The low frequency modes of solvated ions and ion pairs in aqueous electrolyte solutions: iron(ii) and iron(iii) chloride

Abstract

We have investigated the hydration dynamics of solvated iron(II) and iron(III) chloride. For this, THz/FIR absorption spectra of acidified aqueous FeCl₂ and FeCl₃ solutions have been measured in a frequency range of 30–350 cm⁻¹ (≈1–10 THz). We observe a nonlinear concentration dependence of the absorption, which is attributed to the progressive formation of chloro-complexes of Fe(II) and Fe(III), respectively. By principal component analysis of the concentration dependent absorption spectra, we deduced the molar extinction spectra of the solvated species Fe²⁺ + 2Cl⁻ and FeCl⁺ + Cl⁻, as well as FeCl²⁺ + 2Cl⁻ and FeCl₂⁺ + Cl⁻. In addition, we obtain ion association constants log K_FeCl2 = −0.88(5) and log K_FeCl3 = −0.32(16) for the association of Fe²⁺ and Cl⁻ to FeCl⁺ and the association of FeCl²⁺ and Cl⁻ to FeCl₂⁺, respectively. We performed a simultaneous fit of all the effective extinction spectra and their differences, including our previous results of solvated manganese(II) and nickel(II) chlorides and bromides. Thereby we were able to assign absorption peaks to vibrational modes of ion–water complexes. Furthermore, we were able to estimate a minimum number of affected water molecules, ranging from ca. 7 in the case of FeCl⁺ + Cl⁻ to ca. 21 in the case of FeCl²⁺ + Cl⁻.