213 PHC 8 th lecture (1) Gary D. Christian, Analytical Chemistry, 6 th edition 1

Define complexometric reactions. Differentiate between uni- and multi- dentate 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 (NH 3 ) 5

Formation Constant (K f ) 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 K f : Ag + + NH 3  Ag(NH 3 ) + K f1 = ……………… Ag(NH 3 ) + + NH 3  Ag(NH 3 ) 2 + K f2 = ……………… 6

The overall formation constant: K f = K f1 x K f2 K f = ………………… If the equilibrium is in the opposite direction, the constant is the reciprocal of the formation constant and is called the dissociation constant: K d = 1 /K f = ……………… 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 K a values corresponding to the stepwise dissociation of the four protons: H 4 Y  H + + H 3 Y - K a1 = 1.0 x H 3 Y -  H + + H 2 Y 2- K a2 = 2.2 x H 2 Y 2-  H + + HY 3- K a3 = 6.9 x HY 3-  H + + Y 4- K a4 = 5.5 x

H 4 Y has a very low solubility in water, and so the disodium salt Na 2 H 2 Y 2.2H 2 O is usually used, in which two of the acid groups are neutralized 11

The formation of the EDTA chelate with Ca 2+ : Ca 2+ + Y 4-  CaY 2- The formation constant: K f = ……………… 12

The equilibrium in previous equation is shifted to the left as the H + conc.  (Why?).  H + conc. favor formation of the CaY 4- 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

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 Definition of complexometric reactions.  Types of complexing agents.  Mechanism of complex formation. 15

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