Chapter 13 EDTA Titrations
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Chapter 13 EDTA Titrations. Lewis acid-base concept. Lewis acid : electron pair acceptor metal Lewis base : electron pair donor ligand coordinate covalent bond ligand donates both electrons of the electron pair bond. coordinate covalent bond.

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Chapter 13 EDTA Titrations

Lewis acid-base concept

Lewis acid : electron pair acceptor

metal

Lewis base : electron pair donor

ligand

coordinate covalent bond

ligand donates both electrons of the electron pair bond



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Species with two or more nuclei that are good Lewis bases are potential chelate ligands. The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H.D.K. Drew, who stated: "The adjective chelate, derived from the great claw or chela (Greek: chely) of the lobster or other crustaceans, is suggested for the caliper-like groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings." This term is generic for all ligands which have at least two atoms coordinated to the central metal ion.


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Chelate Effect are potential chelate ligands.

  • the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands

  • results from the formation of 5-membered "ring" with metal and two atoms on the ligand


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Metal ion are potential chelate ligands.

Monodentate

Stability 


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Metal ion are potential chelate ligands.

Bidentate

Stability 


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Metal ion are potential chelate ligands.

Tridentate

Stability


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Metal ion are potential chelate ligands.

Tetradentate

Stability


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What forms of EDTA exist? are potential chelate ligands.

At Low pH? Which Functional Groups will be protonated?


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EDTA is a Hexaprotic acid (H are potential chelate ligands. 6Y2+ )

pK1 = 0.0 ,pK2 = 1.5 ,pK3 = 2.0

pK4 = 2.66 ,pK5 = 6.16 ,pK6 = 10.24

The amount of ethylenediaminetetraacetate anion, EDTA-4 => Y-4 in solution is very important.

WHY????


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EDTA are potential chelate ligands. -4


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[Fe(EDTA)(H are potential chelate ligands. 2O]


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Let’s take a look at the are potential chelate ligands. a plot for EDTA


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EDTA are potential chelate ligands.

M+n + Y-4¾ MY (n-4)+

[MY(n-4)+]

KMY = --------------

[M][Y-4]

[Y-4]

a4 = ---------

CT

where

CT = [Y-4] + [HY-3] + [H2Y-2] + [H3Y-1] + [H4Y]


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Minimum pH for Effective EDTA Titrations are potential chelate ligands.


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Metal Ion Indicators are potential chelate ligands.

Erichrome Black T

MgIn + EDTA  MgEDTA + In

(red) (colorless) (blue)


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