XANES analysis of Fe valence in iron gall inks

In this study, we investigate the feasibility and reliability of Fe K-edge XANES spectroscopy as a tool in determination of Fe2+/Fe3+ ratio in historic inks. We have measured Fe K-edge XANES spectra of several historic and model iron gall inks with different Fe2+/Fe3+ ratios, together with some standard reference Fe2+ and Fe3+ compounds with known local atomic structure and symmetry around Fe atoms. We examine different approaches to determine relative amount of Fe2+ in iron gall inks from the Fe K-edge shifts, and demonstrate that a proper choice of the Fe reference compounds with similar symmetry, same type of neighbour atoms in nearest coordination shells, arranged in a similar local structure is crucial for the absolute calibration of the Fe K-edge shift and consequently a reliable determination of Fe2+/Fe3+ ratio in the sample. Best results, with the accuracy of +/- 2%, are obtained by a linear combination fit with XANES spectrum of FeSO4 . 7H(2)O as a reference for Fe2+ and spectrum of an iron gall ink containing predominantly Fe3+. Copyright (c) 2007 John Wiley & Sons, Ltd.