Membrane inlet manometry for the measurement of water activity in aqueous and organic solutions

Abstract An apparatus for continous measurement of water activity is described where a water permeable membrane is used to separate the sample from the vacuum of a continously pumped chamber. When the sample contains no other component than water which permeates through the membrane at a significant rate the steady state pressure in the vacuum chamber is a function of the water activity of the sample. A polypropylene terephthalate membrane was used which is highly permeable to water and virtually impermeable to atmospheric gases and volatile organic liquids. The vacuum chamber is pumped by a turbomolecular pump and the pressure is measured by a Penning gauge. The apparatus is calibrated by the help of aqueous salt solutions of known water activity. Examples of measurements of water activity in binary alcohol-water mixtures are presented and compared with data from the literature.