Thermodynamics of acidic dissociation of ammonium ion in concentrated aqueous solutions of ammonium nitrate

Based on pH-metric measurements, the equilibrium constants for ammonia protonation at 298.15 K and ionic strengths I = 5–9, supported by ammonium nitrate, were calculated. Using direct calorimetric measurements, the enthalpies of this process were determined at 288.15, 298.15 and 303.15 K and I = 5–9 and its full thermodynamic parameters were calculated. The sharp increase in exothermicity and decrease in entropy of this process observed at high ionic strengths are explained by decrease in the contribution of desolvation of reagents in the region of deficiency of solvent molecules and the influence of association of ions of background electrolyte. The obtained thermodynamic characteristics of ammonia protonation clearly indicate that a change in solvation state of reaction participants at high ionic strengths changes the nature of this reaction.