A cation is usually present as a single principal species, which may be a complex ion, free ion, or precipitate. If the reaction goes to completion the principal species is a complex ion. The precipitate is the principal species if most of the precipitate remains undissolved. If a cation forms a stable complex, addition of a complexing agent at 1 M or greater generally will convert the free ion to complex ion.
The dissociation constant Kd can be used to determine the extent to which a cation is converted to a complex ion. The solubility product constant Ksp can be used to determine the fraction of cation remaining in a solution after precipitation. Kd and Ksp are both required to calculate the equilibrium constant for dissolving a precipitate in a complexing agent.
Complexes of Cations with NH3 and OH-
|Cation||NH3 Complex||OH- Complex|