Determination of trace amounts of cobalt using 1-(5-bromo-2-pyridylazo)-2-naphthol-6-sulfonic acid-Cu(II)-Co(II) competitive replacement complexation.

The chromophore 1-(5-bromo-2-pyridylazo)-2-naphthol-6-sulfonic acid (BPANS) has been used to sensitively complex Cu(II) and Co(II) in aqueous solution at pH 9.43. The spectral correction technique was applied to characterize the complexes, and results showed that both Cu(BPANS)2 and Co(BPANS)2 were formed. Trace Co2+ replaced the Cu(II) in the Cu-BPANS complex, so the competitive replacement complexation (CRC) was used to improve the analytical selectivity. A novel light-absorption ratio variation approach (LARVA) was described as one of the most sensitive methods. The combination of CRC and LARVA was applied to the quantitative detection of Co(II) at the ng/mL level. The limit of detection was only 2.0 ng/mL Co, and the analysis of water samples demonstrated satisfactory results.