Cobalt-nickel separation:: The extraction of cobalt(II) and nickel(II) with bis(2-ethylhexyl) phosphinic acid (PIA-8) in toluene

The extraction behaviour of cobalt(II) and nickel(II) from sulfate solutions with bis(2-ethylhexyl) phosphinic acid (PIA-8) in toluene has been studied. Quantitative extraction of Co(II) was observed at pH 5.0-5.9 while that of Ni(II) at pH 6.8-7.0 with 0.03 M PIA-8 in toluene. The difference in pH(0.5) for Co(II) and Ni(II) was 1.9. The stoichiometry of the extracted species were determined by slope analysis method. The extraction reaction proceeds by cation exchange mechanism and the extracted species were Co . R-2(HR)(2) and Ni . R-2 . 2(HR)(2). Temperature dependance of the extraction equilibrium constants were determined to estimate the apparent thermodynamic functions (AG, AS and All). The method was used for separation of cobalt(II) and nickel(II). Cobalt(II) was separated from nickel even at 1:20 (Co:Ni) ratio. The separation of cobalt(II) from nickel(II) was favoured with the increase in temperature.