Practical deviations from Henry`s law for water/air partitioning of volatile organic compounds

A study was conducted to define parameters relating to the use of a down hole submersible photoionization detector (PID) probe to measure volatile organic compounds (VOCs) in an artificial headspace. The partitioning of toluene and trichloroethylene between water and air was studied as a function of analyte concentration and water temperature. The Henry`s law constant governing this partitioning represents an ideal condition at infinite dilution for a particular temperature. The results show that in practice. this partitioning is far from ideal. Conditions resulting in apparent, practical deviations from Henry`s law include temperature and VOC concentration. Thus, a single value of Henry`s law constant for a particular VOC such as toluene can provide only an approximation of concentration in the field. Detector response in saturated humidity environments as a function of water temperature and analyte concentration was studied also.