Acid−Base Chemistry on Crystalline Ice: HCl + NH₃

Abstract

The chemical interaction of (i) HCl, (ii) NH3, and (iii) HCl + NH3 with crystalline ice has been examined as a function of temperature (T = 80−150 K) utilizing FTIR spectroscopy. The infrared spectra reveal the formation of ionic hydrates for HCl and molecular hydrates for NH3. The acid−base reaction leads to the formation of the ammonium ion, NH4+. While it is produced to a limited extent between 80 and 140 K, NH4+ is a dominant species above 140 K. All species appear as lower amorphous hydrates for T = 80−140 K. Dramatic changes in the spectra occur above 140 K as higher crystalline hydrates are formed. This temperature coincides with the amorphous to crystalline phase transformation of ice. This change appears to be due to a greater number of water molecules on the surface that are available for hydration and crystallization.