The apparent molal volumes of aqueous MgSO4 solutions from 0 to 1 m at 0, 25, and 50° have been used to estimate the apparent molal volume of the ion pair MgSO4°, φV(MgSO4°) using the relation φV(obsd) = (1 - α)φV(MgSO4°) + αφV(Mg2+, SO42-) where φV(obsd) is the observed apparent molal volume for MgSO4 solutions at molality mT, α is the fraction of free ions, and φV(Mg2+, SO42-) is the apparent molal volume of the free ions Mg2+ and SO42- at the ionic strength Im = 4αmT. Using known association constants for the formation of MgSO4° and various estimates for φV(Mg2+, SO42-), we have determined the φV of MgSO4° and ΔφV for its formation as a function of temperature and concentration. The infinite dilution values for the φV° of the ion pair MgSO4° appear to be independent of the various estimates of the φV of the free ions Mg2+ + SO42-. The ΔV̄° = ΔφV° for the formation of MgSO4 at 25° is in reasonable agreement with the results calculated from the high-pressure conductance data of Fisher and theoretical estimates. The ΔV̄° calculated from the density data appears to go through a minimum when plotted vs. temperature similar to the volume change for the ionization of weak acids and bases. The φV° and ΔφV° results for MgSO4° are briefly discussed in terms of the structure of the ion pair.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry