Sequential Injection Analysis Ferrozine Spectrophotometry Versus Luminol Chemiluminescence for Continuous Online Monitoring of the Concentration and Speciation of Iron

We designed two sequential injection analysis systems (Luminol chemiluminescent and Ferrozine spectrophotometric) that are capable of monitoring the concentration of soluble iron(II) and reducible iron(III) in natural waters over extended periods (>120 h). The method based on Luminol chemiluminescence estimated the iron(II) concentration as the difference in signal with or without the addition of the strong iron(II) ligand, Ferrozine (TM) (iron complexing reagent). Iron(II) in the sample without the iron complexing reagent induced a chemiluminescence signal whereas iron(II) in the presence of the iron complexing reagent did not induce a signal. The second method, based on Ferrozine (TM) spectrophotometry, measured iron(II) and reducible iron(III) in a 100 cm path length liquid wave capillary cell. Both methods provided stability for >120 h and yielded similar analytical detection limits equal to ten standard deviations of the blank; 2.2 nM for Luminol chemiluminescence with a calibration concentration range of 0 - 12 nM, and 3.9 nM for Ferrozine (TM) spectrophotometry with a calibration concentration range of 0 - 120 nM. The two chemical techniques do not appear to be directly comparable for practical monitoring purposes, and we suggest that Luminol-chemiluminescence is unable to measure strong ligand complexes of iron(II). Both methods used iron(II) as the analytical standard rather than a reduced iron(III) standard. The analysis based on the iron(II) standard was able to be performed without additional buffers and reducing reagents, which significantly minimized contamination.