An improved isopiestic method for measurement of water activities in aqueous polymer and salt solutions

The isopiestic method was described, and its advantages and disadvantages were discussed. A nine-legged glass apparatus was developed and the experimental procedure was improved. The time for equilibration was greatly reduced to 3 or 4 d, even for the high viscous solutions. The average relative error in the observed water activities for standard electrolyte solution was ±0.01%. The water activities of PEG1000 + H2O, PEG4000 + H2O, PEG + MgSO4 + H2O and PEG + (NH4)2SO4 systems were measured at 25°C.