Spectrophotometric Titration of Cobalt(II) with CaCl2 in Mixed Solvents of 2-Propanol and Water for the Analysis of the Extraction Mechanism of Cobalt(II) by Salting-out in the Presence of CaCl2

The spectrophotometric titration of cobalt(II) with CaCl2 was carried out in mixed solvents of 2-propanol and water at different solvent compositions of 2-propannol, water and CaCl2 to analyze the salting-out extraction mechanism of Co(II) by the addition of CaCl2 from the mixed solvents. The formation constants of βCoCl42- = [CoCl42-][Co2+]-1[Cl-]-4 in both the organic and aqueous phases were determined thorough non-liner regression of the spectrophotometric titration data by a computer program SPECFIT/32TM. The values of log βCoCl42- in the aqueous phases were -4.26 ± 0.03, -4.03 ± 0.07, -3.83 ± 0.04, -3.69 ± 0.03 and -3.46 ± 0.01 at mole fractions of 2-propanol of 0.026, 0.023, 0.017, 0.014 and 0.012, respectively, and at [CaCl2]/mol dm-3 values of 3.555 (I = 10.6), 4.276 (I = 12.8), 4.916 (I = 14.7) and 5.444 (I = 16.3), respectively. The formation constants of [CoCl42-] in the organic phase were 5.70 ± 0.06, 5.44 ± 0.03, 5.36 ± 0.06, 5.10 ± 0.04 and 4.84 ± 0.05 at mole fractions of water of 0.431, 0.441, 0.444, 0.447 and 0.451, respectively, and at [CaCl2]/mol dm-3 of 0.941(I = 2.8), 0.943 (I = 2.8), 1.013 (I = 3.0), 1.090 (I = 3.3) and 1.165 (I = 3.5), respectively. These results suggest the formation of [CoCl42-] of 23 - 90% in the aqueous phase at the above mole fractions and the quantitative formation of [CoCl42-] in the organic phase. The extraction percentage of [CoCl42-] increased with an increase in [CaCl2]. The distribution constant, KD (= [CoCl42-]org/[CoCl42-]aq), however, decreased and became constant with [CaCl2]. The detailed extraction mechanism of Co(II) is discussed.