Structure of a 50 mol kg–1 aqueous solution of ammonium nitrate at 373 K by the isotopic difference method of neutron diffraction

Abstract

A neutron diffraction study of a 50 mol kg^–1 aqueous solution of deuterated ammonium nitrate [ca. 1 : 1 molecular ratio of D₂O : N(1)D₄N(2)O₃ which is the saturation solubility at ca. 70 °C] was carried out at 100 °C. The first- and second-order isotopic difference methods of neutron diffraction were applied to the nitrogen nuclei of both the ammonium and nitrate ions. The results derived from the total and first-order difference diffraction functions show the existence of stable ND⁺₄ and NO^–₃ ions in the solution. Structural information concerning the ammonium ion–water and nitrate ion–water coordination was obtained from the first-order differences, and the results indicate a relatively weak hydration shell for these ions.

A calculation of the second order difference function enabled the determination of the ‘cross’ pair radial distribution function g_N(1)N(2)(r) from which it was concluded that the nearest-neighbour N(1)–N(2) distance increases significantly on the addition of D₂O to ND₄NO₃.