5.6: EDTA: Auxiliary Complexing Reagents

EDTA titrations require high-pH conditions, under which the active complexing form of EDTA predominates.

However, under alkaline conditions, many metal ions form hydrous oxide precipitates, which decreases the free metal ion concentration in the solution.

To prevent precipitation and retain the metal ions in solution, auxiliary complexing agents are often used.

For example, when zinc is titrated with EDTA in an ammonia buffer, ammonia acts as an auxiliary complexing agent and binds with zinc ions, forming several stable zinc–ammine complexes.

During titration, EDTA displaces the ammonia ligands to form a more stable zinc–EDTA complex.

The conditional formation constant for the generated zinc–EDTA complex accounts for only the uncomplexed zinc ions.

So, because the solution also contains complexed zinc ions in the form of different zinc–ammine species, the fraction that is uncomplexed zinc ions must be determined.

Rearranging the equation and substituting it into the conditional-formation-constant expression gives a new conditional formation constant that is specific to a fixed concentration of the auxiliary complexing agent and fixed pH.