THE HYDRATION STRUCTURE OF CR3+ IN A CONCENTRATED AQUEOUS-SOLUTION

Neutron diffraction experiments were carried out on aqueous solutions of 2.2M chromium (III) perchlorate in water, heavy water and a 'null' mixture of water and heavy water. The isotopic difference methods of neutron diffraction were applied to the scattering data, and the results were used to obtain structural information concerning the Cr3+ hydration. The first hydration shell of Cr3+ contains six water molecules with oxygen and hydrogen atoms situated at 1.98 (2) and 2.60 (3) angstrom, respectively, consistent with a trigonal pyramidal coordination mode for the water molecules. With a coordinated water molecule geometry appropriate to the crystal structures a mean tilt angle of 34 +/- 6-degrees is calculated. A second-order difference was calculated from the results of the water and heavy water solutions, and g(CrH)(r) was determined. The result for the 'null mixture', which is predominantly g(CrO)(r) is compared with the appropriate combination of the results for the water and heavy water solutions, and in this way it is demonstrated that the Cr3+ ion does not show preferential hydration in perchlorate solutions. There is also clear evidence for orientational correlations in the second hydration sphere with broad peaks centred at 4 and 4.5 angstrom which are assigned to Cr-O and Cr-D correlations respectively.