Analytical Chemiluminescence/Sulfites and persulfates

B11. Sulfites and persulfates
Sulfite is a well-known reductant. Oxidation of aqueous sulfur dioxide by acidified permanganate, cerium(IV) or hydrogen peroxide is feebly chemiluminescent; exploitation of the weak chemiluminescence improved the detectivity of atmospheric sulfur dioxide by a factor of 50. A proposed mechanism comprised an initial oxidation of HSO3― to S2O62―, which then disproportionates to SO42― and excited SO2, which emits visible light. Sulfites undergo an addition reaction with carbonyl compounds and addition of cyclohexanone to protect sulfite solutions against atmospheric oxidation led to the observation that this, at appropriate concentrations, enhanced the oxidative chemiluminescence. Light emission is also sensitized by other cyclohexyl compounds. Paulls and Townshend have suggested that the enhancement depends on β-sultine formation and have shown that the phenomenon occurs generally with higher cycloalkyl compounds, the optimum ring size being nine.

Fused cycloalkane rings also enhance the oxidative chemiluminescence of sulfites and this has given rise to a number of assays for steroids. Thus, a range of corticosteroid drugs have been determined by enhancing the chemiluminescence of sulfite oxidized by cerium(IV). Steroid hormones enhance the chemiluminescence of sulfite oxidized by bromate or by cerium(IV) and an assay based on this effect has been reported. In addition, bile acids sensitize the light emission accompanying the oxidation of sulfites by a variety of oxidants (Ce4+, MnO4―, BrO3― or Cr2O72― and these reactions have been applied analytically.

There is evidence that the chemiluminescence of the permanganate-sulfite reaction has the same emitter as any other permanganate oxidation and the red emission from this persists in the presence of fluorophores as a major contributor to total light output. The cerium(IV)-sulfite reaction does not have any effect on the chemiluminescence spectrum in the presence of fluorophores. The spectra emitted by bromate and dichromate oxidations have not been studied. It is therefore still possible that the chemiluminescence reactions with sulfite might have the mechanism described above, leading to emission from excited sulfur dioxide. There have been persistent reports of emission from the permanganate-sulfite reaction at lower wavelength than can satisfactorily be ascribed to manganese(II) phosphorescence – the usual mechanism – but these can be explained at least partly by the use of spectroscopic data that has not been corrected for the variation in sensitivity of the detector at different wavelengths.

Whereas sulfites promote chemiluminescence due to their reducing properties, persulfates act as oxidizing agents in chemiluminescent reactions. These do not have sulfur in a higher oxidation state than normal sulfates; rather, they contain peroxide units, where two catenated oxygen atoms take the places of two separate oxygen atoms, one in each of the two linked sulfate groups; these oxygen atoms are in oxidation state −I. Chemiluminescence has been reported from persulfates, both by electrochemical reduction at magnesium, silver or platinum electrodes and by thermal decomposition at the surface of magnesium. The light-emitting species in each case are reported to be oxygen radical ions, O•―, and excited peroxide ions, O22―, arising respectively by deprotonation of hydroxyl radicals, HO•, or of hydrogen peroxide or hydroperoxide radicals, HO2•. Persulfates are also used as oxidants in luminol chemiluminescence and as ancillary oxidants in ruthenium chemiluminescence, where they generate the oxidant [Ru(bipy)3]3+ (see eqn. B9.1).