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

Determining the methanol partition coefficient (K_ow) 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 K_ow 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) 0.2 mL of water plus 3 mL of octanol, (2) 1 mL of water plus 3 mL of octanol and (3) 1 mL of water plus 10 mL of octanol] were used in the experiment. After pre-standing at 25 °C for > 10 h and equilibrating in the headspace oven at a temperature of interest for additional 25 min, the methanol signals of the vapor phase (i.e., the headspace) in these vials were measured by HS-GC, from which the K_ow 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 K_ow of methanol and temperature extrapolated the K_ow values in a temperature range of interest. The present method represents a simple and in-situ approach for the determination of the K_ow of methanol and is also suitable to be applied to other volatile compounds.