Ferric ion (Fe(III)) coordination in concentrated aqueous electrolyte solutions

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Abstract
The first-order isotopic difference method of neutron diffraction has been used to study the interatomic coordination of the ferric ion in three concentrated ( approximately 2 M) heavy water solutions, each prepared under different conditions with differing degrees of acidity: (i) 1.52 M Fe(NO3)3; (ii) 1.72 M Fe(NO3)3; and (iii) 2 M Fe(ClO4)3. In all solutions, a well defined hydration structure was obtained, with nearest neighbour Fe3+. . .O and Fe3+. . .D correlations at 2.01(2) and 2.68(3) AA respectively. A variety of coordination numbers were calculated: in the 1.52 M nitrate solution a complete six-fold hydration shell was found, whereas for the 1.72 M nitrate solution, which was heated prior to preparation, significant hydrolysis was observed. By contrast the Fe3+ ion in the perchlorate solution showed six-fold coordination but with significant hydrolysis.