Dissociation Constants of Perchloric and Sulfuric Acids in Aqueous Solution

The experimental dissociation constants of strong acids are notoriously ill-defined, and it is necessary to rely on theoretical methods for their evaluation. We present a methodology for the theoretical evaluation of the dissociation constants, and the values of <i>K</i>_a for perchloric and sulfuric acids have been estimated. It has been shown that the acid dissociation constant <i>K</i>_a can be expressed as a product of two terms, <i>K</i>_a = <i>K</i>_a′ × <i>f</i>_HA^∞, where <i>K</i>_a′ is the apparent dissociation constant and <i>f</i>_HA^∞ is the infinite dilution activity coefficient of undissociated molecule of acid in liquid solution. The values of <i>K</i>_a′ can be computed from readily available reference data. The limiting activity coefficients <i>f</i>_HA^∞ for strong acids can be determined by theoretical methods only. The following estimate for the limiting activity coefficients of perchloric and sulfuric acids has been obtained, −2.5 < log₁₀ <i>f</i>_HA^∞ < −1.3. The ranges of values of the dissociation constants of HClO₄ and H₂SO₄ at 25 °C have been determined; log₁₀<i> K</i>_a(HClO₄) = 10.8–12.3; log₁₀<i> K</i>_a1(H₂SO₄) = 4.5–8.6