Solvent extraction of chromium(III) perchlorate, thiocyanate, nitrate, bromide, and iodide with trioctylphosphine oxide in hexane

Chromium(III) ion was extracted from aqueous 1 moldm(-3) NaX (X- = ClO4-, SCN-, NO3-, Br-, and I-) solutions with trioctylphosphine oxide (TOPO) in hexane at 298 K. The extraction was rapidly equilibrated, except from a NaNO3 solution. The number of TOPO molecules solvating to the extracted chromium(III) perchlorate and thiocyanate was 6. That to the bromide and iodide was 5. That to the nitrate was 5 at the beginning of extraction, and changed to 3 within a few hours of two-phase agitation. The hydration number of extracted Cr(ClO4)(3). 6TOPO was 17.2+/-0.5. The different solvation number of TOPO among these salts is supposed to be an unstable second hydration sphere of the extracted chromium(III) ion, which may be easily removed by the counter anions.