I have had the need recently to calculate the partial vapor pressure vs. temperature of one material in a binary solution. I am only interested in the flammable material, so in this case, it is the ethanol in an ethanol-water system. Since I am not near critical points and my temperature range seems reasonable (8 – 25 C), I am using Raoult’s Law modified by the Activity Coefficient.

y_{ethanol}*P = x_{ethanol}*γ*P_{ethanol}^{sat}

Where:

y_{ethanol}*P: The partial vapor pressure of ethanol (what I am looking for)

x_{ethanol}:Liquid phase mole fraction of ethanol

γ: Activity coefficient for ethanol-water system

P_{ethanol}^{sat}:Vapor pressure of pure ethanol (a function of temperature)

My initial system is 40 volume% ethanol. I calculated the activity-coefficient using the Margules Equation and associated binary-interaction parameters from Perrys. In order to get P_{ethanol}^{sat}, I first tried using the DIPPR 101 equation, but the pressure units were not specified, and my values seemed quite large. I am currently calculating P_{ethanol}^{sat} using Antoine’s Equation, as a result.

Is this an appropriate/correct approach to calculate the partial pressure of the ethanol vapor off of a known ethanol-water system? I’d welcome any improvements or suggestions on “better” equation modifications. Thank you for your time.