COMPLEXOMETRIC
TITRATIONS
• 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.
• 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 : is the type of volumetric
analysis involving the formation of complexes
which are slightly ionized in solution, like weak
electrolyte and sparingly soluble salt.
• The metal ion is known as Central metal
atom-acts as Lewis acid (electron acceptor)
• The anion or neutral molecule is known as
Ligand (L)
•
•
•
•
•
M+ + L
Ag+ + 2 CNCu2+ + 4 CNAg+ + 2 NH3
Cu2+ + 4 NH3
ML
[Ag(CN)2][Cu(CN)4]2[Ag(NH3)2]+
[Cu(NH3)4]2+
• * Coordination number = The no. of
coordinate bonds formed to a metal ion by its
ligands.
• The higher the valence of metal ion the more
stable the complex e.g.Ferricyanide is more
stable than Ferrocyanide
Types of complexing agents (( Classification of ligands according
to the no. of sites of attachment to the metal ion ))
• Unidentate (Monodentate) -attached to metal at one
site (i.e. forming one coordinate bond, or capable of
donating one unshared pair of electrons)
• e.g. H2O , NH3 , CN - , Cl - , I - , Br - ,
• Bidentate Ligand
The ligand attached to metal at two sites.
NH2
H2C
NH2
+ Cu2+
2
H2C
H2C
Ethylene diamine
CH2
Cu
CH2
H2C
NH2
H 2N
NH2
H 2N
• Tridentate Ligand:
The Ligand attached to metal at 3 sites
Diethylene triamine
• Tetradentate Ligand:
The Ligand attached to metal at 4 sites
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
• [A]- 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 [hexacyanoferrate III] is more stable than
Ferrocyanide [hexocyanoferrate II].
• (3) 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+
• [B]- 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.
•
Na2H2Y.2H2O
O
OH
O
Na+
-
O
N
N
OH
.2H2O
OO
Na+
O
• Titration involving EDTA known as complexometric
titration.
• EDTA is a hexadentate ligand, containing 4 oxygen
and 2 nitrogen donor.
• 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.
Detection of End Point
•
•
•
•
•
Metal indicator ((Metallochromic Ind.)).
Acid-base Indicator.
Specific Indicator.
Turbidity end point (appearance of turbidity).
Instrumental method.
Metal Indicators (Metallochromic 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 -
• H4 Ind.-
OH -
H3 Ind.2H+
Reddish violet
pH :
<9
H2 Ind.3H+
Violet
9-11
Blue
> 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 - 
Pink
CaY 2- + H3Ind.2- + H2O
Violet
Metal
Colour of complex
Colour of indicator
Ca 2+
Pink
violet
Cu 2+
Orange
Violet
Co 2+
Yellow
violet
Ni 2+
yellow
violet
• Eriochrome Black T (EBT)
• It can be represented by H2Ind • The color of Ind. change with the change of pH.
• EBT contains 2 replaceable phenolic hydrogen.
OH • H2 Ind.-
OH H Ind.2-
Ind.3-
Blue
H+
Yellow
> 11
H+
Wine red
pH :
<7
7-11
• It is used for the determination of Mg 2+ , Zn 2+ , Cd
2+ , pb 2+ , Hg 2+ & Mn 2+ salt at pH 7 – 11 using
ammonia buffer (pH = 10)
• M 2+ + H Ind.2-  M. Ind.- + H +
• M. Ind.- + H2Y 2-  MY 2- + H Ind.2- + H +
Wine red
Blue
• Mg 2+ + H Ind.2-  Mg Ind.- + H+
Wine red
• Mg Ind.- + H2Y 2-  MgY 2- + H Ind.2- + H +
Wine red
Blue
• EBT cannot be used for the determination of Cu 2+ , Fe 3+
, Al 3+ , Co 2+ and Ni 2+
Specific Indicator
•
•
•
•
Examples:
(1)- Thiocyanate (CNS -)
a)- It is specific ind. for Fe 3+
b)- When sample of Fe 3+ is treated with CNS – a
blood red complex is formed.
• c)- Upon titration against EDTA, the end point is
detected by decolorization of the blood red color.
•
• (2)- Salicylic acid
• a)- It is specific for Fe 3+ , gives violet color.
• b)- The end point is detected by decolorization of
violet color
Applications of Complexometric
Titrations
EDTA Titration
Requirements for direct EDTA titrations:
• M-EDTA complex must be more stable than MInd. 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 ppt. at the pH of titration. If
Mn+ is ppt. as MOH, auxiliary reagent must be
added to prevent pptn. of M n+.
• 1- pb2+ salt is ppt. as pb(OH)2 at the pH suitable for
titration.
• add tartaric acid (auxiliary reagent) which converts
pb(OH)2 to soluble lead tartarate complex.
• 2-Sometimes buffer acts as auxiliary reagent.
• during titration of Cu2+ salt in alkaline medium,
Cu(OH)2 is ppt. and the reaction with EDTA becomes
slow.
• upon 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.
• Direct determination of water hardness
• Water hardness is due to the presence of Ca2+ & Mg2+ salts.
• EDTA forms complex with Ca2+ & Mg2+, Ca-EDTA complex is more
stable than Mg-EDTA complex.
• At pH 12 EDTA forms complex with Ca2+ only.
•
• Total Ca2+ & Mg2+:
• Total Ca2+ and Mg2+ determined by titration with EDTA at pH 10
using ammonia buffer and EBT as ind.
• Upon titration with EDTA, Ca2+ will be chelated first, then Mg2+.
•
• For Ca2+ only:
• Direct titration with EDTA at pH 12 using 8% NaOH and Murexide.
• Mg2+ is pptd. as Mg(OH)2 leaving Ca2+ which is titrated with EDTA
•
• For Mg2+ :
•
Total – Ca2+ = Mg2+
• Direct determination of Cu2+ with EDTA
• The complex of Cu2+ with EDTA is more stable than its
complex with murexide ind.
• Cu2+ + H3Ind.2-  CuH2Ind.- + H+
• CuH2Ind.- + H2Y2-  CuY2- + H3Ind.2- + H+
yellow
violet
•
• Direct determination of Zn2+ by EDTA
• - The complex of Zn2+ with EDTA is more stable than its
complex with EBT ind.
• Zn2+ + H Ind.2-  Zn Ind.- + H+
• Zn Ind.- + H2Y2-  ZnY2- + H Ind.2- + H+
wine red
Blue
Back Titration
• Addition of known excess of st. EDTA to the sample
• The medium is buffered.
• excess EDTA is titrated with standard soln. of another metal
ion e.g. Mg2+ or Zn2+
• It is used in the following cases:
• Insoluble substances e.g. BaSO4 , Ca(C2O4)2 , PbSO4 ,
Mg3(PO4)2 … etc. Usually soluble in hot EDTA.
• The reaction between Mn+ & EDTA is slow (incomplete) e.g.
Fe3+ , Al3+ , Cr3+ , Th4 , … etc.
• The Mn+ is pptd. at the pH suitable for titration e.g. Al(OH)3.
• The colour change at the end point :
•
From free ind. colour  to M-Ind. complex
(opposite that direct titration)
• Det. of Aluminium salts:
• Sample of Al3+ is heated with known xss. of st. EDTA at
pH 7-8.
• The soln. is then adjusted to pH=10 using ammonia
buffer.
• The residual EDTA is titrated against st. Zn2+ using EBT
indicator.
• The colour change from blue to wine red.
pH 7-8
• Al3+ + H2Y2-  AlY- + 2 H+
• Boil
pH 10
• Zn2+ + H2Y2-  ZnY2- + 2 H+
•
• Zn2+ + H Ind.2-  Zn-Ind.- + H+
Blue
wine red
Titration of Mixtures
• EDTA is not a selective reagent (it chelates
with most metal ions)
• Selectivity of EDTA can be increased by one of
the following procedures:
– Control of pH of the medium
– Adjustment of oxidation number of metal ion
– Masking and demasking agent
• Control of pH of the medium
• First group: Trivalent & tetravalent cations e.g. (Bi3+ , Fe3+ ,
Th4+) and Hg2+ titrated (form stable complex) at pH 1-3 using
conc. HNO3.
• Second group: Divalent metals e.g. (Co2+ , Ni2+ , Cu2+ , Zn2+ ,
pb2+ and Cd2+) titrated (form stable complex) at pH 4-6 using
acetate buffer.
• Third group: Alkaline earth metal e.g. (Ba2+ , Sr2+ , Ca2+) and
Mg2+ titrated (form stable complex) at pH=10 using
ammonia buffer or 8% NaOH.
• From the mentioned above, we can titrate Mn+ of the first
group at pH 1-3 without interference of the second and third
groups or at pH 4-6 we can titrate Mn+ of the second group
without interference of the third group.
• e.g. Mixture of Bi3+ & pb2+: First titrating Bi3+ at pH = 2 using
xylenol orange as ind., then increased pH to 5 by adding
hexamine and titrating pb2+.
• Adjustment of oxidation number of metal ion
• - This solves the interference between Mn+ of the same
group of pH.
• Examples:
• Ascorbic acid (vit. C) is reducing agent used in:
• Removal of interference of Fe3+ in first group (pH 1-3)
 reduced to Fe2+
• Removal of interference of Hg2+ in first group (pH 1-3)
 reduced to Hgo (pptd.).
• Removal of interference of Cu2+ in second group (pH 46)  reduced to cuprous.
alkaline
• Oxidation of Cr3+  to CrO42+
H2O2
• Fe2+ , Hgo, Cuprous , CrO42- do not react with EDTA
• Masking and demasking agent
• Masking agents: are reagents which prevent interfering ion from
reaction without physical separation.
• These reagents form complexes with interfering ions which are more
stable than complexes formed with ind. & EDTA.
• Examples of masking agent:
• (A)- KCN
• It is used as masking agent for Ag+ , Cu2+ , Cd2+ , Co2+ , Ni2+ , Zn2+ , … etc.
•
M+ + 2 CN -  [M(CN)2] •
M+ + 4 CN -  [M(CN)4]2•
(B)- Triethanolamine :
CH2CH2OH
•
N
CH2CH2OH
•
CH2CH2OH
•
- It is used as masking agent for Fe3+ , Al3+ and Sn2+
• (C) Fluoride (e.g. NH4F):
• - It is used as masking agent for Fe3+ and Al3+ to give hexafluoro complex
[FeF6]3- and [AlF6]3• (D)- Iodide (KI):
• - It is used as masking agent for Hg2+ to give tetraiodo complex (HgI4)
• Demasking agent : are reagents which regain the ability
of masked ion to enter the reaction with ind. and EDTA.
• Example:
• - The masking by CN– can be removed by:
• - mixture of formaldehyde – acetic acid
• - on addition of demasking agent to [Zn(CN)4]2- , Zn is
liberated and titrated.
• [Zn(CN)4]2- + 4 HCHO + 4 CH3COOH
• (less stable)
CN
•
 Zn2+ + 4 CH2 + 4 CH3COO•
OH
Cyanohydrin
(more stable)