Edta Titrations

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  • Topic: Ligand, Chelation, Titration
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Chapter 13: EDTA titrations
Complexation Reaction: A reaction between two species having a well-defined stoichiometry. The resulting bond is not permanent from a covalent standpoint. Complex: The resulting structure formed during a complexation reaction. Coordination Center: Metal ion in a complex (Lewis acid) Ligand: The species that complexes the metal center. A single species can form one or more bonds with a single coordination center (Lewis base) Coordination Number: Number of ligand bonds formed around the coordination center. Chelate: Ligands that form multiple bonds (multidentate; bi, tri, tetra, penta)

Ethylenediaminetetraacetic acid EDTA CO 2

CO2NH+ CO2-

NH+ CO2-

Ethylenediamine

NH2+

NH2+

• EDTA is the most commonly used chelating agents as it can form complexes with a wide range of metals. • The ability of EDTA to complex is dependent on its form. The most desirable state is the Y4- form. • As the pH increases, more EDTA becomes Y4-. [Y 4- ] = [H 6 Y 2+ ] + [H5Y + ] + [H 4 Y] + [H 3 Y - ] + [H 2 Y 2- ] + [H Y 3- ] + [Y 4- ] a Y 4[Y 4- ] = [EDTA ]

a Y 4-

Equil. Concentrations

• The formation constant for metal-EDTA complexes is:
M n + + Y 4- = MY n - 4 [MY n - 4 ] Kf = [M n + ][Y 4- ]

• It is important to note the requirement for the charge state of EDTA. Leads to a conditional (effective) formation constant

[MY ] K 'f = a Y 4 - K f = n+ [M ][EDTA ]

n -4

Equilibrium Concentrations
• Again we must consider equilibrium reactions and concentrations in analysis using complexation. • Equilibrium constants are referred to as formation constants, Kf. • For simple complexes (1:1) we can make some similar assumptions and generate similar equations as we did for monoprotic acids • For more complex systems we must deal with step-wise formations and step-wise formation constants.

EDTA titrations
1. Before the equivalence point there is excess M in solution At the equivalence point, treated as dissolving pure MY...
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