Page 62 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Complexation Titrations

Analytical importance of complexes

Complexation reaction is widely used in analytical chemistry. One
of the first uses of the complexing agents was for titrating metal ions,
which is the topic of this chapter. In addition, many complexes can be
used for the spectrophotometric or gravimetric determination of samples.

Formation of complexes

Coordination complexes are neutral or ionic compounds formed by
the union of a metal ion with either other ions of opposite charge or with
neutral molecules. The group attached to the metal ion is called ligand or
complexing agent or coordinating agent. The number of bonds formed
between the metal ion and the ligand is called the coordination number
of the metal. The chemical bond between the metal ion and the ligand is a
coordination covalent bond.

In fact, most metal ions in aqueous solution actually exist as aquo
complexes e.g. [Cu(H2O)4]2+ . We often simplify such complexes in
chemical equations by writing the metal ion as if it were uncomplexed
Cu2+. A complexation reaction of a metal ion involves the replacement of
one or more H2O molecule by ligands.

M(H2O)n + L ↔ M(H2O)(n-1) L + H2O

Nature of ligands

Ligands may be classified on the basis of the number of points of
attachment to the metal ion to:

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