Thermodynamic equilibria of the extraction of chromium(VI) with tri-n-octylphosphine oxide from aqueous solutions

The distribution of chromium(VI) between aqueous solutions and the kerosene solutions of tri-n-octyl-phosphine oxide (TOPO) have been studied at 25 degrees C and pH 2-4, in which range the predominant forms of Cr(VI) are HCrO4- and Cr2O72-. 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-Huckel 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 X 10(8), respectively.