4. Complexometric Titration.ppt

Complexes
◦ Complexation reactions are widely applied through complexometric
titration in order to determine the metal ions, present in the
solution.
◦ Metals ions, especially transition metals, act as Lewis acids, because
they accept electrons from Lewis bases.
◦ When metal cations combine with Lewis bases, the resulting species
is called a complex ion. This also called coordination complex. The
base is called a ligand.
◦ When the metals are covalently bonded with surrounding ions or
molecules the resulting species are called metal complexes or
coordinate complex
◦ The surrounding ions or molecules are called ligands

Complexometric titration
• Complexometric titration is a form of volumetric analysis in which the
formation of a colored complex is used to indicate the end point of a
titration. Complexometric titrations are particularly useful for the
determination of a mixture of different metal ions in solution.
An indicator capable of producing an unambiguous color change is
usually used to detect the end point of the titration.
Complexometry
• Complexometry is the type of volumetric analysis involving the formation
of complexes which are slightly ionized in solution, like weak electrolyte
and sparingly soluble salt.
• Complex is formed by the reaction of metal ion (Mn+) with either an
anion e.g. [Ag(CN)2]- or neutral molecule e.g. [Ag(NH3)2]+
• The metal ion is known as Central metal atom.
• The anion or neutral molecule is known as Ligand (L)

◦ Ag+ + 2 CN - [Ag(CN)2] -
◦ Cu2+ + 4 CN - [Cu(CN)4] 2-
◦ Ag+ + 2 NH3 [Ag(NH3)2] +
◦ Cu2+ + 4 NH3 [Cu(NH3)4] 2+
◦ Central metal atom = acts as Lewis acid (electron acceptor)
◦ Ligand = acts as Lewis base (electron donor)
◦ Coordinate bond (Dative) = The bond formed between central metal
atom (ion) (acceptor) and the Ligand (donor)

◦ Dative bond is similar to covalent bond (formed of two electrons) But in
dative bond the electrons pair are donated from one atom to the other.
The atom gives electron pair is known as donor, while the atom accept
electron pair is known as acceptor. The bond is represented by an
arrow () from donor to acceptor.
NH3

NH3  Cu  NH3

NH3

Coordination Number
◦ Coordination number = the number of ligands surrounding a central
cation in a transition metal complex.
◦ Common coordination numbers are 2 (Ag+),
4 (Ni2+ , Cu2+) and 6 (Fe3+ , Cr3+).
◦ The geometries of the ligands about the central atom are shown in the
figure.
◦ It is usually double the charge of the metal.
◦ The charge of a complex is the algebraic sum of the charges of the
central ion and ligand .. e.g.
◦ [Ag(CN)2] -  Ag+ + 2 CN -
1 (+ve) + 2 (-ve) = 1 (-ve)
◦ e.g. [Fe(CN)6]3-  Fe3+ + 6 CN -
3 (+ve) + 6 (-ve) = 3 (-ve)
◦ The higher the valence of metal ion the more stable the complex e.g.
Ferricyanide is more stable than Ferrocyanide

Types of complexation
1. Unidentate (Monodentate) Ligand (Simple Ligand):
◦ The ligand attached to metal at one site e.g. H2O , NH3 , CN- , Cl - , I-,
Br -, (i.e. forming one coordinate bond, or capable of donating one
unshared pair of electrons)

2. Bidentate Ligand:
◦ The ligand attached to metal at two sites.
H2C
H2C
NH2
NH2
H2C
H2C
NH2
NH2
CH2
CH2
H2N
H2N
Cu
+ Cu2+
2
Ethylene diamine

3. Tridentate Ligand:
The Ligand attached to metal at 3 sites
4. Tetradentate Ligand:
The Ligand attached to metal at 4 sites
Diethylene triamine
Triethylene tetramine

Chelation
Chelate : It is a complex formed between the ligand containing two or
more donor groups and metal to form ring structure. (heterocyclic
rings or chelate rings).
Chelating agents: organic molecules containing two or more donor
groups which combine with metal to form complex having ring
structure. Chelates are usually insoluble in water but soluble in
organic solvent.
Sequestering agent: Ligands which form water soluble chelates e.g.
EDTA.

Factors affecting stability of complex
 Effect of central metal ion:
1. Ionic size (metal radius):
◦ Smaller an ion (small radius of metal), greater its electrical field
more stable complex
2. Ionic charge (metal charge):
◦ Metal of higher charge give more stable complexes. e.g.
Ferricyanide is more stable than Ferrocyanide.
3. Electronegativity:
◦ The higher acidity (electronegativity) of metal (Mn+), the higher
stability of complex.
4. Metal which has incomplete outer shell (has high acidity) have
more tendency to accept electrons, more stable complex. e.g. Ca2+ ,
Ni2+ , Zn2+ , Mn2+ , Cu2+

Factors affecting stability of complex
 Effect of Ligand:
1. Basic character:
◦ The higher the basicity (strong base is good electron donor), the
higher the ability of ligand to form complex. e.g. ligand contain
electron donating atom.
e.g. N > O > S > I- > Br- > Cl- > F-
2. The extent of chelation:
◦ Multidentate ligands form more stable complexes than
monodentate.
3. Steric effect:
◦ Large, bulky ligand form less stable complexes than smaller ones
due to steric effect. e.g. ethylene diamine complexes are more
stable than those of the corresponding tetramethyl ethylene
diamine.

Complexones
◦ Amino poly-carboxylic acid compounds used as complexing agents for
many metal ions.
◦ Complexone I:
◦ Complexone II:

◦ Titration involving EDTA - complexometric titration.
◦ EDTA is a hexadentate ligand, containing 4 oxygen and 2 nitrogen
donor.
Na+
Na+
-
O N
O-
O
N
OH
O
OH
O
O

◦ It reacts with most cations (divalent, trivalent, tetravalent) forming
freely sol. stable complexes.
◦ The formed complexes contain the metal and EDTA in the ratio of
1:1 irrespective to the charge of the metal ion.
◦ The general reactions for the formation of metal – EDTA complexes
as follows
◦ Mn+ + H2Y2 (MY)n-4 + 2 H+
◦ M2+ + H2Y2 MY2- + 2 H+
◦ M3+ + H2Y2 MY- + 2 H+
◦ M4+ + H2Y2 MYo + 2 H+

◦ 2 moles of hydrogen ions are formed in each case.
◦ EDTA is not selective chelating agent.
◦ Formation or dissociation of complexes is affected by pH.
1) In acidic medium:
 [H+]
 ionization of EDTA
 stability of metal – EDTA complex
shift the reaction backward.

2) In slightly alkaline solution:
the reaction is forward
 stability of complexes (chelates).
3)- In strong alkali:
precipitation of metal as hydroxide.

M n+ + H2Y 2- MY n-4 + 2 H +
[MY(n-4)] [H+]2
Keq. = -------------------------
[Mn+] [H2Y2-]
◦ In buffered system
[MY(n-4)]  undissociated complex
Keq. = -----------------------
[Mn+] [H2Y2-]  dissociated species

Detection of End Point
◦ Metal indicator (Metallo-chromic Indicator)
◦ Acid-base Indicator.
◦ Specific Indicator.
◦ Turbidity end point (appearance of turbidity).
◦ Instrumental method.

Metal Indicators (Metallo-chromic Ind.)
◦ They are organic dyes which form colored chelates (complexes), which
exhibit a color in the free form and a different color in the complex form.
◦ Mn+ + Ind.  M – Ind.
◦ M – Ind. + EDTA  M – EDTA + free Ind.
◦ Act as ligand to form complex with metal (act as Lewis base and the metal
acts as Lewis acid).
◦ The reaction between metal and ind. must be reversible.
◦ The metal-ind. complex should be less stable than the metal-EDTA
complex.
◦ The color of free form different than color of complex one.
◦ Changes its color according to the pH of the medium.

◦ Murexide: Ammonium salt of Purpuric acid or ammonium purpurate
◦ It can be represented by H4 Ind. -
OH - OH -
◦ H4 Ind.- H3 Ind.2- H2 Ind.3-
H + H +
Reddish violet Violet Blue
pH : < 9 9-11 > 11

It is used for the determination of
◦ Ca 2+ , Co 2+ , Ni 2+ , & Cu 2+ salts at pH 9-11
◦ M 2+ + H3 Ind.2-  M. H2 Ind.- + H +
◦ M.H2 Ind.- + H2Y 2 + OH -  MY 2- + H3Ind.2- + H2O
◦ Ca 2+ + H3Ind.2-  Ca H2Ind.- + H+
◦ Ca H2Ind.- + H2Y 2- + OH -  CaY 2- + H3Ind.2- + H2O
Pink Violet

Metal Color of complex Color of indicator
Ca 2+ Pink violet
Cu 2+ Orange Violet
Co 2+ Yellow violet
Ni 2+ yellow violet

Specific Indicator
1. Thiocyanate (CNS--)
• It is specific ind. for Fe 3+
• When sample of Fe 3+ is treated with CNS – a blood red complex is
formed.
• Upon titration against EDTA, the end point is detected by
decolorization of the blood red color.
2. Salicylic acid
◦ It is specific for Fe 3+, gives violet color.
◦ The end point is detected by decolorization of violet color

◦ M-EDTA complex must be more stable than M-Ind. complex in
buffered medium.
◦ The compound to be determined is water soluble.
◦ The reaction between EDTA and metal must be rapid. If the reaction
is slow it must be catalyzed.
◦ Mn+ should not be precipitated at the pH of titration. If Mn+ is
precipitated as MOH, auxiliary reagent must be added to prevent
precipitation of M n+.
Requirements for direct EDTA titrations:

◦ pb2+ salt is ppt. as pb(OH)2 at the pH suitable for titration. In that
case, we have to add tartaric acid (auxiliary reagent) which converts
pb(OH)2 to soluble lead tartarate complex. Sometimes buffer could be
added because it acts as auxiliary reagent.
◦ During titration of Cu2+ salt in alkaline medium, Cu(OH)2 is ppt. and
the reaction with EDTA becomes slow. We can solve this problem by
using ammonia instead of alkali hydroxides, the soluble [Cu(NH3)4]2+
is formed which is less stable than Cu-EDTA and hence the reaction
forward rapidly.

4. Complexometric Titration.ppt

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4. Complexometric Titration.ppt