Complexometric titration involves the titration of a metal ion solution with a chelating agent or ligand until the metal ion forms a stable complex. It is useful for determining mixtures of metal ions. The document discusses various types of complexometric titrations including direct titration, back titration, and replacement titration. It also covers the use of metal ion indicators, masking and demasking reagents, and provides examples of complexometric titration for determining compounds like magnesium sulfate, calcium gluconate, and auric ions in ores.

1. Complexometric titration
PRESENTED BY:-
Miss.- Harshada R. Bafna.
M. Pharmacy.(Q.A.)

2. Contents -
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Classification Complexometric titration.
Metal ion indicators
Masking and demasking reagents.
Estimation of Magnesium sulphate, and calcium gluconate.

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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.
 PRINCIPLE -
 The main principle is the reaction between the ligand and the metal ion to form a
complex.
M+2 + Ln (MLn)+2
 The metal ion acts as Lewis acid and the ligand acts as Lewis base which is the
complexing agent.
 Examples:
1. Ag+ + 2CN− Ag(CN)2−

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Metal ion + chelating agent /complexing agent Metal ligand complex or ligands
LIGAND - A species which donates lone pair of electrons and forms a coordinate
linkage with metal ion.
 Classification of Ligand-
1. UNIDENTATE : If a single site is present in ligand to form a complex.
Eg: Cyanide ion (CN-)
2. BIDENTATE : Two sites are present in a ligand
Eg: Glycine and oxalic acid
3. MULTIDENTATE : When more than 2 or 3 sites are present.
Eg: EDTA

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 Chelating agents: organic molecules containing two or more donor groups
which combine with metal to form complex having ring structure.
 They are also known as chelants, chelators, or sequestering agents.
 The most effective chelating groups in ligand as are amino and carboxylate ions.
 Other chelating agents are the following:

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Classification Complexometric titration
Direct
Titration
Back titration
Alkalimetric
titration of
metals
Indirect
titration
Replacement
Titration

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Direct
Titration
 It is simplest and the most convenient method.
 The standard chelating agent solution is added to the metal ion solution until the
end point is detected.
 In this method, metal ion is added to the suitable buffer solution and appropriate
indicator solution and the resulting solution is titrated with the EDTA solution.
 Cu+2, Zn+2, and Ni+2 can be determine by using direct titration method.
Example: Calcium gluconate injection is assayed for determining the calcium
chloride.
 Disadvantage is the time consumption of time is more for the complex
formation and also the interference of the other ions are observed.

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In this method, excess of complexing agent is back titrated with the standard
solution of the second metal ion.
In this method, excess of standard EDTA solution is added to the sample
solution and the pH is adjusted. Then the resulting solution is back titrated with
the appropriate titrant.
ZnCl2 , ZnSO4 , MgCl2 , MgSO4 is used as standard metal ion solution.
 Al+3, Co+2, Pb+2, Mn+2, Hg+2, and Ni+2 can be determine by using Back
titration method
 Example: Mn determination and ZnO determination.

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Replacement
Titration
 By name itself it indicates the displacement of the metal ion with other
metal ion takes place in this method. But it does not give the sharp end
points.
 This method is useful for the determination of Ca+2 ion.
 Example:
Mn+2 + MgEDTA−2 Mg+2 + MnEDTA−2

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Alkalimetric
titration of
metals
 This method, use the principle of liberation of free H+ ions during the
complexation.
 The H+ ions can also be determined by instrumental method.

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Indirect
titration
 Here protons from the complexing agent are displaced by the heavy metal
and titrated with the sodium alkali.
 This method is used in the analysis of Na, K, Ag, Au, As, Cl, Br and F.
 Example:
Mn+ + H2X−2 MX(n−4) + 2H+

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Metal ion indicators
 Definition- It is a substance that changes color when it binds to metal ions in
solution.
 Example- 1) Eriochrome Black T 2) Murexide
3) Xylenol orange 4) Pyridylazonaphthol (PAN)
 Requirement for Metal ion indicator -
1. The colour must be sufficiently intense, so that a minimum amount of indicator
can be used.
2. The colour contrast between the indicator and Metal-indicator complex should
be readily observable.
3. The Metal-indicator complex should possess sufficient stability to ensure a sharp
colour change, however it should be less stable than Metal-EDTA complex.

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4. The change in equilibrium from metal-indicator complex to the Metal-EDTA
complex should be sharp and rapid.
5. The colour reaction of the indicator should be selective.
6. The indicator must be very sensitive to metal ions so that the colour change
occurs at near the equivalence point.
7. The indicator must be stable in the titration medium.
8. The indicator must be stable on storage also.
9. All the above requirements must be fulfilled in the pH range in which the
proposed titration is to carried out.
10. It should be commercially available in adequate purity

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Example of metal ion indicator-
 EBT (Eriochrome Black T) is used to estimate Mg, Zn, Mn, Cd, Hg, Pb, Cu,
Al, Fe, Ti , Co, Ni and Pt metal ions .
 Murexide is used to estimate Ca, Ni, Co, Zn, Cd, Cu metal ions in EDTA
titrations.
 Xylenol orange is used to estimate Bi, Cd, Pb, Sc, Th, Zn metal ions in EDTA
titrations.
 Pyridylazonaphthol (PAN) is used to estimate Cd, Cu, In, Sc, Zn, metal ions
in Back and replacement EDTA titrations.

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Masking and demasking reagents
 Masking agent is a reagent used in chemical analysis which reacts with
chemical species that may interfere in the analysis.
 Examples of masking agent-
1) Triethanolamine : CH2CH2OH
N CH2CH2OH
CH2CH2OH
It is used as masking agent for Fe3+ , Al3+ and Sn2+
2) Fluoride : F−
It is used as masking agent for Fe 3+ and Al 3+ to give hexafluoro complex.
3) Iodide : I-
- It is used as masking agent for Hg 2+ to give tetraiodo complex.

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 Demasking agent is the technique of releasing the masked species so that it is
free to react with the principal reagent
 Examples of Demasking agent-
1) The mixture of Formaldehyde – acetic acid.
 The ideal requirements for these agents are the following:
1. It should be readily reacted by precipitation.
2. It should form stable complexes.
3. The colour developed by these agents should not interfere with the end point.

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Application of complexometric titration
 Used for determining of the hardness of the water.
Example: Gold ions in ores.
 Used in the determination of the metal ions.
Example: Used in the determination of the auric ions.
 Used in the preparation of calcium assays.
 Used in the determination of the magnesium sulphate.
 Used in the determination of the aluminium glycinate.

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Reference
1. A Textbook of Pharmaceutical Analysis (volume – I) by DR A V
Kasture, DR H N More, DR K R Mahadik, DR S G Wadodkar, Nirali
Prakashan, Pg.no. 99-106.
2. A Textbook of Pharmaceutical Analysis by David. G. Watson, Churchill
Livingstone, Pg.no. – 58..
3. www.google.com

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