Thermodynamics of the system HCl + SmCl3 + H2O from 5 to 55°C. Application of Harned's rule and the Pitzer formalism

Rabindra N. Roy, Lakshmi N. Roy, Darin R. Gregory, Andria J. VanLanduyt, Denis Pierrot, Frank J. Millero

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


A comprehensive array of electrochemical cell measurements for the system HCl + SmCl3 + H2O was made from 5 to 55°C using a cell without liquid junction of the type: Pt; H2(g, 1 atm)|HCl (mA) + SmCl3 (mB)|AgCl, Ag (A) The present study, unlike previous studies of trivalent ions, are not complicated by hydrolysis reactions. Measurements of the emf were performed for solutions at constant total ionic strengths of 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mol-kg-1. The mean activity coefficients of HCl (γHCl) in the mixtures were calculated using the Nernst equation. All the experimental emf measurements (about 850) were first treated in terms of the simpler Harned's rule. Harned interaction coefficients (αAB and βAB) were calculated. The linear form of Harned's rule is valid for most ionic strengths, but quadratic terms are needed at I = 1.5 and 3 mol-kg-1. The Pitzer model was used to evaluate the activity coefficients using literature values, β(0), β(1), and Cφ, for HCl from 0 to 50°C and 25°C for SmCl3. The effect of temperature on the parameters for SmCl3 has been estimated using enthalpy and heat-capacity data. The mixing parameter ΘH.Sm was determined at 25°C. The addition of the ψH.Sm.Cl coefficient did not improve the fit significantly and no temperature dependence was found to be significant. The value of ΘH,Sm = 0.2 ± 0.01 represented the values of γHCl with a standard deviation of σ = 0.009 over the entire range of temperatures and ionic strength. The use of higher-order electrostatic effects (EΘH,Sm, EΘH.Sm) was included as it gave a better fit of the activity coefficients of HCl.

Original languageEnglish (US)
Pages (from-to)1211-1227
Number of pages17
JournalJournal of Solution Chemistry
Issue number12
StatePublished - Dec 2000
Externally publishedYes

ASJC Scopus subject areas

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


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