Complexometric titration Principle, Types, Method and MCQ-Part -1

Introduction :- 

• Complexometric titration is a form of volumetric titration in which the formation of a colored complex is used to indicate the end point of a titration.
• The complexes are formed by the reaction of a metal ion (an acceptor, a central atom or a cation) with an anion, a neutral molecule or very rarely a positive ion.
• Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution.
• An indicator capable of producing an distinct 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.
• 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).

• M⁺ + L ⇔ML
• Ag⁺ + 2CN^–      ⇔[Ag(CN)₂]^–
• Cu²⁺ + 4CN^–  ⇔  [Cu (CN)₄]^2-
• Ag⁺ + 2NH₃  ⇔    [Ag(NH₃)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)
• Classification of ligands :-

  Unidentate :- If a single site is present in ligand to form a complex e.g. cyanide ion (CN¯)

  Bidentate :- Two sites are present in a ligand e.g. glycine & oxalic acid

  Multidentate :- When more than 2 or 3 sites are present. e.g. EDTA

EDTA

• Ethylene diamine tetra acetic acid (EDTA) and its disodium salts are verstile complexometric agents.
• EDTA form complexes with all metal ions.
• The complexes formed by using EDTA are of chelate type.
• Alkali medium is required for complex ion becoz EDTA  will ionizes more in alkali medium.

WERNER′S CO-ORDINATION NUMBER (WCN) :- 

It is the number of small group which can be attached to the central atom in a complex.

TYPES OF COMPLEXOMETRIC TITRATION :-

1.Direct Titration Method:- It is the simplest and the most convenient method in which the standard solution of EDTA is slowly added to metal ion solution till the end point is achieved.
2. Back Titration Method:- In this method, an excess of a standard solution of EDTA is added to the metal solution being determined to complex all the metal ions present in the solution. The excess EDTA left after the complex formation with the metal is back tit with a standard solution of a second metal ion.
3. Displacement titration:- The metal to be analyzed is added to metal-EDTA complex. The analyte ion (with higher Kf’) displaces from the metal and the metal is subsequently titrated with standard EDTA.

4.Indirect Titration:-The anion is first precipitated with a metal cation and the precipitate is washed I boiled with an excess of disodium EDTA solution to form the metal complex.

MCQ 

1.Complexing agent is 

a. Electron donating ions

b. Electron accepting ions

c. a and b

d. None of the above

2. What is WCN ?

a. Number of group that are attached to complex

b. Number of group that are attached to central atoms

c. a and b

d. None of the above

3. Which sentence is false about ligand ?

a.  It is act like lewis base

b. CN¯ is example of unidentate.

c.It is electron donor group.

d. EDTA is example of unidentate.

4. Which sentence is false about complexometric titration ?

a. It is formed netural molecule.

b.The anion is first precipitated with a metal cation in indirect titration.

c.The anion and netural molecule is known as ligand.

d. It is used to study for compound solubility.

5.Complexometric titrations are useful for the determination ?

a. Non metal ions

b. Basic drug

c.Metal ions

d. None of the above

6.EDTA is a  

a. Tetradentate ligand

b.Hexadentate ligand

c.Octadentate ligand

d.Pentadentate ligand

7.The metal indicator complex must possess sufficient stability to 

a. Resist dissociation

b. Sharp color change

c. a and b

d. None of the above

8. Which sentence is flase about EDTA ?

a.Alkali medium is required for complex ion becoz EDTA  will ionizes more in alkali medium.

b.EDTA form complexes with all metal ions.

c.Ethylene diamine tetra acetic acid (EDTA) and its disodium salts are verstile complexometric agents.

d.Alkali medium is required for complex ion becoz EDTA  will ionizes more in acidic medium.

ANSWER KEY

1.a

2.b

3.d

4.d

5.c

6.b

7.c

8.d

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REFERENCE 

TEXT BOOK OF PHARMACEUTICAL ANALYSIS THIRD EDITION BY DR. S. RAVI SANKAR