Raman study of ferric chloride solutions and hydrated melts

Abstract

A Raman study of ferric chloride hexadydrate (FCHH) in 11.5M HCl solutions has shown the presence of tetrahedral ${\mathrm{FeCl}}_4^{-}$ ion as the most prominent species. An appreciable concentration of a polymeric species, which is composed of higher chloroferrate groups of the type FeCl₅ · H₂O, and a small concentration of lower chloro‐aquoferrate complex ion of the type ${\mathrm{Fe(H}}_2{\mathrm{O)}}_4{\mathrm{Cl}}_2^{+}$ are also present in the solutions. The saturation of FCHH‐11.5M HCl solution with dry HCl is observed to convert both the polymeric species and the lower chloro‐aquoferrate ions into ${\mathrm{FeCl}}_4^{-}$ ions. In hydrated ferric chloride melts the most prominent species exists in the form of tetrahedral ${\mathrm{FeCl}}_4^{-}$ ion along with an appreciable concentration of ${\mathrm{Fe(H}}_2{\mathrm{O)}}_4{\mathrm{Cl}}_2^{+}$ ion. Hydrated ferric chloride melts saturated with LiCl or dry HCl gas, contains only ${\mathrm{FeCl}}_4^{-}$ ions. In the case of moderately acidic (≤ 6M HCl) ferric chloride solutions Raman evidence has revealed the presence of a polymeric species along with significant concentrations of lower chloro‐aquoferrate complexes. For neutral ferric chloride solutions evidence indicates that the most prominent species exists in the form of a polymeric species with structural characteristics similar to those observed in moderately acidic solutions. It has been proposed that these polymeric species are composed of randomly oriented higher chloroferrate groups of the type FeCl₅ · H₂O and FeCl₆. Higher stability of the polymeric species in the neutral solutions has been attributed to the presence of a strong hydration layer around the polymeric species.