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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