Experiment Overview

Complexometry is a titration technique based on the formation of stable coordination complexes between metal ions and chelating agents (strong ligands). The most commonly used complexing agent is EDTA (ethylenediaminetetraacetic acid).

What This Experiment Demonstrates

• Formation of metal–ligand coordination complexes
• Use of metallochromic indicators
• Quantitative determination of metal ion concentration

Apparatus Required

• Burette
• Pipette
• Conical flask
• Volumetric flask
• White tile

Chemicals Required

• EDTA solution
• Metal ion solution (e.g. calcium or magnesium ions)
• Buffer solution (pH ≈ 10)
• Eriochrome Black T or suitable metallochromic indicator
• Distilled water

Theory

EDTA is a hexadentate ligand that forms highly stable 1:1 complexes with most metal ions. During the titration, EDTA binds to the free metal ions present in the solution.

The general complex formation reaction can be represented as:

\[\ce{M^{n+} + EDTA^{4-} -> [M(EDTA)]^{(n-4)-}}\]

A buffer solution is used to maintain a constant pH, ensuring that EDTA remains in its active deprotonated form and that the metal–EDTA complex is stable.

Metallochromic indicators such as Eriochrome Black T (EBT) form weak complexes with metal ions. When being added, EDTA displaces EBT from the metal ion. At the endpoint, there is practically no metal-EBT complext left, so its color disappears. Depending on the specifics, after the end point, we either see the color of the M-EDTA complex, or that of the free indicator.

Method

1. Rinse and fill the burette with the EDTA solution.
2. Pipette a known volume of the metal ion solution into a conical flask.
3. Add the buffer solution to maintain the required pH.
4. Add a few drops of the metallochromic indicator.
5. Titrate with EDTA until the endpoint color change is observed.
6. Record the final burette reading.

Observations

A distinct color change occurs at the endpoint when all metal ions have formed complexes with EDTA, indicating completion of the reaction.

Calculations

• Use the 1:1 stoichiometry of metal ion to EDTA.
• Calculate the moles of EDTA used.
• Determine the concentration of the metal ion solution.

Result

The concentration of the metal ions in the solution is determined using the volume of EDTA required to reach the endpoint.

Determining water hardness

The main ingridients in all soaps are sodium and potasium salts of fatty acids. When there's too much calcium or magnesium in a water sample, calcium and magnesium soaps precipitate, making the water unable to foam up as much due to the soap being added. The presence of calcium and magnesium is called water hardness.

Both Ca and Mg form a wine red complex with EBT. After adding EDTA, the release of blue free EBT leads to a visible color change. This allows water hardness to be easily determined by complexometry.

Water hardness is a very important topic in environmental science, and finding ways to measure and reduce it is of much interest to environmental chemists.

Video Demonstration

Video courtesy of Apoptosis (YouTube). Video used for educational purposes under YouTube’s embedding policy.