# What is the role of buffer solution in complexometric titrations?

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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.

• Show 8 more events
• Student from Berlin, Germany requested an answer.
1 year ago ·
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11 months ago ·
• Student from India thankyou.
EDTA, often written as H₄Y, is a common ligand in complexometric titrations. It has four carboxyl groups and two amine groups that can act ...
5 months ago ·
• Chemically, what happens if the pH falls above or below ~10.3?
3 months ago ·
• As the pH falls below 10.3, the solution becomes more acidic. We get less of the Y⁴⁻ form and more of the HY³⁻ form of EDTA. The metal will still bind, but not as well. As the pH gets lower, so does the binding constant K_f of the metal. The rule of thumb is that we must have K_f > 10^8. The trick is to calculate the minimum pH that will make K_f > 10^8. Some metals have such an affinity for EDTA that they will bind even at pH 2.
3 months ago ·
• Student from Bishops Stortford, United Kingdom Thanks
3 months ago ·
• Where does the sodium come from?
EDTA is often used as the disodium salt, Na₂H₂Y. It reacts with many metal ions to form a complex:
4 weeks ago ·
• Although H₂Y²⁻ is the active ingredient in the buffer, you can't buy a bottle of H₂Y²⁻. It has to be paired up with a positive ion like Na⁺. The function of the Na⁺ is simply to balance the charges of the negative ions. In the buffer it acts merely as a spectator ion.
3 weeks ago ·

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