Journal of Solution Chemistry

, Volume 41, Issue 6, pp 994–1007 | Cite as

Correlation of the Extraction Constant Values of Zn2+ and Cd2+ from Phosphoric Acid Media by Bis(2,4,4-trimethylpentyl)dithiophosphinic Acid Dissolved in Toluene

  • A. Ocio
  • M. P. Elizalde
  • J. L. Aparicio


Values of the extraction constants of Zn2+ and Cd2+ from aqueous phosphoric acid solutions (0.36 to 7.31 mol⋅L−1) by Cyanex 301 in toluene, involving formation of the complexes ZnR2 and CdR2 with R being bis(2,4,4-trimethylpentyl)dithiophosphinate, have been correlated at T=298 K as a function of the ionic strength. For this purpose the activity coefficients of all of the aqueous species have been calculated taking into account both the protolytic equilibria of concentrated phosphoric acid and complexation reactions between the cations and the phosphoric acid species. Good correlations have been obtained for the extraction constant values with the ionic strength, provided the release of water molecules during the extraction processes is considered. Finally, extraction constant values are reported at infinite dilution.


Phosphoric acid Activity coefficients Cyanex 301 Zinc Cadmium Bis(2,4,4-trimethylpentyl)dithiophosphinic acid 

List of Symbols


Bis(2,4,4-trimethylpentyl)dithiophosphinic acid


Cd or Zn


Stoichiometric equilibrium constant for the formation of the complex M(H2PO4)+


Infinite dilution equilibrium constant for the formation of the complex M(H2PO4)+


Stoichiometric equilibrium constant for the formation of the complex M(H2PO4)2


Infinite dilution equilibrium constant for the formation of the complex M(H2PO4)2


Conditional equilibrium constant for the extraction of MR2 from phosphoric acid solutions


Stoichiometric equilibrium constant for the extraction of MR2 from phosphoric acid solutions


Infinite dilution equilibrium constant for the extraction of MR2 from phosphoric acid solutions


Activity coefficient (molal scale)


Side reaction coefficient of cadmium or zinc


Average number of water molecules


Water activity


Ionic strength (mol⋅kg−1)


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© Springer Science+Business Media, LLC 2012

Authors and Affiliations

  1. 1.Department of Analytical Chemistry, Faculty of Science and TechnologyUniversity of the Basque CountryBilbaoSpain

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