Improved apparatus and procedures for isopiestic studies at elevated temperatures

A significantly improved apparatus for the isopiestic measurement of thermodynamic activities at elevated temperatures is described. Details of the new design and of an analysis of the propagation of the experimental errors are presented. The apparatus was tested by measurements of calcium chloride against sodium chloride at 353.15 K. Accuracies of better than 0.1% in the osmotic coefficient are routinely achieved. Data are also given for the osmotic coefficient of aqueous sodium hydroxide solutions at 353.15 K as a function of molality (up to 13 mol/kg), measured with respect to both sodium chloride and calcium chloride solutions as reference electrolytes. (C) 1997 American Institute of Physics.