Determination of methanol partition coefficient in octanol/water system by a three-phase ratio variation headspace gas chromatographic method

Abstract

Determining the methanol partition coefficient (Kow) in octanol/water system is important to assess the risk of its impact to humans and the environment. In this paper, we report a novel method for the determination of the Kow of methanol with headspace gas chromatography (HS-GC), using a three-phase equilibrium-based phase ratio variation approach. Three headspace vials with different phase ratios [(1) 0.2mL of water plus 3mL of octanol, (2) 1mL of water plus 3mL of octanol and (3) 1mL of water plus 10mL of octanol] were used in the experiment. After pre-standing at 25 °C for > 10h and equilibrating in the headspace oven at a temperature of interest for additional 25min, the methanol signals of the vapor phase (i.e., the headspace) in these vials were measured by HS-GC, from which the Kow of methanol can be calculated based on the established equations. The results showed that the method has good precision (within 0.06 log unit in standard deviation) and acceptable accuracy (with method error within 0.5 log unit). A correlation between the Kow of methanol and temperature extrapolated the Kow values in a temperature range of interest. The present method represents a simple and in-situ approach for the determination of the Kow of methanol and is also suitable to be applied to other volatile compounds.