Hydration of ferric chloride and nitrate in aqueous solutions: water-mediated ion pairing revealed by Raman spectroscopy.

Iron is the most abundant transition metal in the earth's crust and is important for the proper functioning of many technological and natural processes. Despite the importance, a complete microscopic understanding of the hydration of ferric ions and water mediated ion pairing has not been realized. Hydrated Fe(iii) is difficult to study due to the process of complexation to the anion and hydrolysis of the hydrating water molecules leading to a heterogeneous solution with diverse speciation. Here, ferric chloride and nitrate aqueous solutions are studied using polarized Raman spectroscopy as a function of concentration and referenced to their respective sodium salt or mineral acid. Perturbed water spectra (PWS) were generated using multivariate curve resolution-alternating least squares (MCR-ALS) to show the residual spectral response uniquely attributable to the hydration of ferric speciation. The hydrogen bonding network associated with the hydrating water molecules in ferric chloride solutions are found to be more similar to hydrochloric acid solutions, whereas in ferric nitrate solutions, the network behaves more similar to sodium nitrate, despite increased acidity. Thus, in the FeNO3 and FeCl3 solutions, ion pairing and coordination, respectively, are significantly influencing the hydration spectra signature. These results further reveal concentration dependent changes to the hydrogen bonding network, hydrating water symmetry, and changes to the relative abundance of solvent shared ion pairs that are governed primarily by the ferric salt identity.