text: For reactions in aqueous solution, such as an acid dissociation reaction 
  :AH + HO  A + HO 
  the concentration of water may be taken as being  constant and the formation of the hydronium ion is implicit. 
  :AH  A + H
  Water concentration is omitted from expressions defining equilibrium constants, except when solutions are very concentrated.
  : (K defined as a dissociation constant)
  Similar considerations apply to  metal ion hydrolysis reactions.