Complexometric titrations involve the titration of a metal ion with a complexing agent or chelating agent such as EDTA. EDTA is a hexadentate ligand that forms very stable 1:1 complexes with metal ions. It is often used as the titrant in complexometric titrations due to the stability of the metal-EDTA complexes formed. The endpoint of the titration can be detected using metallochromic indicators that change color when the metal ion is completely bound to EDTA instead of the indicator. Complexometric titrations have many applications such as determining water hardness and analyzing metals in pharmaceuticals, cosmetics, urine and other samples.

2. COMPLEXOMETRIC
TITRATION
Topic:
Presented By:
Roll no. 002
Roll no. 014
Roll no. 025

3. Titration:
A technique in which a solution of known
concentration is used to determine the concentration of unknown
solution.
Titrant:
It is the compound in the titration buret, mostly its concentration is exactly known.
Titrand:
It is the substance which is being analysed in the titration.
Conditions for Titration:
1.Reaction Proceed rapidly
2.Well-defined stoichiometry
3.Large equilibrium Constant

4. Complex formation Titration:
A Titration based on formation of a complex between
analyte and titrant is called a Complexometric
Titration.
Complex: Metal ion + Ligand
Ø Metal ions are Lewis acids
Ø ligands are Lewis bases.

5. Ligands:
Ligands are electron-pair donor and form coordinate covalent
bond with Metal ion.
Ligands Types:
On the basis of number of
donating group:
§ Monodentate
§ Bidentate
§ Polydentate

6. Examples:
Monodentae ligands
Bidentae

7. Chelating Agent:
Multidentate ligands
are chelating Agent.
Chelate Effect:
The Multidentate
ligands form strong 1:1
complexes with many
metal Ions.

8. Ammonia is a simple complexing agent with one pair of unshared electrons that will
complex copper ion:
Here, the copper ion
acts as a Lewis acid,
and the
ammonia is a
Lewis base.
Ammonia will also complex with silver ion to form a colorless complex.
Two ammonia molecules complex with each silver ion in a stepwise fashion, and we
can write an equilibrium constant for each step, called the formation constant Kf :

9. The overall reaction is the sum of the two steps, and the overall formation
constant is the product of the stepwise formation constants:
The formation constant is also called the stability constant Ks, or Kstab.
We could write the equilibria in the opposite direction, as dissociations.
If we do this, the concentration terms are inverted in the equilibrium constant
expressions.
The equilibrium constants then are simply the reciprocals of the formation
constants, and they are called instability constants Ki, or dissociation constants
Kd

10. Simple complexing agents such as ammonia are rarely used as titrating
agents because a sharp end point corresponding to a stoichiometric
complex is generally difficult to achieve. This is because the stepwise
formation constants are frequently close together and are not very large,
and a single stoichiometric complex cannot be observed.
Certain complexing agents that have two or more complexing groups on
the molecule, however, do form well-defined complexes and can be used as
titrating agents.
Schwarzenbach demonstrated that a remarkable increase in stability is
achieved if a bidentate ligand (one with two complexing groups) is used
For example,
He showed replacing ammonia with the bidentate ethylenediamine,
NH2CH2CH2NH2 (en), results in a highly stable
Cu(en)2 2+ complex.

11. The most widely used chelating agent in titrations is
ethylenediaminetetraacetic acid (EDTA).
Each of the two nitrogens and each of the four carboxyl groups
contains a pair of unshared electrons capable of complexing with a
metal ion. Thus, EDTA contains six complexing groups.

12. EDTA Structure:
EDTA Stands For: ethylenediaminetetraaceti cacid.
Ø EDTA is a chelating agent or sequestering agent as it binds the metal ion through several
coordinate bonds.
Ø EDTA is a tetrabasic acid that has six potential sites for bonding with a metal ion, four
carboxyl groups and the two amino groups so it is a Hexadentate ligand.
Ø EDTA reacts with any metal ion
Ø within the ratio of 1:1
Ø regardless of the charge on
Ø the cation, thus EDTA is not
Ø a selective reagent.
Ø e.g. Ag+ + EDTA [Ag-EDTA]3-
Al3+ + EDTA [Al-EDTA]-

13. The complex formed is called chelate, it is a cage like structure that
contains cyclic rings in which the metal ion is effectively surrounded and
isolated from the surrounding media, thus EDTA forms stable complexes
with metal ions.
Mode of chelation:

14. Forms of EDTA:
EDTA has different forms at different pH.
Ø The various EDTA species are often abbreviated as
H4Y, H3Y-, H2Y2-, HY3- and Y4-.
Ø The fully protonated form (H4Y) is only a major component in very acidic
solution(pH<3).
Note that H4Y exists as a Zewitter ion.
Ø The species H3Y- and H2Y2- are predominant throughout the pH range of 3 to 10 .
Ø The fully unprotonated form Y4- (General Form) is dominant only in very
basic solutions (pH>10).

17. Ø EDTA is slightly soluble in water so its disodium salt (Na2H2Y) is commonly
used instead as a titrant.
Ø The reaction between metal ion and EDTA is usually written as:
Mn+ + H2Y2- [MY]n-4 + 2H+
Ø EDTA is produced as several salts such as :
Disodium EDTA
Sodium Calcium edetate (Used to treat lead poisoning).
Tetrasodium EDTA.

18. Functions:
Ø EDTA basically a chelating agent make colour complexes with metals ions ,
and the amount of colour change indicate the amount of metal ions present.
Ø Used for removing heavy metals from body and from hard water.
Ø EDTA is used as an anticoagulant for stored blood in blood banks; it
prevents coagulation by sequestering the calcium ions required for
clotting.
Ø EDTA is frequently used in soaps and detergents, because it forms a
complexes with calcium and magnesium ions. These ions are in hard
water and interfere with the cleaning action of soaps and detergents.
The EDTA binds to them, sequestering them and preventing their
interference.

19. Effect of pH on stability of EDTA complexes:
Ø All the forms of EDTA exist at particular pH
Ø As pH decreases we move to highly protonated form H6Y+2.

20. pH is highly acidic (1-3) pH is slightly acidic (4-6) pH is alkaline (10)
FREE EDTA (H4Y) FREE EDTA (H3Y-)
Some divalent metal ions
such as Pb2+, Cd2+ and
Zn2+ are able to form stable
complexes with EDTA at this
pH.
FREE EDTA (H2Y2- and
HY3-)
Most of the metal ions
are able to form stable
complexes with EDTA at
this pH.
Only the trivalent and tetravalent
metal ions are able to form a stable
complex at this pH (log K>20) in the
presence of high conc. of H+ which
compete with the metal ion for the EDTA.

21. The stability of the formed complexes is highly dependent on the pH of the
medium:
Ø Tri and tetravalent valent metal ions such as Bi3+, Fe2+, Cr3+, Th4+, V4+
form highly stable EDTA complexes having log K>20 . These can be titrated with
EDTA in acid medium pH 1-3 using 0.2N HNO3 to adjust pH.
Ø Some divalent metal ions such as Pb2+, Cd2+, Zn2+ can be titrated in acidic
medium pH 4-6 using hexamine buffer and xylenol orange as indicator.
Ø Ca2+ and Ba2+ can be titrated in highly alkaline medium pH 12 using sodium
hydroxide and murexide as indicator.
Ø Ca2+ ,Mg2+, Ba2+, Pb2+, Zn2+ can also be titrated in alkaline medium pH 10
using ammoniacal buffer and Eriochrome black T as indicator.

22. EDTA Equillibria :
We can represent EDTA as having four Ka values corresponding to the
stepwise dissociation of the four protons :

23. Ø In general ,Chemical Equillibrium is reached when the forward reaction
rate is equal to the reverse reaction rate and can be described by using a
equillibrium constan K . Complex ion equilibria are no exception to this
and have their own unique equilibrium constant.
Ø This Formation Constant Kf describes the formation of a compex ion
from its central ion and attatched ligands. This constant may be called as
stability contant .

24. Formation Constant:
The equilibrium constant for the reaction of a metal with a ligand is called the
Formation Constant , Kf.
Formation constant are written for the Y4- species of EDTA.
But when the pH becomes low that is below the 10 (<10) than the formation
constant is not applicable.
Then we use Conditional Formation Constant ,

25. EDTA Titration Curve:
Ø A titration is performed by adding the chelating agent to the sample.
Ø Take EDTA Conc. in mL on x-axis and metal Conc. on y-axis.
Ø We report metal conc. In pM, where
pM=_log[Mgn+]
e.g pCA2+ = _ log[Ca2+].
§ In region 1 excess of metal ion present.
We added EDTA some react but most of the
metal left behind
§ In region 2 at equivalence point
we added enough EdTA to form complex
with all of the metal ions left over.
So the conditional formation constant is :
The Kf' = Ca2+ = EDTA.
§ In region 3 we have of EDTA
which will complex with all of the metal ions
that are left over. So
The Kf' = Ca2+ = EDTA
We could calculate [EDTA] and [Mn+] at
every point in the titration curve.

26. Methods of
detecting of
end point
Instrumenta
l method
Spectrophotometric Potentiometric
Indicators
Metallochromi
c
indicators
Detection of the end point:-

27. Indicators :
The substances undergoes colour change on approaching of
completion reaction are called indicators.
Types of indicators:
• Acid base indicators (phenolphthalein , methyl orange)
• Redox indicators (diphenyl amine , diphenyl benzidine)
• Complexometric indicators (Eriochrome black T , calagmite)

28. Metallochromic Indicators :
Metal + Indicator Metal -Indicator complex Metal-EDTA
+ EDTA complex
Metallochromic Indicators are organic dyes whose Colour change shows that the
indicator has been displaced (usually by EDTA) from the metal cations in solution
when the endpoint has been reached.

29. Properties of metallochromic indicators :
• The free indicator must posses a different colour from the metal indicator
complex.
• The indicator must be sensitive towards the metal ion so that only small
amount of it is necessary for a titration.
• The metal indicator complex must be less stable then metal EDTA complex.
• It should form 1 : 1 complex which must be weaker then the metal chelate
complex.
• The metal indicator complex must be formed in the same pH of the metal
indicator complex.

30. For example: titration of Magnesium ion by EDTA, using Eriochrome
black-T as indicator, in order to determine the hardness of water.

31. Metal (analyte)
Indicator
buffer solution
EDTA solution
(titrant)

32. Some others indicators used in complexometric titrations:

33. Fractions of dissociating species in polyligand complexes:
ß- values
Dissociation of polyligand complexes is occur by same method as done
by polyprotic acids in stepwise manner i. e. :

34. The fractions of each species of silver is defined as follows:

35. As we want to calculate the fractions , we will use the equilibrium
constants of formation which are as follows:

36. Now putting the values of equation no (6) and (7) into equation no (1),(2),(3)
and by dividing nominator and denominator by concentration of silver ions to
get the values of fractions:

39. Effect of complexation on solubility:
The solubility is defined as the concentration of solute in solution when it is
equilibrium with the solid substance.
Complexing agents (ligand) can compete for the metal ion in precipitates
just as acids compete for the anion. A precipitate of MA that dissociates to
give M+ and A- and whose metal complexes with ligand L to form ML+
having equilibria :

40. For example, consider the solubility of AgBr in the presence og NH3.
The equilibria are:

41. Applications of complexometric titrations:
§ The major application of complexometric titrations is for the
determination of concentrations or amounts of metallic elements in
water , food and other industrial samples.
For example:
The amount of calcium and magnesium ions can be measured by
complexometric titrations by measuring hardness of water .
§ Complexometric titration is used for the estimation of the amount
of total hardness in water.
§ It is widely used in the pharmaceutical industry to determine the
metal concentration in drugs.
§ Titanium dioxide is used in many cosmetic products. This can be
analysed by complexometric titration.
§ It is used to analyse urine samples.
§ It is widely used in analytical chemistry.