The hydration of divalent cations in aqueous solution. An X-ray investigation with isomorphous replacement

With the method of isomorphous replacement direct information about the hydration of cations has been obtained. The study comprises two groups of isomorphous aqueous solutions, viz. one group composed of cadmium and calcium nitrate solutions and another with solutions of the nitrates of zinc, nickel, cobalt and magnesium. The divalent cations appear to have a clear-cut shell of 6 neighbouring water molecules. The cation-water distance is in close agreement with the distances mentioned in the literature for the unit M(H₂O)₆²⁺. in crystalline substances. A second hydration shell of about 12 molecules of water can be discerned at a distance of about 4.2 Å of the cation.