Structure and Population of Complex Ionic Species in FeCl 2 Aqueous Solution by X-ray Absorption Spectroscopy.

Technologies for mass production require cheap and abundant materials such as ferrous chloride (FeCl 2 ). The literature survey shows the lack of experimental studies to validate theoretical conclusions related to the population of ionic Fe-species in the aqueous FeCl 2 solution. Here, we present an in situ X-ray absorption study of the structure of the ionic species in the FeCl 2 aqueous solution at different concentrations (1-4 molL -1 ) and temperatures (25-80 °C). We found that at low temperature and low FeCl 2 concentration, the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33 (±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å. The structure of the ionic complex gradually changes with an increase in temperature and/or concentration. The apical water molecule is substituted by a chlorine ion to yield a neutral Fe[Cl 2 (H 2 O) 4 ] 0 . The observed substitutional mechanism is facilitated by the presence of the intramolecular hydrogen bonds as well as entropic reasons. The transition from the single charged Fe[Cl(H 2 O) 5 ] + to the neutral Fe[Cl 2 (H 2 O) 4 ] 0 causes a significant drop in the solution conductivity, which well correlates with the existing conductivity models.