Unsaturated Solutions of Sodium Chloride as Reference Sources of Water Activity at Various Temperatures

In view of the uncertainty aboui the exact water activity.(a w ) of some solutions (saturated and unsaturated) used as references sources, the use of unsaturated NaCl solutions as isopiestic standards for calibration of hygrometers is proposed. It is shown that there is an excellent agreement on the exact value of NaCl solutions between various literature compilations and theoretical models. Further, it is also shown that in the important range (for food related applications) of 15–50°C the a w of NaCl solutions is almost invariant.