Chapter 13 “EDTA” Titrations It’s a Complex World Out There.
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Transcript of Chapter 13 “EDTA” Titrations It’s a Complex World Out There.
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Chapter 13
“EDTA” TitrationsIt’s a Complex World Out There
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Terms 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
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Terms 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.
Chelate - Having chelæ or prehensile claws. Most Transition metals bind six ligand atoms.
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Examples of Ligands and complexes.
Cyanide Ethylenediamine EDTA (ethylenediaminetetraacetic acid)
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Example of Complexing ligands in nature and Medicine Nonactin - an ionophore
Ferrioxamine B
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EDTA Acid Base Properties 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
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EDTA The fraction in that form is:
][][][][][][][][
4322345
26
4
4
YYHYHYHYHYHYHY
Y
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Fractional Form Hn +++ K Format
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Conditional Formation Constants
]][[][ 4
'4
EDTAMMYKK n
n
fYf
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Conditional Formation Constants 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+
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Most Metal Ions will form Hydroxides Auxiliary Complexing agents are used.
Ammonia for zinc for example.
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Indicators Same function as with acid base
indicators but with excess EDTA you remove a metal from the indicator thus changing its color.
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Techniques 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
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Techniques Masking
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.