Simultaneous Spectrophotometric Measurement of Iron(II) and Iron(III) in Atmospheric Water

A new analytical method employing di-2-pyridyl ketone benzoylhydrazone (DPKBH) as a colorimetric chelating agent for the simultaneous determination of iron(II) and iron(III) in cloudwater has been developed. A spectrophotometric detection limit of 4 nM for both Fe(III) and Fe(II) with a linear response from 4 nM to 0.1 μ was established for samples extracted with CHCl₃-H₂O. DPKBH chelation without CHCl₃ extraction showed a linear response from 0.1 to 30 μM. The molar extinction coefficients of the Fe(Il)-bis(DPKBH) (E₁) and Fe(III)-bis(DPKBH) (E₂) complexes are and E₂ = 3.6 X 10⁴ L mol^-1 cm^-1 at 370 nm and e₂ = 1.1 X 10⁴ L mol^-1 cm^-1 at 660 nm. Analytical interference studies on the possible changes in the oxidation state of iron with S(IV), oxalate, and other potential electron donors have also been carried out. This analytical method has been used to determine iron(II) and iron(III) simultaneously in cloudwater samples collected within the Los Angeles basin airshed. The concentration of Fe(II) varied from 0.3 to 5 μM, and the concentration of Fe(III) varied from 0.6 to 1.4 μM during several stratus cloud events.