Chemical state analysis of iron(III) in perchloric, nitric, sulfuric and hydrochloric acid solutions by means of Mossbauer spectrometry

Abstract

The chemical state of ferric ion has been studied through the analysis of Mosbauer spectrum. The aqueous 0.5 N iron(III) solution was placed and rapidly frozen in an acrylate regin cell cooled by dry ice, and the Mossbauer spectrum of the frozen solution was taken at 78°K.
In 8 N perchloric acid solution, iron(III) ion shows an weak and broad peak, which was assumed to be octahedral hexaaquo iron(III) ion; Fe(H₂O)₆³⁺. This spectrum appears until the pH of solution reaches 0.3. At pH 0.35, doublet peak with Q. S. 0.67 mm/sec was obtained, which is assigned to the dihydroxo-bridging dimer (H₂O)₄Fe(OH)₂Fe(H₂O)₄⁴⁺.
Mossbauer spectrum obtained in 6 N nitric acid solution is weak and broad, suggesting the existence of very weak coordination between iron(III) and nitrate ions. Around pH 1.0, the change of coordination was thought to occur, because the overlapping two doublet peaks appear. The dihydroxo-bridging species, (H₂O)₄Fe(OH)₂Fe(H₂O)₄⁴⁺, and the doublet peak with larger Q. S. (1.70 mm/sec) was attributed to either the oxo-bridging species, (H₂O)₄Fe-O-Fe(H₂O)₄⁴⁺, or other hydrolysis products.
As the pH of the solution increases, the outer doublet peak tends to disappear and inner doublet peak prevails, indicating that the iron(III) ion changes from oxo-bridging species (or hydrolysis products) to the dihydroxo-bridging species.
In hydrochloric acid media, fairly complicated spectra are obtained. The spectrum in 6 N hydrochloric acid solution shows a singlet peak of isomer shift 0.70 mm/sec (compared with sodium nitroprusside). At pH 1.20, clear doublet peak with Q. S. 0.35 mm/sec is observed. This peak is considered to result by the superposition of complicated singlet peak of chloro-complex and doublet peak of hydroxo-bridgeing species (Q. S. 0.67 mm/sec), which prevails over pH 1.33.
The spectrum obtained for sulfuric acid solution assured the existence of strong coordination between iron(III) and sulfate ions. This assumption is also confirmed by the evidence that the appearance of doublet peak due to dihydroxo-bridging species shifted to high pH, i.e., pH 2.4.
The effect of ligand concentration on the retardation of ferric hydroxide formation is also investigated in an iron(III)-water-chloride system, and the hindrance of precipitation is attributed to the equilibrium chloro-complexes formation.