A New Method for the Measurement of the Water Activity or Relative Humidity by Fourier Transform Infrared Spectroscopy

Water activity is an important property of aqueous solutions and humid solids. It can be measured via the relative humidity of a gas which is in equilibrium with the solution or solid. A new method was developed to measure the water activity via the relative humidity by fourier transform infrared (FTIR) spectroscopy. This method is based on earlier work on the measurement of the dew point of compressed humid air (Koglbauer and Wendland, J. Chem. Eng. Data 2007, 52, 1672-1677). The accuracy of the method was tested on salt fixed points which are widely used as humidity fixed points for the calibration of water activity sensors. The estimated uncertainty of the new method and the deviation of the new data from the reference values of the humidity fixed points are within the uncertainty of the reference values.