Solute structuring in aqueous iron(III) sulphate solutions. Evidence for the formation of iron(III) –sulphate complexes

Abstract

Ferric sulphate solutions of high concentration have been investigated by an x-ray scattering technique. The experimental distribution curves show the main peaks at about 1.5, 2.0, 2.4, 2.8, 3.3, and 4.2 Å. The presence in solution of sulphate groups in tetrahedral configuration and of hydrated iron complexes in octahedral configuration is in agreement with the peaks found at 1.5, 2.0, and 2.4 Å. Water–water interactions from the solvent give the main contributions to the 2.8 Å peak. The 3.3 Å peak reveals the formation of inner iron–sulphate complexes of average form Fe(H2O)6−n(OSO3)+3−2nn, in which oxygens from sulphate groups substitute n water molecules of the hydrated Fe(H2O)3+6 ions. Least squares refinements of the i (s) curves are consistent with a structural unit in which about one sulphate tetrahedron shares a corner with one iron octahedron with an Fe–O–S angle of 135 deg. The same structural unit found for the iron–sulphate complexes in solution can be identified as the building unit of the crystalline structures of several iron–sulphate solid compounds. The large peak at about 4.2 Å in the distribution curves strongly suggests a hydrogen bonded structure outside the inner coordination sphere of Fe3+. The introduction into the model of (H2O)I– (H2O)II interactions between water molecules pertaining to two subsequent hydration shells of Fe3+ resulted in a marked better agreement between experimental and calculated i (s) curves. Some indications have been obtained supporting the presence of hydration water around the sulphate groups.