213 PHC
8th lecture
(1)
Gary D. Christian, Analytical
Chemistry,
6th edition
1
•
•
•
Define complexometric reactions.
Differentiate between uni- and multidentate complexing agents.
Understand the mechanism of complex
formation and the effect of pH on it.
2
It is a reaction involves
the formation of a
substance called
complex which is slightly
ionising in solution.
3
Complex Formation
 Many
metal ions form slightly dissociated
complexes with various complexing agents.
 The
no. of molecules of the complexing agent
(ligand) depend on the coordination no. of
the metal and on the no. of complexing gps.
on the ligand.
 The
complexes formed are stable.
4
 Many
cations will form complexes in solution
with substances that have a pair of unshared
electrons (complexing agents).
 The
metal complexes formed with these
complexing agents are 1:2, 1:4, 1:6, or 1:8
according to the coordination no. of the metal.
 There
is only one complexing group on these
complexing agents (unidentate).
 e.g.Ammonia
(NH3)
5
Formation Constant (Kf)
2 ammonia molecules will complex with silver ion
to form a colorless complex in a stepwise
fashion.
The equilibrium constant for each step, called the
formation constant Kf :
Ag+ + NH3  Ag(NH3)+
Kf1 = ………………
Ag(NH3)+ + NH3  Ag(NH3)2+
Kf2 = ………………
6
The overall formation constant:
Kf = Kf1 x Kf2
Kf = …………………
If the equilibrium is in the opposite direction,
the constant is the reciprocal of the formation
constant and is called the dissociation
constant:
Kd = 1 /Kf = ………………
7
 It
is an organic agent that has two or more
groups capable of complexing with a metal
ion.
 The
complex formed is called a chelate.
 The
chelating agent is called the ligand.
8
 The
metal complexes formed with these
complexing agents are often 1:1, regardless
of the coordination no. of the metal ion.
 There
are sufficient complexing groups on
one chelating agent (multidentate).

e.g.ethylenediaminetetraacetic acid (EDTA).
9
EDTA have four Ka values corresponding to
the stepwise dissociation of the four
protons:
H4Y  H+ + H3YH3Y-  H+ + H2Y2H2Y2-  H+ + HY3HY3-  H+ + Y4-
Ka1 = 1.0 x 10-2
Ka2 = 2.2 x 10-3
Ka3 = 6.9 x 10-7
Ka4 = 5.5 x 10-11
10
H4Y has a very low solubility in water,
and so the disodium salt
Na2H2Y2.2H2O is usually used, in
which two of the acid groups are
neutralized
11
The formation of the EDTA chelate with Ca2+:
Ca2+ + Y4-  CaY2-
The formation constant:
Kf = ………………
12
The equilibrium in previous equation is shifted
to the left as the H+ conc.  (Why?).
 H+ conc. favor formation of the CaY4- chelate
(Why?).
The pH can affect also the metal ion.
That is, OH- competes for the metal ion just as
H+ competes for EDTA.
13
14



Definition of complexometric reactions.
Types of complexing agents.
Mechanism of complex formation.
15
16