Optical absorption spectra of Fe2+ and Fe3+ in aqueous solutions and hydrated crystals

Absorption spectra of aqueous solutions containing Fe2+ and Fe3+ ions and hydrated crystals are studied up to 50000 cm(-1). Charge-transfer transitions for both hydrated ions are here measured for the first time. The weak Fe2+ spin-forbidden transitions which up to now have received little attention in diluted systems, are put in evidence: as a consequence, we have proposed for the lower energy transitions different attributions respect to the commonly accepted ones. It is further shown that the step by step oxidation process of Fe2+ to Fe3+, starting from Fe(BF4)(2), solution, allows us to quantitatively correlate the oscillator strength of the various spin-allowed, spin-forbidden and charge-transfer transitions of both ions. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.