Determination of Formation and Partition Constants of Ion Associates in Extraction of Cationic Cobalt(III) Complexes with Pyridylazo Compounds

The extractions of cationic cobalt(III) complexes with four kinds of pyridylazo compounds using alcohol or chloroform were carried out. The formation and partition constants of ion associates formed with the cationic cobalt(III) complex and a counter anion (ClO4-, SCN-, Br-, Cl-) were determined by an analysis of the extraction equilibria. By using 1-nonanol instead of 1-octanol as the organic solvent, the formation constants of ion associates increased (Δ log Kf ? 0.4), while their partition constants decreased (Δ log KD ? -0.3) in all present systems. Moreover, only in the 4-methyl-2-(2-pyridylazo)phenol (PAC) system, the formation constant for the chloride ion system (log Kf = 1.7) was observed to be greater than that for the bromide ion system (log Kf = 1.0). The formation constants of ion associates were found to be mostly dependent on the bulkiness and the polarity of the pyridylazo compounds. These results provide fundamental information on the retention behaviours of cationic complexes in the RP-HPLC