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

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.

Chelate Effect

• 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

Metal ion

Monodentate

Stability

Metal ion

Bidentate

Stability

Metal ion

Tridentate

Stability

Metal ion

Tetradentate

Stability

What forms of EDTA exist?

At Low pH? Which Functional Groups will be protonated?

EDTA is a Hexaprotic acid (H6Y2+ )

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

WHY????

pK1 = 0.0 ,pK2 = 1.5 ,pK3 = 2.0pK4 = 2.66 ,pK5 = 6.16 ,pK6 = 10.24

EDTA-4

[Fe(EDTA)(H2O]

Let’s take a look at the plot for EDTA

EDTA

[MY(n-4)+]KMY = --------------

[M][Y-4]

[Y-4]4 = ---------

CT

where

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

M+n + Y-4 ¾ MY (n-4)+

Minimum pH for Effective

EDTA Titrations

Metal Ion Indicators

Erichrome Black T

MgIn + EDTA MgEDTA + In

(red) (colorless) (blue)