Very accurate measurements of ionic activities of aqueous electrolyte solutions may be obtained from moderately accurate pressure differences between pure water (or a reference liquid) and the aqueous electrolyte solution. A flask containing deionized double distilled water is connected to another flask containing the electrolyte solution by way of a commercially available differential pressure transducer (e.g., Model No. 315 Bh-10, MKS Instruments, Inc., Burlington, Massachusetts). Means are provided for evacuating the flasks simultaneously to remove dissolved gases and for maintaining the pure water and electrolyte solution at a temperature accurate to 0.01 Deg c. Stirrers are provided in the flasks and in the constant temperature bath to maintain a uniform temperature. By controlling the temperature very accurately and obtaining the vapor pressure of the pure water from tables known to be accurate, the activity of water in the electrolyte solution is given by aw = (p deg - ap)/p deg, where aw is the activity of water in the electrolyte solution, p deg is the vapor pressure of water vapor in the reference flask, and ap is the differential pressure as read on the transducer's readout.
U.S. Pat. 4,301,676, Nov. 24, 1981; Chem. Abstr. Not Found