STANDARD ENTHALPY OF FORMATION OF AMMONIUM-NITRATE AND OF THE NITRATE ION

The standard enthalpy of formation of ammonium nitrate in its stable reference state at 298.15 K has been derived from measurements of the energy of combustion of samples of hydrocarbon oil and polyethene sheet in the presence of ammonium nitrate. Under the conditions chosen, the last substance decomposed to N2(g), O2(g), and H2O(l); the only significant by-product found was HNO3·nH2O. Special care was devoted to preparation of the NH4NO3 sample in its stable state (crystalline form IV), free of water. Combination of the resulting value of ΔtHo with recent data on the enthalpies of solution and dilution of NH4NO3(cr, IV), together with key values from the CODATA compilation, gave ΔtHo(NO3-, aq) = -(206.81 ± 0.41) kJ mol-1. © 1979.