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

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) (approximate to 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-.