What is the role of buffer solution in complexometric titrations?

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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).

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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

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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, #pK_(a4)# = 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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