VOLUMETRIC PROPERTIES OF AQUEOUS 1-1-ELECTROLYTES NEAR AND ABOVE THE CRITICAL-TEMPERATURE OF WATER .3. EXPERIMENTAL DENSITIES AND APPARENT MOLAR VOLUMES OF CSBR(AQ) TO THE TEMPERATURE 725.5-K AND THE PRESSURE 38.0 MPA, COMPARISON WITH OTHER 1-1-ELECTROLYTES, AND EXTRAPOLATIONS TO INFINITE DILUTION FOR NACL(AQ)

A vibrating-tube densitometer was used for measuring differences in de nsities between CsBr(aq) and water from molalities of 0.0024 mol.kg-1 to 0.50 mol.kg-1, at temperatures between 604.4 K and 725.5 K, and at pressures from 18.4 MPa to 38.0 MPa. The temperature and molality depe ndences of the derived apparent molar volumes are compared with those for the other 1-1 electrolytes. Anomalous behavior was observed for di lute solutions in the region of low density of water (rho0 < 350 kg.m- 3) where the salts with the lowest ionic radii had the least negative values of the apparent molar volumes. This behavior indicates that the ion pairs with the smallest distance of closest approach have the lea st negative apparent molar volumes. Using the Pitzer parametric equati on and a simplified model describing ion-pairing in dilute solution, t he partial molar volumes at infinite dilution were estimated when the density of water was greater than 280 kg.m-3. Under these conditions, there was no evidence that critical-point phenomena invalidated the es timate.