Download Chapter 12 (Complexometric Titration)

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 5membered "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+ )
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????
EDTA-4
[Fe(EDTA)(H2O]
Let’s take a look at the a plot for
EDTA
EDTA
M+n + Y-4 ¾ 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]
Minimum pH
for Effective
EDTA
Titrations
Metal Ion Indicators
Erichrome Black T
MgIn + EDTA  MgEDTA + In
(red)
(colorless) (blue)