EDTA Tit Rations Analytical Chem II

8/3/2019 EDTA Tit Rations Analytical Chem II

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EDTA Titrations

Chapter 13


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Metal-Chelate Complexes

Metal ions are Lewis acids, thecorresponding Lewis bases are electrondonating ligands.

Monodentate ligand: binds to a metal ionthrough only one metal ion.

Multidentate or chelating ligand attachesto a metal ion through more than oneatom.


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Chelate Effect

Chelating is the ability of multidentateligands to form more staple metalcomplexes than those formed by

monodentate ligands.

These reactions happen over themonodentate 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 metalwith 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 calciumcomplex


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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 whenthey bind to a metal ion.

The color change singles the end point

Most indicators can only be used in acertain pH range.


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EDTA Titration Techniques

Direct titration: analyte is titrated withstandard EDTA.

Back titration: a known excess of EDTA isadded to the analyte.

Displacement titration: For metal ions thatdo not have a satisfactory indicator.

Indirect titration: Anions that precipitatewith certain metal ions.


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Masking: reagent that protects somecomponent of the analyte from reactionwith EDTA.

Demasking: releases metal ion from amasking 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

EDTA Tit Rations Analytical Chem II

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