Chemistery 221
Chapter 14:
Complex-Formation Titrations
Complex-formation reagents aere used widely for titrating cations.
Most metal ions react with electron-pair donors to form coordination compounds or
complexes. Many of us have utilized the physical properties (color) of such complexes
in the laboratory. Ammonia complexes are widely used to test for cations. During the
Electrogravimetric Determination of Copper and Lead in Brass experiment, ammonia
was added to an analyte solution to test for the presence of copper (II), which in the
presence of ammonia would produce the copper-ammonia complex, Cu(NH3)42+, a bright
blue complex. The donor species, or ligand, must have at least one pair of unshared
electrons available for bond formations. Ligands are defined as ions or molecules that
form covalent bonds with a cation or a nuetral metal atom by donating a pair of
electrons, which are then shared by the two. Ammonia, as mentioned above, along with
water and halide ions are common inorganic ligands.
The number of covalent bonds that a cation forms with a ligand is referred to as
its coordination number. For example, copper (II) has coordination number of four. The
species formed from such coordination or complexing, can be electrically positive,
nuetral or negative. Copper when complexed with ammonia results in a cationic
complex, Cu(NH3)42+, when complexed with glycine, a nuetral complex,
Cu(NH2CH2COO)2, and when complexed with chloride, an anionic complex, CuCl42-.
A chelate is produced when a metal ion coordinates with two or more donor groups of a
single ligand to form a five or six membered heterocyclic ring. The copper complex of
glycine, is an example of a chelate:
2
+H
NH2
C
H
C
O
OH
O
O
NH2
Cu2+ O
O
NH2
Cu
