What is the dissociation constant (Ka)?

  • In solution, acids tend to dissociate to form an H+ ion and a negative ion of the conjugate base (HA →  H+ + A–)
    • Reaction is freely reversible
    • At equilibrium, a constant ratio is reached between dissociated (A–) and undissociated (HA) particles
  • The dissociation constant (Ka) represents the ratio of both sides at equilibrium given fixed analytical conditions:
    • Expresses how easily acid releases a hydrogen ion (its strength as an acid)
    • Is dependent upon the molecular structure of the molecule
    • Can be determined by the following formula: 

K_{a}=\frac{[A^{-}][H^{+}]}{[HA]}

[H+] is the concentration of hydrogen ions
[A-] is the concentration of anions or conjugate base
[HA] is the concentration of undissociated molecules