Relative proton affinities of hydrogen sulfide and water

The difference in proton affinities for H₂S and H₂O has been measured in a photoionization mass spectrometer by studying the following equilibrium ${\mathrm{H}}_3{\mathrm{S}}_{(\mathrm{g})}^{+}+{\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{g})}\rightleftarrows {\mathrm{H}}_3{\mathrm{O}}_{(\mathrm{g})}^{+}+{\mathrm{H}}_2{\mathrm{S}}_{(\mathrm{g})}$ . Mixtures of H₂S and H₂O are photolyzed at 10.63 eV in the source of the mass spectrometer. H₂S⁺, which is the only ion produced, reacts with H₂S to produce H₃S⁺. H₃S⁺ reacts with water to produce H₃O⁺ which further hydrates to $\mathrm{H}({\mathrm{H}}_2\mathrm{O}{)}_n^{+}$ . The equilibrium constant is measured as a function of temperature and a van't Hoff plot produces ΔH^o=2.9 kcal/mole ΔS^o=5.5 eV. Thus the proton affinity of H₂S is 3 kcal/mole greater than the proton affinity of H₂O. Species such as ${\mathrm{H}}_3\mathrm{S}({\mathrm{H}}_2\mathrm{S}{)}_n^{+}$ and ${\mathrm{H}}_3\mathrm{S}({\mathrm{H}}_2\mathrm{O}{)}_n^{+}$ are not observed while $\mathrm{H}({\mathrm{H}}_2\mathrm{O}{)}_n^{+}$ hydrates are highly favored. A possible explanation is offered which involves the weakness of hydrogen bonds to sulfur.