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 K_a for perchloric and sulfuric acids have been estimated. It has been shown that the acid dissociation constant K_a can be expressed as a product of two terms, K_a = K_a' × f_HA^∞, where K_a' is the apparent dissociation constant and f_HA^∞ is the infinite dilution activity coefficient of undissociated molecule of acid in liquid solution. The values of K_a' can be computed from readily available reference data. The limiting activity coefficients f_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  f_HA^∞ < -1.3. The ranges of values of the dissociation constants of HClO₄ and H₂SO₄ at 25 °C have been determined; log₁₀  K_a(HClO₄) = 10.8-12.3; log₁₀  K_a1(H₂SO₄) = 4.5-8.6.