Flow injection spectrophotometric determination of ng ml−1 levels of cobalt(II) using the photochemical decomposition of a cadmium(II)-water-soluble porphyrin complex

Cadmium(II)-5,10,15,20-tetrakis(4-sulfophenyl)porphine complex (CdL) (Soret band: maximum wavelength (λmax) = 432 nm, molar absorptivity (ϵ) = 4.8 × 105 l mol−1 cm−1) was found to be uniquely decomposed by irradiation with blue light around 432 nm. However, the cobalt complex of the same ligand (λmax = 432 nm, ϵ = 2.8 × 105 l mol−1 cm−1) was stable to visible light. A highly sensitive flow injection system for cobalt(II) ions was developed by the use of these photochemical properties which allows an elimination of the absorbance blank of the reagent CdL in the spectrophotometric detection of Co(II) following the exchange reaction: CdL + Co2+→ Co(III)L + Cd2+. The calibration graph for cobalt(II) was linear in the range 1 × 10−7–1 × 10−6 mol l−1. The apparent molar absorptivity of the cobalt complex obtained from the calibration slope was 1.2 × 104 l mol−1 cm−1. The relative standard deviation was 0.6% (10 determinations) for 6.8 × 10−7 mol l−1 cobalt. The detection limit (SN = 3) was 3.45 × 10−8 mol l−1 (2 ng ml−1).