title: 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.
abstract: 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 betaCoCl4(2-) = [CoCl4(2-)][Co2+](-1)[Cl-](-4) in both the organic and aqueous phases were determined thorough non-linear regression of the spectrophotometric titration data by a computer program SPECFIT/32. The values of log betaCoCl4(2-) 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 [CoCl4(2-)] 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 [CoCl4(2-)] of 23-90% in the aqueous phase at the above mole fractions and the quantitative formation of [CoCl4(2-)] in the organic phase. The extraction percentage of [CoCl4(2-)] increased with an increase in [CaCl2]. The distribution constant, KD (= [CoCl4(2-)]org/[CoCl4(2-)]aq), however, decreased and became constant with [CaCl2]. The detailed extraction mechanism of Co(II) is discussed.
authors.affiliation: Department of Chemistry, Faculty of Science and Engineering, Saga University, Japan.
authors.forename: Nguyen Huu
authors.initials: NH
authors.lastname: Chung
authors.forename: Ying Guang
authors.initials: YG
authors.lastname: Wu
authors.forename: Masaaki
authors.initials: M
authors.lastname: Tabata
mesh_terms: D019840:2-Propanol / Q000737:chemistry*; D000465:Algorithms; D002122:Calcium Chloride / Q000737:chemistry*; D002413:Cations, Divalent / Q000032:analysis / Q000737:chemistry; D002712:Chlorides / Q000737:chemistry; D003035:Cobalt / Q000032:analysis* / Q000737:chemistry / Q000302:isolation & purification; D012997:Solvents / Q000737:chemistry*; D013053:Spectrophotometry; D017186:Titrimetry / Q000379:methods; D014867:Water / Q000737:chemistry*
chemical_list: D002413:Cations, Divalent; D002712:Chlorides; D012997:Solvents; D014867:Water; D003035:Cobalt; D002122:Calcium Chloride; D019840:2-Propanol