Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

NW2A/2005G234

Structural Study of Iron(III) Complexes with Bromide Ion in Solution
Yasuhiro NIWA1*, Yasuhiro INADA1, Masaharu NOMURA1, Masao KIMURA2
1KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
2
Adv. Tech. Res. Lab., Nippon Steel Corp., Futtsu, Chiba, 293-8511, Japan
Introduction
Stainless steels remain essentially passive, but theytend to corrode at specific areas and form deep pits inenvironments containing appreciable concentrations of Clor Br . To elucidate the mechanism, it is important tounderstand the reactions between Fe, which is one of themain species in stainless steels, and water including
anions. The complexation equilibrium and the structure of
the component species are important to understand thereactivity of metal ions. In this study, the complexation
reactions of Fe(III) with bromide ion have been
investigated in aqueous and aqueous acetone solutions
using XAFS technique.
Experiments
Sample solutions were prepared by dissolving iron(III)perchlorate hydrate and lithium bromide monohydrate
into distilled water or aqueous acetone. The molar ratios
3+of [Br ]/[Fe ] for samples were 0 ~ 13.5 for aqueous
solution and 0 ~ 4.0 for aqueous acetone solution,respectively. XAFS spectra around the Fe K-edge weremeasured by the transmission mode at the NW2A
beamline of PF-AR using an Si(111) double-crystal
monochromator.
Results and Discussion
Radial structure functions of samples are shown in
Figure 1, without the correction for the phase shift. The
peaks around 150–160 pm in Fig. 1 (a) and (b) are
assigned to the interactions between the Fe(III) ion andthe oxygen atom of water molecules in the first hydration
1stsphere: Fe-O(H2O ). The interactions corresponding tothe second hydration sphere are observed as broad peaks
around 350 pm in Fig. 1 (a). With the increase in the3+molar ratio of [Br ]/[Fe ], new peaks appear around210–220 pm in Fig. 1 (a) and (b), and peak intensities for
1stFe-O(H2O ) are decreased. This tendency is more clear inaqueous acetone solution than in aqueous solution. These
new peaks can be assigned to the Fe-Br interaction. The
different results in two solutions indicate that the2+[FeBr(H2O)5] complex is formed in aqueous solution andthat the higher order complexes with more bromide ionsare also formed in aqueous acetone solution. This isconsidered to be mainly due to the lower dielectric
property of acetone than water and the stronger solvationof bromide ion with water than acetone.
3+The Fe-O bond length of [Fe(H2O)6] in aqueous
solution was obviously shorter than that in aqueous
acetone solution, suggesting that the coordinating water

40

Users' Report

Figure1. Radial structure functions for various molar
ratios of [Br-]/[Fe3+] in aqueous solution (a) and inaqueous acetone solution (b).
molecules to the Fe(III) ion are polarized by the watermolecules in the second hydration sphere due to thehydrogen bonding interactions. Furthermore, it is foundthat the peak around 150–160 pm is shifted to longer
3+
distance with the increase in the molar ratio of [Br ]/[Fe ]in aqueous solution. The Fe-O bond length (204 pm) in2+[FeBr(H2O)5] is found to be longer than that (201 pm) in3+
[Fe(H2O)6] in aqueous solution according to the curve
fitting analysis. The elongation is accounted by theelectron donation from coordinating bromide ions toFe(III) center, resulting in charge neutralization.
Authors have been investigating the corrosion
mechanism with dispersive XAFS (DXAFS) instrument
at NW2A. This study is expected to reveal dynamics ofthe corrosion mechanism.
*yasuhiro.niwa@kek.jp