Analytical Chemistry Lecture 8: Volumetric Analysis

Study Notes

Visual detection of end point can be achieved by measuring potential with an indicator electrode relative to a reference electrode and plotting it against the volume of titrant.
There are three methods for visual detection:
- Self indication
- Starch detection
- Redox indicators

Self indication is used when the reducing or oxidizing agent is colored.
The end point can be detected by the appearance or disappearance of the color of the reagent.
Example: KMnO4, where the end point can be detected by the pink color of permanganate.

Starch indicator is used for titrations involving iodine.
Starch forms a dark blue complex with iodine (I2).
Starch is not a redox indicator; it responds specifically to the presence of I2, not to a change in redox potential.
The active fraction of starch is amylose, a polymer of the sugar α-d-glucose.
In the presence of starch, iodine forms I6 chains inside the amylose helix, and the color turns dark blue.

Redox indicators are highly colored dyes that are weak reducing or oxidizing agents.
They can be oxidized or reduced, and the color of both forms are different.
There are not many good redox indicators.
Ferroin is one of the best indicators that can be used in cerium(IV) titrations. It is oxidized from red to pale blue at the end point.
Diphenylaminesulphonic acid is used as an indicator for titrations with dichromate in acid solution. The color at the end point is purple.

Potassium permanganate (KMnO4) is a widely used oxidizing agent.
In acidic medium, potassium permanganate is reduced to Mn2+.
In basic medium, it is precipitated as brown MnO2.
Potassium permanganate is standardized by titration with primary standard sodium oxalate.
The solution must be heated for rapid reaction, and the reaction is catalyzed by the Mn2+ product.

It is a strong oxidant with an intense violet color.
It is not a primary standard.
Aqueous solutions of KMnO4 are not entirely stable because the ions tend to oxidize water.
Standardization of potassium permanganate is done with sodium oxalate.
Conditions for titration with potassium permanganate:
- Titration is carried out in acidic solution (1 M H2SO4).
- Titration is carried out at 70°C to expel CO2 and to catalyze the reaction.
- The reaction occurs slowly even at elevated temperature, but once Mn is formed, it acts as a catalyst for the reaction.

KMnO4 is a strong oxidant with an intense violet color.
In strongly acidic solutions (pH ≤ 1), it is reduced to Mn2+.
In neutral or alkaline solution, it is reduced to brown solid MnO2.
In strongly alkaline solution (2 M NaOH), green manganate ion (MnO42-) is produced.
Reduction half-reactions:
- Strongly acidic solutions (pH ≤ 1): MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
- Neutral or weakly alkaline solutions: MnO4- + 4H+ + 3e- → MnO2 (s) + 2H2O
- Strongly alkaline solutions (2 M NaOH): MnO4- + e- → MnO42-

Iodide ion is a weak reducing agent and will reduce strong oxidizing agents.
Iodometry is a method of titration where an excess of iodide is added to a solution of an oxidizing agent.
The resulting I2 is then titrated with a reducing agent.
The titrating agent is sodium thiosulphate.
This method is used for the determination of dichromate.

We can't titrate strong oxidizing agents directly with thiosulphate because it might be oxidized to higher states.
Starch, used to detect the end point, should not be added from the beginning of the titration but near the end of titration where the color of iodine is pale yellow.

Example 4: Titration of 0.1467 g of primary standard Na2C2O4 required 28.85 ml of potassium permanganate. Calculate the molar concentration of KMnO4 in this solution.