Login processing...

Chapter 5: Complexometric Titration, Precipitation Titration, and Gravimetry Back To Chapter

5.6: EDTA: Auxiliary Complexing Reagents

TABLE OF
CONTENTS

JoVE Core
Analytical Chemistry

A subscription to JoVE is required to view this content.

Education

EDTA: Auxiliary Complexing Reagents

Previous Video Next Video

Previous Video
5.5: EDTA: Conditional Formation Constant

Next Video
5.7: EDTA: Direct, Back-, and Displacement Titration

Embed Languages Add to Favorites ADD TO PLAYLIST

TRANSCRIPT

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.

5.6: EDTA: Auxiliary Complexing Reagents

EDTA titrations are usually carried out in highly basic conditions, where the fully deprotonated form of EDTA, Y⁴⁻, actively complexes with the free metal ions in the solution. Several metal ions precipitate as hydrous oxide (hydroxides, oxides, or oxyhydroxides) under these conditions, lowering the concentration of free metal ions in the solution. For this reason, auxiliary complexing agents or ligands such as ammonia, tartrate, citrate, or triethanolamine are used in EDTA titrations to prevent unwanted precipitation. These ligands bind strongly to the metal ions to form metal−ligand complexes that are less stable than the metal−EDTA complexes. The addition of EDTA during titration displaces ligands from the metal−ligand complexes, forming more stable metal−EDTA complexes.

X

Simple Hit Counter