log10Ka2

Log10 Ka2 is the common logarithm (base 10) of the second acid dissociation constant, Ka2, for a polyprotic acid. Ka2 quantifies the strength of the second deprotonation step, such as HA− ⇌ H+ + A2− in a diprotic acid H2A, or the corresponding step in triprotic systems that have Ka3 as well. In general, Ka1 characterizes the first proton loss, Ka2 the second, and Ka3 any further deprotonation.

The value log10 Ka2 is related to the second pKa by pKa2 = -log10 Ka2, or equivalently log10 Ka2 = -pKa2. This means the log10 Ka2 value is typically negative for acids with a positive pKa2. For example, phosphoric acid has Ka2 ≈ 6.2 × 10^-8, which yields log10 Ka2 ≈ -7.21 and pKa2 ≈ 7.21.

Applications of log10 Ka2 include speciation calculations in solutions with multiple protonation steps, interpretation of titration curves, and modeling using logarithmic parameters. Using log10 Ka2 can simplify comparisons of second-deprotonation strength across different acids and facilitate numerical stability in computational chemistry and biochemical software that operate on log-scaled constants.

Notes and caveats: Ka values are formally defined using activities rather than concentrations; at infinite dilution, activities approximate concentrations. Temperature, ionic strength, and the medium can affect Ka values, so log10 Ka2 is context-dependent and should be reported with conditions when used in calculations.