X-ray Diffraction Study of the Coordination of Fe(III) in a Highly Hydrolyzed Solution of Iron(III) Chloride

The hydrolysis of iron (III) has been largely investigated because of the intrinsic interest and the technological importance of this subject, and detailed results have been reported in some reviews [1, 2]. One of the less clear points in this matter is the relationship between the hydrolysis products and the solid phases obtained from iron (III) solutions. It has been observed [1] that, in some conditions, the addition of a base to Fe (III) solutions gives rise to rapid formation of precipitates while, in different situations, hydrolysis phenomena result in the growth of polymers from which solid phases are produced very slowly. According to Spiro et al. [3], "precipitation occurs most readily via low molecular weight species and thus polymerization and precipitation are competing processes". The hypothesis has been advanced [3] that structural differences between polymers and precipitates are responsible for this competition effect; as a consequence structural information on the polymeric species is badly needed. Further interest for these problems comes from the observation that some Fe (III) hydrolyzed products are analogs of natural compounds of biological importance, like the ferritin, so that they have been used as model compounds in investigations on these natural substances. To date, however, only two studies [4, 5] have been performed by X-ray diffraction on solutions containing polymeric species obtained from alkalized iron (III) nitrate solutions. These studies agree as far as the kind of short and medium range order is concerned (that is the succession of peaks appearing in the radial distribution function calculated from experimental data). But a