Volume change for the formation of magnesium sulfate ion pairs at various temperatures

Frank J. Millero, William L. Masterton

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40 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)1287-1294
Number of pages8
JournalJournal of physical chemistry
Issue number13
StatePublished - Jan 1 1974

ASJC Scopus subject areas

  • Engineering(all)
  • Physical and Theoretical Chemistry


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