Analytical Chemiluminescence/Oxygen radicals
B10. Oxygen radicals
Modest chemiluminescence occurs when solutions of iron(II) ions or titanium(III) ions are added to carbonate buffer at alkaline pH, the intensity increasing with the metal ion concentration. This occurs even in solutions that have been deaerated with nitrogen. Surprisingly, the chemiluminescence of deaerated solutions sometimes exceeds that observed in oxygenated solutions. If luminol is also present the intensity of the chemiluminescence is increased (by a factor of about 100 for 1 x 10-5 mol dm-3 luminol), even though the only oxidant present is dissolved oxygen. The presence of the fluorophore rhodamine B also increases the chemiluminescence intensity, but the enhanced chemiluminescence is always more intense in oxygenated solutions. It is possible that other metal ions of low oxidation number, having reducing properties, will also induce this effect. Cobalt(II) ions or copper(II) ions have been shown to give rise to chemiluminescence when added to alkaline solutions of luminol with no added oxidant.
The phenomenon can be rationalized in terms of the well-established chemistry of single electron oxidation of iron(II) in solution.
(B10.1) Fe2+ + O2 → Fe3+ + O2•―
(B10.2) Fe2+ + O2•― + H+ → Fe3+ + HO2―; followed by HO2― + H+ → H2O2
(B10.3) Fe2+ + H2O2 → Fe3+ + HO• + HO―
(B10.4) Fe2+ + HO• → Fe3+ + HO―
The oxygen radicals so produced are the effective chemiluminescence reagent. Radicals can recombine to generate products in excited states, which emit light. The surprising result that chemiluminescence is more intense when the solutions are de-aerated may be due to the more rapid oxidation of iron(II) in oxygenated solutions, leading to initially high concentrations of radicals which fall rapidly as they are converted to hydroxyl ions, so that transient high chemiluminescence would occur too soon to be detected in the flow system used. Luminol chemiluminescence initiated by iron(II) is no doubt due to primary oxidation by hydroxyl radicals (alone or in association with Fe2+), followed by secondary oxidation by superoxide. The light emission occurring when reductants are added to an alkaline solution of luminol and potassium ferricyanide is a special case of this reaction.
The iron(II)-luminol reaction has been applied to the determination of iron(II) in water under natural conditions at nanomolar and micromolar concentrations. It is claimed to be a better assay than ultraviolet/visible spectrophotometry, titrimetry or polarography, having the advantages of high sensitivity, extreme rapidity and simplicity of operation, low cost and avoiding pre-treatment of the sample. It distinguishes iron(II) from iron(III) and can be adapted to measure total iron. Titanium(III)-luminol chemiluminescence has been applied to the determination of titanium(IV) which was converted to titanium(III) by on-line reduction. Fenton’s reagent, a mixture of aqueous iron(II) ions and hydrogen peroxide, has been used to promote chemiluminescence by oxidation. An example is the determination of amines and amino-acids after derivatization to Schiff bases. A selective determination of adrenaline has also been reported.
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