What is the role of buffer solution in complexometric titrations?

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Ernest Z. Edit Share
Jun 29, 2014

The buffer adjusts the pH to ensure that the reaction goes to completion.

A complexometric titration uses the formation of a coloured complex to indicate the endpoint.

EDTA, often written as H₄Y, is a common ligand in complexometric titrations. It has four carboxyl groups and two amine groups that can act as electron pair donors (Lewis bases).

EDTA is often used as the disodium salt, Na₂H₂Y. It reacts with many metal ions to form a complex:

M²⁺ + H₂Y²⁻ ⇌ MY²⁻ + 2H⁺

The complex has the structure

Carrying out the reaction in a basic buffer solution removes H⁺ as it is formed. This moves the position of equilibrium to the right and favours formation of the complex (Le Châtelier's Principle).

Also, for EDTA, $p {K}_{a 4}$ = 10.26. Thus, if the solution is buffered to about pH 10.3, most of the EDTA will exist as Y⁴⁻ ions.

The metal ions will not have to remove the hydrogen ions from H₂Y²⁻. They can react directly according to the equation

M²⁺ + Y⁴⁻ → MY²⁻

Above pH 10.3, most metal ions react quantitatively with EDTA.

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