The partial molal volume and compressibility change for the formation of the calcium sulfate ion pair at 25°C

Frank J. Millero, Francis Gombar, Jeffrey Oster

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The apparent molal volumes (φv) and compressibilities (φK) of CaSO4 solutions have been determined at 25°C from precise density and sound-speed measurements. The large deviations of the values of φv and φK from the limiting law and various additivity estimates for the free ions (Ca2+, SO42-) have been used to estimate the partial molal volume ( {Mathematical expression}) and compressibility ( {Mathematical expression}) for the formation of the CaSO40 ion pair. Values of {Mathematical expression}= 25 ± 3 cm3-mole-1 and {Mathematical expression}= (54±21)×10-4 cm3-mole-1-bar-1 were found. Since these values are larger than the value for the formation of MgSO40, the results indicate that more inner-sphere ion pairs are formed when SO42- complexes with Ca2+ than with Mg2+. Using a simple model for ion-water interactions, the percent of inner-sphere or contact ion pairs in CaSO4 solutions is estimated to be 36 to 37%.

Original languageEnglish (US)
Pages (from-to)269-280
Number of pages12
JournalJournal of Solution Chemistry
Issue number4
StatePublished - Apr 1 1977



  • Calcium sulfate
  • ion pair
  • molal volume and compressibility

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Molecular Biology
  • Physical and Theoretical Chemistry

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