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Chapter 13
“EDTA” Titrations
It’s a Complex World Out There
Terms
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Ligand - Each of the atoms or groups
attached to the central (usually the metal)
atom of a co-ordination complex. (OED)
Lewis Acids. Electron pair acceptors (Metal
Ions)
Lewis Bases. Electron pair donators – the
ligands
Terms
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Monodentate – one atom per ligand
molecule.
Multidentate – more than one ligand atom
per ligand molecule. (Chelating ligand)
Applied to a group (ligand) that loops round a central metal ion to be attached
at two or more points, and also to the co-ordination compound so formed.
Hence as n., a chelate compound.
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Chelate - Having chelæ or prehensile claws.
Most Transition metals bind six ligand atoms.
Examples of Ligands and complexes.
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Cyanide
Ethylenediamine
EDTA (ethylenediaminetetraacetic acid)
Example of Complexing ligands in nature
and Medicine
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Nonactin - an ionophore
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Ferrioxamine B
EDTA Acid Base Properties
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EDTA is hexaprotic.
pKa’s are 0.0, 1.5, 2.0, 2.66, 6.16 and 10.24
First four are for carboxylate protons
The neutral compound is tetraprotic.
A common salt is Na2H2Y.2H2O
The Y4- form is the one that binds
EDTA
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
Y4

The fraction in that form is:
[Y 4 ]

[ H 6Y 2 ]  [ H 5Y  ]  [ H 4Y ]  [ H 3Y  ]  [ H 2Y 2 ]  [ H Y 3 ]  [Y 4 ]
Fractional Form
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Hn +++ PK
Format
Conditional Formation Constants
n4
[ MY ]
K  Y 4 K f 
n
[ M ][ EDTA]
'
f
Conditional Formation Constants
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Since these are a function of pH the control
of pH allow us to determine the strength of
the metal ligand complex.
This can be seen in the EDTA titration of Ca2+
Most Metal Ions will form Hydroxides
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Auxiliary Complexing agents are used.
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Ammonia for zinc for example.
Indicators
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Same function as with acid base indicators
but with excess EDTA you remove a metal
from the indicator thus changing its color.
Techniques
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Direct Titration
Back Titration (Al ion with excess EDTA) Titrate
excess EDTA with Zn
Displacement Titration – Metal ion is place in a
solution with excess CaEDTA. The Ca displaced is
then titrated with standard EDTA.
Indirect Titration – SO4 _- Add Excess Ba – filter and
boil solid with excess EDTA. Amount of excess EDTA
determined by titration with Mg
Techniques
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Masking
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Other ligands are added and block an interfering
metal ion from reacting with the EDTA
These systems can get very complex but and
work quite well - Follow cook book
procedures. You would want to validate you
procedure.