A calorimetric method to determine water activity

Research output: Contribution to journalArticle


A calorimetric method to determine water activity covering the full range of the water activity scale is presented. A dry stream of nitrogen gas is passed either over the solution whose activity should be determined or left dry before it is saturated by bubbling through water in an isothermal calorimeter. The unknown activity is in principle determined by comparing the thermal power of vaporization related to the gas stream with unknown activity to that with zero activity. Except for three minor corrections (for pressure drop, non-perfect humidification, and evaporative cooling) the unknown water activity is calculated solely based on the water activity end-points zero and unity. Thus, there is no need for calibration with references with known water activities. The method has been evaluated at 30 °C by measuring the water activity of seven aqueous sodium chloride solutions ranging from 0.1 mol kg−1 to 3 mol kg−1 and seven saturated aqueous salt solutions (LiCl, MgCl2, NaBr, NaCl, KCl, KNO3, and K2SO4) with known water activities. The performance of the method was adequate over the complete water activity scale. At high water activities the performance was excellent, which is encouraging as many other methods used for water activity determination have limited performance at high water activities.


Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Physical Chemistry
  • Materials Engineering


  • vaporisation, nitrogen, sodium compounds, solutions, thermal variables measurement, calorimetry, chemical variables measurement, calorimeters
Original languageEnglish
Article number114903
JournalReview of Scientific Instruments
Issue number11
Publication statusPublished - 2011
Publication categoryResearch

Related research output

Björklund, S., 2013, Department of Chemistry, Lund University. 188 p.

Research output: ThesisDoctoral Thesis (compilation)

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