THERMODYNAMIC EQUILIBRIA OF THE EXTRACTION OF CHROMIUM(VI) WITH TRI-N-OCTYLPHOSPHINE OXIDE FROM AQUEOUS SOLUTIONS

ABSTRACT

The distribution of chromium( VI ) between aqueous solutions and the kerosene solutions of tri-n-octyl-phosphine oxide (TOPO) have been studied at 25 °C and pH 2-4, in which range the predominant forms of Cr(VI) are HCrO₄ and Cr₂O₇ ²⁻. The extraction equilibrium is influenced by the total chromium(VI) concentration, aqueous pH value, and total extractant concentration. The thermodynamic equilibria among all species in the aqueous phase were used for the analysis of distribution data, and the activity coefficients of ionic species were estimated according to Debye-Hiickel or Bromley equation. By numerical method, the complexes of Cr(VI ) with TOPO formed in the organic phase were found to be H₂CrO₄ (TOPO) and H₂Cr₂O₇ (TOPO) ₃. The corresponding extraction constants were 2019 and 3.69× 10⁸, respectively.

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Ting-Chia Huang

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