PHOTODECOMPOSITION OF IRON(III) HYDROXO AND SULFATE COMPLEXES IN AQUEOUS-SOLUTION - WAVELENGTH DEPENDENCE OF OH AND SO4- QUANTUM YIELDS

Effective quantum yields for the production of OH radicals from iron(I II) hydroxo species in aqueous solution were measured in the wavelengt h range 280-370 nm at pH 2 and 3 under aerated conditions and at pH 3 in argon-saturated solutions of ferric perchlorate. 2-Propanol was use d as radical scavenger; the yield of product acetone was measured. Spe ctra of ferric perchlorate solutions were taken and evaluated to deter mine absorption coefficients at various pH values and to calculate K-4 , the first hydrolysis constant of Fe(III), at 46 wavelengths in an at tempt to obtain evidence for the presence of Fe(OH)(2)(+) in addition to FeOH2+ in the spectra. Whereas the influence of the dimer Fe-2(OH)( 2)(4+) was clearly evident, no indication for Fe(OH)(2)(+) was found. Values obtained for the first hydrolysis constant K-4, corrected to 25 degrees C, were (2.76 +/- 0.19) x 10(-3) M and (3.63 +/- 0.15) x 10(- 3) M at ionic strengths of 0.1 and 0.05, respectively, in excellent ag reement with literature values. These data served to derive absolute O H quantum yields for FeOH2+(H2O)(5), which were found to rise from 0.0 7 at 370 nm to 0.31 at 280 nm.,At wavelengths below 300 nm the OH prod uction from Fe3+(H2O)(6) contributed markedly with an estimated quantu m yield of approximate to 0.05. Quantum yields of SO4- from the FeSO4 complex were determined in solutions of ferric perchlorate, to which of sodium sulfate was added at pH 2. Benzene was used as scavenger, an d sufficient 2-methyl-2-propanol was added to quench OH radicals resul ting from iron(III) hydroxo species. Absolute SO4- quantum yields were found to rise from 1.6 x 10(-3) at 350 nm toward 7.9 x 10(-3) at 280 nm.