Hammett acidity function

From Wikipedia, the free encyclopedia

The usual measure used for superacids is the Hammett acidity function:

$H_{0} = \mbox{p}K_{BH^+} - \log \frac{[BH^+]}{[B]}$

where B is the base and BH⁺ its protonated form. pK_BH⁺ is −log(K) for the dissociation of BH⁺. By using bases with very negative pK_BH⁺ values, the H₀ scale may be extended to negative values. The H₀ scale becomes identical to the pH scale in dilute aqueous solution. On this scale, pure H₂SO₄ (10¹² M) has a H₀ value of −12, and for oleum, about H₀ = −15, etc. Take note that the Hammett acidity function clearly avoids water in its equation.