Nickel and cadmium phosphates in aqueous solution. Cation–anion complex formation and phosphate–H2O interactions

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Abstract
Two highly concentrated phosphate solutions have been investigated by an x‐ray scattering technique. The experimental correlation functions show that the main peaks for the nickel phosphate solution are at about 2.05, 2.70, 3.5, and 4.15 Å, and, those for the cadmium at about 2.35, 2.75, 3.55, and 4.3 Å. The presence of hydrated nickel and cadmium complexes in octahedral configuration in our solution is in agreement with the peaks found at 2.05 and 2.35 Å. Water–water interactions from the solvent give the main contributions to the peak at about 2.70 Å. The 3.5–3.6 Å peak in the solutions reveals the formation of inner cation–phosphate complexes of average form Z(H2O)6−x(OPO3H2)+2−x(Z=Ni, Cd), in which oxygens from phosphate groups substitute x water molecules of the hydrated Z(H2O)2+6 ions. Least squares refinements of the structure function i(s) are consistent with a structural unit in which about one phosphate tetrahedron shares a corner with one nickel or cadmium octahedron with Z–O–P angle of 136° and 130°, respectively. The large peak at about 4.2 Å in the correlation functions suggests a hydrogen bonded structure outside the inner coordination sphere of Ni2+ and Cd2+. Some indications have been obtained supporting the presence of hydration water around the phosphate groups.

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