Acid strength of tetrafluoroboric acid The hydronium ion as a superacid and the inapplicability of water as an indicator of acid strength

Abstract

The Hammett acidity function, H ₀ , of 50.7% to 79.2% aqueous fluoroboric acid has been determined by ¹³ C NMR spectroscopy (the Δδ ⁰ method) with mesityl oxide as indicator. At all concentrations, the H ₀ values are more negative than those of sulfuric or trifluoromethanesulfonic acid solutions, increasingly so at higher concentrations. The acid strengths measured represent the acidities of various clusters of hydronium ions and water, ion-paired with fluoroborate anions; in such systems the acid strength is also dependent upon the anion. At 79.2% concentration (0.28 mol excess water per mol hydronium ions) the solution is as strong as, or slightly stronger than, pure sulfuric acid, meaning that mixtures with less excess of water are superacidic. The hydronium ion (as fluoroborate) being a stronger acid than 100% sulfuric acid means that water dissolved in hydronium fluoroborate is a weaker base than the hydrogen sulfate ion in sulfuric acid. At the same concentration (wt.%), ‘fluoroboric acid’ is stronger than perchloric acid, but the two are equally strong when concentration is measured in molar ratios water:acid, meaning that perchloric acid is also fully converted in concentrated solutions to hydronium perchlorate. In acetic acid solution, hydronium perchlorate and fluoroborate are again equally strong, as expected, but they are significantly weaker than phosphotungstic acid, even though the hydron-donating species should be H ₃ O ⁺ ·xH ₂ O, in all cases. The acid strength of H ₃ O ⁺ is thus strongly dependent upon the nature of the anion, which means that the basicity of water is strongly dependent upon the acid. Water is, therefore, not useful as a probe base for estimating acid strengths.