The spectrophotometric determination of micromolar concentrations of Co²⁺ using o-phenanthroline

Co²⁺ and o-phenanthroline formed a 1:3 complex with absorption maxima at 346, 332, 313, and 301 nm. The complex obeyed Beer's Law at the first three maxima. Standard curves constructed by monitoring the E₃₄₆ at different concentrations of Co²⁺ had a maximum sensitivity of about 1 μm Co²⁺. At this concentration of Co²⁺ the ΔE₃₄₆ was 0.003 absorption units. This assay was not affected greatly by Ca²⁺, Mg²⁺, K⁺, Na⁺, or Tris. Erbium ions (Er³⁺) produced a small, nonspecific increase in absorbance at all wavelengths. Zn²⁺ also formed a complex with o-phenanthroline with maxima at 343, 328, and 313 nm. The absorbance of the Zn²⁺-o-phenanthroline complex was about 10% that of the Co²⁺-o-phenanthroline complex at 346 nm, but was still sufficient to cause interference at Zn²⁺ concentrations above 10 μm.