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EDTA Titrations

Chapter 13

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Metal-Chelate Complexes

Metal ions are Lewis acids, the

corresponding Lewis bases are electron

donating ligands.

Monodentate ligand: binds to a metal ion

through only one metal ion.

Multidentate or chelating ligand attaches

to a metal ion through more than one

atom.

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Chelate Effect

Chelating is the ability of multidentate

ligands to form more staple metal

complexes than those formed by

monodentate ligands.

These reactions happen over the

monodentate because of favored

thermodynamics.

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Thermodynamic favorable

The delta Hs for mono and multidentates

are generally comparable.

However, the delta S s (entropy) favors a

reaction with the multidentate.

G= H - TS

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Chelation in Medicine

Chelation therapy was first introduced inGermanyin the 1930s to combat heavymetal poisoning.

First used for good in the U.S.A. in the lateforties.

Utilized in medical treatments to help

remove lead from the systems of patients. EDTA is the chelation agent of choice.

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EDTA

Ethylediaminetetraacetic acid

Most widely used chelating agent

Forms 1:1 complexes Through titration or indirect reactions,

EDTA can quanitatively measure nearly all

elements.

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EDTA Structure

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EDTA Complexes

Equilibrium constant for the reaction of a metal

with a ligand is called the formation constant.

M+n + Y-4 MYn-4

Kf= (MYn-4)/(M+n)(Y-4)

Pb+2 + CaY-2 PbY- + Ca+2

K ~ 108

The Pb+2 ion replaces the Ca+2 ion because K ffor the lead complex is greater than the calcium

complex

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EDTA Titration CurveRegion 1

Excess Mn+

left after each additionof EDTA. Conc. of free metal

equal to conc. of unreacted Mn+.

Region 2Equivalence point:[Mn+] = [EDTA]

Some free Mn+ generated by

MYn-4 Mn+ + EDTA

Region 3

Excess EDTA. Virtually all metal

in MYn-4 form.

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Metal Ion Indicators

Compounds whose color changes when

they bind to a metal ion.

The color change singles the end point

Most indicators can only be used in a

certain pH range.

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EDTA Titration Techniques

Direct titration: analyte is titrated with

standard EDTA.

Back titration: a known excess of EDTA is

added to the analyte.

Displacement titration: For metal ions that

do not have a satisfactory indicator.

Indirect titration: Anions that precipitate

with certain metal ions.

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Masking: reagent that protects some

component of the analyte from reaction

with EDTA.

Demasking: releases metal ion from a

masking agent.

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Sources

I will never tell !

Textbook, Quantitative Chemical Analysis

http://www.chem.wits.ac.za/chem201/Complexation%20reactions.ppt#52