Complexometric Titration: Principles, Applications, and Limitations - Prof. Soma, Slides of Pharmaceutical Analysis

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COMPLEXOMETRIC

TITRATION

Definition

• Complexometric titration is a titrimetric method

through which unknown concentration of a

metal ion is determined by using

specific/particular type of complexing agent.

• It should be noted that the metal should exist in

the ionic form (obviously in the cationic form).

Classification of ligands

a) Monodentate ligand/unidentate ligand : They have the capacity to donate a single electron pair to form a complex. They can further be divided into- i. Anionic : They contain negative charge. E.g. *NCS

• , *OH - , CH 3

COO*

• , *NO 2 - , I - , CN - , Br - , Cl - , F - . ii. Neutral : They contain lone pairs of electrons but no charge. E.g. *NO, *H 2

O, *NHR

2

, *NH

3

, *NH

2

R.

b) Multidentate ligand : These ligands have the capacity to donate more than one electron pair to form a complex. They can also be further divided as anionic and neutral. These include bidentate ( 2 donor atoms- 2 pair of electrons donated), tridentate, hexadentate etc.

Examples of multidentate ligand Type Name Structure Bidentate Ethylenediamine Tridentate Diethylenetriamine Tetradentate Nitrilotriacetic acid (NTA) Triethylenetetramine (trien)

• The most common and widely used complexing

agent in complexometric titration is EDTA

(Ethylene Diamine Tetra Acetate) which is a

hexadentate ligand.

Examples of multidentate ligand

Reaction and calculation

• As EDTA is added to the analyte solution, EDTA forms

complex with the metal ions present in the solution.

Usually EDTA can react with the analyte metal in 1 : 1 ratio,

so the reaction is-

• We take the metal ion containing solution in the conical

flask and use Na

2

H

2

EDTA solution in the burette. The

end point of the titration is determined by using an

indicator.

• Since, the reaction occurs in 1:1 ratio thus 1 mole EDTA

will form complex with 1 mole metal ion. i.e.

• So, if we know the conc. of the EDTA solution and volume of EDTA required then we can determine the weight of the metal ion in the sample. For example, if 14 mL of 0. 5 M EDTA solution is required to form complex with all the metals of the solution, then we can determine the weight of the metal as follows-

End point determination

• At the beginning, the metal ion is in the free cationic form. Then we add few drops of indicator to the conical flask containing the metal ion solution. The indicator will form a colored complex with metal –
• Since the indicator is added in very small amount, most of the metal ions remain in the free cationic state and very few metal ions exist as the metal-indicator complex. Now, we add EDTA solution. It will form metal-EDTA complex until all the free metal ions are complexed. Then EDTA will replace the indicator from the metal-indicator complex and free indicator will be obtained.

Chelate effect

• Multidentate ligands form more stable metal-

ligand complex than monodentate ligand. This is

called chelate effect.

• The stability is due to the formation of ring

complex.

Required conditions of complexometric titration There are some factors that should be maintained during complexometric titration-

1. Water:

Water used as solvent should be free from any type of metal. We know that hard water contains metals (e.g. Ca 2 + , Mg 2 + ). Thus, soft water must be used. This is because if metal is present in the water, then EDTA will react with the metal and thus more EDTA will be required to from complex with all the metal ions of the sample. So the strength obtained will be higher than the original value.

3. EDTA:

EDTA is most widely used complexing agent because it reacts in 1 : 1 ratio with the metal ions of sample, so the calculation is simple. In basic medium, the reaction lies to the right, thus the reaction is considered complete. The EDTA used is called Disodium EDTA ( Na 2 H 2 EDTA). It should be-

• Non-hygroscopic
• Chemically and physically stable
• Strength/concentration can be properly known
• Non-irritable (for the safety of handling)

4. pH of the medium: EDTA is used as Na 2 H 2 EDTA which contain two H that can be removed. Thus the reaction is as follows- As we can see from the reaction, proton is formed in the reaction. Now if the pH is acidic then the conc. of proton will be high and the reaction equilibrium will shift to the left (unreacted metal and EDTA will remain in the reaction medium). Thus, the pH of the medium should be controlled. Different metal ions give best results at different specific pH. pH is controlled by the addition of buffers e.g. acetate buffer.

Methods/types of complexometric titration

Based on the manner in which the reagents are used and

mixed to perform the titration, complexometric titration

can be of following types –

✓ Direct titration

✓ Back titration

✓ Replacement titration

✓ Indirect titration

1. Direct titration

Method: In this method following procedure is followed-

• The analyte is taken in a conical flask.
• 2 - 3 drops of suitable indicator is added. Indicator will form complex with the metal of the analyte.
• The titrant EDTA is added from the burette. EDTA will first form complex with the free metal present in the analyte solution.
• Once all the free metal is complexed with EDTA, EDTA will then replace the indicator from Metal-Indicator complex and indicator will become free.
• Free indicator will show a different color which indicates the end point.