Measurement of water activity of saturated salt solutions with the vapour pressure manometer

Abstract

The factors which can affect the results of water activity measurement with the vapour pressure manometer (VPM) are analysed. The manometer reading (h l ) is dependent on the volume (V c ) and water activity (a w ) of the sample. The value of h 2, on the other hand, is affected by the solubility of gases in the solution and the volume of the sample. Hence, it is recommended that the volume of the sample be kept constant when using the VPM technique. The time to reach equilibrium is dependent on the a w of the solution and the temperature of measurement. However, these variables do not explain all the variability of the equilibration time observed in this work. A marked effect of the evacuation operation on the solution temperature as a result of adiabatic evaporation of water was observed. The effect of the evacuation of the manometer on the water content of the sample is not clear. However, it is recommended that the water content of the sample be measured after its a w measurement, and not before. The reading of h 2 is shown to have a negligible effect on the value of the correction factor and so a simplified equation for the correction factor is developed.