Aqueous ammonia and ammonium chloride hydrates: Principal infrared spectra

The infrared (IR) spectra of aqueous ammonia (NH3) and aqueous ammonium chloride (NH4Cl) were recorded by attenuated total reflectance to obtain their molecular organizations. Factor analysis (FA) of the spectra revealed two hydrates for each species: (NH3)2⋅H2O and NH3⋅3H2O; NH4+,Cl-2·H2O; and (NH4+,Cl-)·3H2O, respectively. The hydrate spectra and species abundances were obtained as a function of total concentrations. From this the equilibrium equation between the two ammonia hydrates was determined: 2[(NH3)2·H2O]+5(H2O)2⇌4[NH3·3H2O] with its equilibrium constant Kα = (2.3 ± 0.6) × 10−5 L3 mol−3. Similarly, for the two ammonium chloride hydrates the equation is 2[(NH4Cl)2·H2O]+5(H2O)2⇌4[NH4Cl·3H2O] with its equilibrium constant: Kβ = (4 ± 1) × 10−7 L3 mol−3. Band simulations of the hydrate spectra were compared to that of pure liquid water and parent molecules. For aqueous ammonium chloride solutions the water and all ammonium hydrate bands are slightly displaced from that of pure water and pure ammonium chloride, respectively. However, for ammonia hydrates the situation is different: compared to the gas situation the hydrate water bands have similar displacements as that of pure liquid water; the ammonia deformation bands are also little displaced but the stretching bands are strongly red shifted. These shifts, which are even greater than that in pure liquid water, are attributed to strong hydrogen bonding situations: water–H with N–ammonia and ammonia–H with O–water. This explains the high solubility of ammonia in water. The comparison between the spectra of aqueous ammonium chloride and ammonia hydrates indicates that ammonium ion is not present in aqueous ammonia from 11.3 M down to at least our detection limit of 3 mM NH3.