The isothermal compressibilities (β's) of aqueous NaCl, Na2SO4, MgSO4, and MgCl2 solutions have been measured at 0, 15, 30, and 45° by a piezometric technique. The compressibilities were determined at applied pressures P = 8.7, 16.8, and 25.7 bars and the data were extrapolated to 1 atm. The β's have been fitted to an equation of the form, β = β0+ AKc + BKc3/2, where β0 is the compressibility of pure water, c is the molar concentration, and AK and BK are temperature-dependent parameters. The equation fits the compressibility data to ±0.07 × 10-6 bar-1 for all the salts. The apparent molal compressibilities, φK, have been calculated from the compressibility data. At high concentrations the concentration dependence of the (φK)'s were found to follow the equation, φK = φK0 + SK*IV1/2, where φK0 and SK* are empirical constants and IV is the molar ionic strength. To obtain reliable infinite dilution (φK0)'s (that agree with values derived from sound velocity measurements) it was necessary to use the equation, φK = φK0 + SKIV1/2 + bKIV, where φK0 is the infinite dilution apparent molal compressibility, SK is the Debye-Hückel limiting law slope, and bK is a temperature-dependent parameter related to deviations from the limiting law. The (φK0)'s from all the electrolytes increase with increasing temperature and appear to go through a maximum between 40 and 50°. The deviations from the limiting law, bK, are all positive at low temperature and decrease with increasing temperatures for all the salts. The (φK)'s are briefly discussed in terms of the ion-water and ion-ion interactions responsible for the observed behavior The (φK)'s for MgSO4 solutions have been analyzed by assuming the ion pair, MgSO40, is formed. The ΔφK for the formation of the ion pair and the φK of MgSO40 have been calculated at various concentrations and temperatures. The results are used to examine the structure of the ion pair.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry