Complexometric Reactions (1)

Report
213 PHC
8th lecture
(1)
Gary D. Christian, Analytical
Chemistry,
6th edition
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Define complexometric reactions.
Differentiate between uni- and multidentate complexing agents.
Understand the mechanism of complex
formation and the effect of pH on it.
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It is a reaction involves
the formation of a
substance called
complex which is slightly
ionising in solution.
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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.
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 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)
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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 = ………………
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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 = ………………
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 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.
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 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).
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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
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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
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The formation of the EDTA chelate with Ca2+:
Ca2+ + Y4-  CaY2-
The formation constant:
Kf = ………………
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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.
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Definition of complexometric reactions.
Types of complexing agents.
Mechanism of complex formation.
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