X-RAY AND NEUTRON-DIFFRACTION STUDIES OF THE HYDRATION OF SEO42- AND CRO42- ANIONS BY ISOMORPHIC SUBSTITUTION

The structure in aqueous solutions of the hydrated selenate and chromated ions SeO42- and CrO42- have been derived from X-ray and neutron diffraction measurements using CrO42- as an isomorphous substitute for SeO42-. The Se(Cr)-O bond lengths are 1.63 +/- 0.02 angstrom and do not differ from those found in crystal structure. A well defined hydration shell of about 12 water molecules surrounds the XO42- ions at a Se(Cr)-H2O distance of 3.96 angstrom. The diffraction results indicate a geometrical orientation of the water molecule around the tetrahedral anions very similar to that one found in the case of simple anions such as Cl-, with Se(Cr)-D distances of 3.25 and 4.4 angstrom and with a Se(Cr)-O distance of 3.96 angstrom.