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COMPLEXOMETRIC TITRATION

❖ COORDINATION COMPOUND OR COORDINATION COMPLEX

• Definition-A coordination complex is the product of a Lewis acid-base reaction in which neutral
molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent
bonds.
Metal + ligand → coordination complex
Ex- K2[Pd(Cl4)], K3[Fe(CN)6]
• Ligands are Lewis bases - they bound to central metal by donating pair of electrons to metal
atom/ion. Ligands are also called complexing agents.
• Types of ligands-
i) Unidentate- single donor atom, ex- Cl− , H2O, NH3
ii) Didentate – two donor atom , ex- ethane-1,2-diamine, oxalate
iii) Polydentate- more than two donor atom, ex- ethylenediaminetetraacetate ion (EDTA4-)
• Metal atoms/ions are Lewis acids - they can accept pairs of electrons from Lewis bases.
• Within a ligand, the atom that is directly bonded to the metal atom/ion is called the donor atom.
• A coordinate covalent bond is a covalent bond in which one atom (i.e., the donor atom) supplies
both electrons. This type of bonding is different from a normal covalent bond in which each atom
supplies one electron.
• If the coordination complex carries a net charge, the complex is called a complex ion.
• Compounds that contain a coordination complex are called coordination compounds.
Q- Why EDTA is preferred in complexometric titration?
Ans-
• When di or Poly dentate ligand is used it’s two or more donor atom binds to a single metal ion
and it is called chelate ligand such complex is called chelate complex and these are more stable
than unidentate ligands complex because of ring formation (chelation). Therefore, it is also
called chelating agent
• EDTA chelates with almost all the metal ions and this forms the basis for complexometric or
chilometric or EDTA titration.

❖ Complexometric titration
• Complexometric titration (also called chelatometry) is a form of volumetric analysis in which the
titrant(complexing agent or chelating agent) and analyte form a complex and the formation of a
coloured 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.
• The most common type of complexometric titration is called EDTA (ethylene diamine tetra acetic acid)
titration.
• Sequestering agent: Ligands which form water soluble chelates (e.g. EDTA), they are used to liberate or
solubilize metal ions
• The agents which form water insoluble chelates are used to remove the metal ions from solution by
precipitation.
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❖ Complexometric Indicator
• An indicator capable of producing an unambiguous colour change is usually used to detect the end-point of
the titration.
• A complexometric indicator is a molecule that changes colour in the presence of metal ions.
• The indicator forms a complex with the metal ion, and the complex has a different colour than the individual
indicator molecule.
• Common indicators are organic dyes such as Fast Sulphon Black, Eriochrome Black T(EBT), Eriochrome Red
B, Patton Reeder, or Murexide. Color change shows that the indicator has been displaced (usually by EDTA)
from the metal cations in solution when the end point has been reached.

❖ Principle
When the indicator Eriochrome Black T binds with metal ions, it turns wine-red, however, when it is free
from metal ions, it remains blue in colour. While ethylene diamine tetra acetic acid (EDTA) is colourless
regardless of whether it is coupled or not to a metal ion.
Adding an EBT indicator to the sample containing metal ions, turns it wine-red in colour. Eriochrome Black T
has loosely bound(less stable) with metal ions, whereas EDTA has strongly bound (stable) with metal ions.
Therefore, when all the metal ions are bound to EDTA, the EBT indicator in the sample solution remains free
and the solution appears blue.
Mn+ + chelating agent→ [M-chelate]n+
❖ Procedure-
1. Take 25ml of the given sample in a conical flask.
2. Now add 1ml of buffer solution.
3. Now add 2 drops of Eriochrome Black – T solution in the mixture which turns the colour of the solution win
red.
4. Fill the burette with an EDTA solution.
5. Now start titrating the mixture with a standard EDTA solution.
6. An endpoint can be detected by a colour change from wine red to blue.
7. Repeat the titration three times to obtain 3 concordant reading

❖ Mechanism of action of indicator:

• Let the metal be denoted by M, indicator by I and chelate by EDTA.
• At the onset of the titration, the reaction medium contains the metal-indicator complex (MI) and
excess of metal ion.
• When EDTA titrant is added to the system, a competitive reaction takes place between the free
metal ions and EDTA. Since the metal-indicator complex (MI) is weaker than the metal-EDTA chelate,
the EDTA which is being added during the course of the titration is chelating the free metal ions in
solution at the expense of the MI complex.
• Finally, at the end point, EDTA removes the last traces of the metal from the indicator and the
indicator changes from its complexed colour to its metal free colour.

❖ Types of complexometric titration

1. Direct Titration –
• It is the most convenient and simple method of complexometric titration using EDTA.
• It is similar to the acid–base titration technique.
• In this titration standard, EDTA solution is added to give sample containing metals using burette till the
endpoint is achieved.
• Copper, barium, zinc, mercury, aluminium, lead, bismuth, chromium, etc. are metals that can be
determined by using direct complexometric titration.
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2. Back Titration –
• It is called back titration as in this type of titration, we perform back titration for the excess amount of
EDTA.
• In this, an excess amount of standard solution of EDTA is added to the metal solution being examined.
Then the excess amount of EDTA is back titrated by the solution of the second metal ion.
3. Replacement Titration –
• It is used when direct titration or back titration don’t give sharp endpoints. This analyte (containing
metal) is added to the metal–EDTA complex.
• The metal present in the analyte displaces another metal from the metal–EDTA complex.
4. Indirect Titration –
• This is also known as Alkalimetric titration.
• It is used for the determination of ions such as anions, which do not react with EDTA chelate.
• Protons from disodium EDTA are displaced by a heavy metal and titrated with sodium alkali.

❖ Masking and demasking

5. Masking agents
• They act either by precipitation or by formation of complexes more stable than the
interfering ion-EDTA complex
• Examples of masking agent
i) KCN: It is used as masking agent for Ag+, Cu2+, Cd2+, Co2+, Ni2+, Zn2+, … etc.
ii) Triethanolamine:It is used as masking agent for Fe3+, Al3+and Sn2+
iii) Fluoride (e.g. NH4F): It is used as masking agent for Fe3+and Al3+ to give [FeF6]3-
and [AlF6]3-
iv) Iodide (KI): It is used as masking agent for Hg2+to give tetraiodocomplex (HgI4)
6. Demasking:
• It is the process in which the masked substance regains its ability to enter into a particular
reaction. This enables to determine a series of metal ion in one solution containing many
cations.
• Methods of demasking-
i) Use of masking agents- HCHO, Chloral hydrate
ii) pH control method-The formation of a metal chelate is dependent on the pH of the
reaction medium. In weakly acid solution, the chelates of many metals are
3+
completely dissociated such as alkaline earth metals, whereas chelates of Bi, Fe or
Cr are readily formed at this pH.
iii) Use selective metal indicators- these indicators react with different metal ions under
various conditions
iv) Classical separation
v) Solvent extraction
vi) Removal of anion
vii) Kinetic masking- because kinetic inertness metal ion do not enter into complexometric
titration
➢ Example of using masking and demasking agents in complexometry is the analysis of three metals Cd
and Ca. the following method of analysis is followed:
1. Direct titration of the mixture with the EDTA gives the sum of the 3 metals.
2. Cu and Cd may be masked with the addition of cyanide to the solution, leaving only Ca ion.
3. When formaldehyde or chloral hydrate is added to the cyanide containing mixture, only Cd is
demasked and the EDTA titrates the sum of Ca and Cd. In this manner, the concentration of three ions
is determined by 3 individual titrations.
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Oxidation with H2O2releases Cu2+from complex.

❖ EDTA titratrion curve

❖ Applications of complexometric titrations:

• The complexometric titration is used to determine the concentration of metal ions in sample solutions.
• It is used for the estimation of the amount of calcium present in food products.
• It is used to find out the total hardness of the water.
• Complexometric titration is commonly used in the pharmaceutical industries to determine the concentration
metal is in a pharmaceutical dosage form (Drug).
• It is used in analytical chemistry to perform the assay.
• Complexometric titration is used in environmental analysis to determine bad metals.
• Cosmetic products are also analysed by complexometric titration.

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