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

5.4: EDTA: Chemistry and Properties

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EDTA: Chemistry and Properties

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In complexation reactions, although ligands of various denticities can be used, polydentate ligands are generally preferred, as they form highly stable metal–ligand complexes due to the chelate effect.

EDTA, a well-known chelator, contains six potential coordinating sites with lone pairs that can chelate with metal ions to form metal–EDTA complexes.

These complexes are cage-like structures with geometries that differ depending on the size of the metal ions.

Neutral EDTA is a tetraprotic acid, although, at very acidic pH, the last two carboxyl groups undergo protonation to give a hexaprotic system.

This species undergoes successive dissociation into various forms, and their relative amounts vary with the pH of the solution.

At very low pH, hexa-, penta-, and tetraprotic species predominate. As the pH increases, the tri-, di- and monoprotic species gain predominance.

Finally, above pH 10, the most dominant form is the fully unprotonated species, which is the active form of EDTA and the most important complexing species.

5.4: EDTA: Chemistry and Properties

Polydentate ligands are most widely used in complexometric titrations because they form more stable complexes with the metal ions than mono- or bidentate ligands due to the chelate effect. Examples of polydentate ligands are ethylenediaminetetraacetic acid (EDTA), crown ethers, and cryptands. The most important feature of optimal polydentate ligands is the ability to form 1:1 complexes in a single-step process. Amino carboxylic acid derivatives are frequently used as complexing agents. EDTA is among the most versatile ligands, and it is widely used as a chelating agent in analytical chemistry because most elements can be measured with EDTA through different titration methods.

EDTA is a hexadentate ligand consisting of six complexing groups: four carboxylate oxygens and two amine nitrogens. They coordinate with the metal ion by sharing the lone pairs with the metal. EDTA forms stable cage-like structures with most metal ions and is usually represented with octahedral geometry. However, the geometry of the different complexes depends on the size of the metal ions.

EDTA is a neutral tetraprotic acid, represented by the abbreviation H₄Y. (The disodium salt of EDTA is the preferred laboratory reagent due to its higher solubility than the parent acid.) EDTA dissociates into various species, and their relative amounts depend on the pH of the solution. At low pH, the amine nitrogens are protonated, making EDTA a hexaprotic system (H₆Y²⁺). The fully deprotonated form (Y⁴⁻) exists at high pH and can generate hexadentate complexes with metal ions. This is considered the 'active' form of EDTA.

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