.99 ± 0.04; and for ethylbenzene, 5.14 ± 0.06.