Dissociation constants and the Henderson-Hasselbalch equation Different definitions of dissociation constants
Acids For an acid with a dissociation costant pKa (or written as pKa,a): Neutral acid + H2O = Ionized conjugate base + H3O+
Given Ka = Ka,a = [Ionized conjugate base] * [H3O+] / [Neutral acid], we have Fraction of neutral base = 1/(1+10^(pH – pKa)), or Fraction of neutral base = 1/(1+10^(pH – pKa,a))
Bases For a base with a dissociation constant pKb, its conjugate acid has a dissociation constant pKa,b (pKb + pKa,b = 14 for 25 °C): Neutral base + H2O = Ionized conjugate acid + OH- Or, Ionized conjugate acid + H2O = Neutral base + H3O+
Given Kb = [Ionized conjugate acid] * [OH-] / [Neutral base], and Ka,b = [Neutral base] * [H3O+] / [Ionized conjugate acid], we have Fraction of neutral base = 1/(1+10^(pH – pKb)), or Fraction of neutral base = 1/(1+10^(pKa,b – pH))