CALCULATION OF DENSITIES OF AQUEOUS-ELECTROLYTE SOLUTIONS AT SUBZERO TEMPERATURES

We developed a FORTRAN program based on the Pitzer equations to calcul ate densities of electrolyte solutions at subzero temperatures. Data f rom the published literature collected at -28.9, -17.8, -12.2, -6.7, 0 , and 25 degrees C were used to calculate the Pitzer-equation paramete rs and to evaluate model performance. Three approaches to estimating t he molar volume of the solute at infinite dilution were evaluated: (1) extrapolation of apparent molar volumes to zero square-root ionic str ength; (2) calculation with the Tanger and Helgeson model; and (3) glo bal fit of the data in which the molar volume of the solute at infinit e dilution was estimated along with the Pitzer-equation parameters. Th e last approach gave parameter estimates that reproduced the experimen tal data most accurately. The parameterized model predicted accurately densities of single-electrolyte and multielectrolyte solutions at -28 .9, -17.8, -12.2, -6.7, 0, and 25 degrees C. Available experimental da ta are generally quite poor. Accordingly, Pitzer-equation parameters e stimated for subzero temperatures should be viewed as conditional unti l improved measurements of single-electrolyte solution densities at su bzero temperatures are made.