The hydration of divalent cations in aqueous solution : an x-ray investigation with isomorphous replacement

The x-ray diffraction patterns of M solns. of the nitrates of Co2+, Ni2 +, Zn2+, Mg2+, Cd2+, and Ca2+ were recorded with an Enraf-Nonius liq . jet diffraction camera. These cations were paired into 4 isomorphous groups based on the similarities of the cation-water distances in their cryst. hydrates. A theoretical expression was derived relating the electron distribution function to the difference in the intensities diffracted by a pair of isomorphous solns., and this function was obtained by calcg. the Fourier transform, using the measured intensities. The resulting electron distribution function exhibited a peak near 2 g, ascribed to the primary hydration layer, and a 2nd, less clear-cut, peak near 4 .ANG., attributed to the secondary hydration layer. Least-squares fits of these peaks yielded primary coordination nos. and cation water distances at 25°, and a secondary coordination no. of 12 ± 2. Cation-water distances were also obtained when minor deviations from isomorphism were taken into account