Water Activity Calculation by Direct Measurement of Vapor Pressure

An improved method for calculation of water activity from vapor pressure manometric data, which takes into account the volume expansion of nonadsorbing gases, the initial pressure in the desiccant flask and the pressure exerted by the desiccant, was developed. The water activity of pure water was measured over a range of nonadsorbing gas pressures and initial pressures in the desiccant flask. Standard deviations in a_w of pure water calculated using no corrections, corrections for volume expansion only, and full corrections were 0.221, 0.011, and 0.005, respectively. Correction only for volume expansion may under some conditions result in larger errors than when no corrections are made at all. Based on the improved equation, an experimental modification which reduces loss of water from samples during measurement was proposed.