The dissociation of phosphoric acid in NaCl and NaMgCl solutions at 25°C

J. Peter Hershey, Marino Fernandez, Frank J. Millero

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31 Scopus citations


The pK1*, pK1* and pK3* for the dissociation of H3PO4 have been measured in NaCl solutions from 0.5 to 6m at 25°C. The results have been used to evaluate Pitzer interaction parameters ψ(NaClH2PO4)=-0.028±0.005, λ(NaH3PO4)=-0.075±0.025, θ(HPO4Cl)=0.105±0.009, θ(PO4Cl)=-0.59±0.02 and ψ(NaClHPO4)=-0.003±0.004, ψ(PO4NaClH)=0.110±0.008. These parameters yield values of pK1*, pK2* and pK3* in NaCl that agree with the measured values with average deviations of ±0.04, ±0.03 and ±0.05 in pK1*. Measurements of pK1* and pK2* were also made in NaMgCl solutions. These results have been used to evaluate β(O)(MgH2PO4)=-3.55±0.07,β(1)(MgH2PO4=-16.9±0.03, β(O)(MgH2PO4=-17.5±0.03 and β(1)(MgH2PO4)=27.4±0.8 at 25°C. The results for pK2* in NaMg-Cl solutions were also used to calculate log KMX*=3.2±0.1 for the formation of the ion pair MgHPO4o.

Original languageEnglish (US)
Pages (from-to)875-891
Number of pages17
JournalJournal of Solution Chemistry
Issue number9
StatePublished - Sep 1989


  • Phosphoric acid
  • Pitzer equations
  • ion pairing

ASJC Scopus subject areas

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


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