Redox Titrations: MCQ 2

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Questions and Answers

Why is excess potassium iodide used in the preparation of iodine solution?

To act as a solvent to keep the iodine in solution (correct)

To ensure all of the I₂ from the KIO₃ solution reacts with thiosulphate (correct)

To dilute the potassium iodate solution

To react with thiosulphate directly

What is the purpose of adding starch indicator in the titration reaction?

To indicate the end-point of the titration (correct)

To change the colour of the solution

To react with the iodine solution

To react with the thiosulphate solution

What is the application of iodine-thiosulphate reactions in the Winkler method?

To prepare iodine solutions

To titrate thiosulphate solutions

To determine the percentage of hypochlorite in bleach

To find the amount of dissolved oxygen in water (correct)

Why is the bleach diluted before titration?

Because household bleach is very concentrated Signup and view all the answers

What is the role of chlorine in the reaction with potassium iodide?

To displace iodine from potassium iodide Signup and view all the answers

What is the primary function of potassium permanganate in redox titrations?

To act as an oxidising agent Signup and view all the answers

What is the purpose of adding dilute sulphuric acid to the solution of ammonium iron (II) sulfate?

To prevent the iron (II) from reacting with the air to form iron (III) Signup and view all the answers

Why is potassium permanganate not used with hydrochloric acid in redox titrations?

Because HCl reacts with potassium permanganate Signup and view all the answers

What is the oxidation state of manganese in potassium permanganate?

+7 Signup and view all the answers

What is the colour of potassium permanganate in its +7 oxidation state?

Purple Signup and view all the answers

What is the purpose of adding dilute H₂SO₄ to the iron tablets during the experiment?

To prevent Fe(II) being oxidised to Fe(III) Signup and view all the answers

Why is it necessary to prepare a fresh solution of Potassium permanganate just before titrating?

Because it reacts with both water and sunlight in solution Signup and view all the answers

In the titration of potassium permanganate with iron solution, what is the end-point of the reaction?

When a pale pink colour persists Signup and view all the answers

What is the purpose of rinsing the burette with KMnO₄ solution before titration?

To wash away any impurities in the burette Signup and view all the answers

What is the concentration of the iron solution in moles per litre if 25 cm³ of the solution requires 5.75 cm³ of a 0.015M KMnO₄ solution for a complete reaction?

0.00431M Signup and view all the answers

Study Notes

Potassium Permanganate in Redox Titrations

Potassium permanganate (KMnO₄) is a useful oxidizing agent in redox titrations due to its complete reaction and ability to act as its own indicator, showing a clear color change.
However, it has disadvantages, such as being unstable and requiring standardization against a standard solution of ammonium iron (II) sulfate prior to titration.

Preparing Ammonium Iron (II) Sulfate Solution

Ammonium iron (II) sulfate is a primary standard that can be weighed and diluted to make a standard solution.
Dilute sulfuric acid is added to prevent the iron (II) in the solution from reacting with air to form iron (III).
The solution is prepared by washing the powder into a beaker of dilute sulfuric acid, stirring well, and then transferring it to a volumetric flask.

Potassium Permanganate Solution

Potassium permanganate contains Mn in the +7 oxidation state, which is purple in color.
In acidic conditions, Mn is reduced completely, and no indicators are needed for the titration.
However, if acidic conditions are not maintained, an intermediate oxidation state will be reached, resulting in a brown color.
To prevent this, dilute sulfuric acid is added to the KMnO₄ solution.

Titration of Potassium Permanganate with Ammonium Iron (II) Sulfate

The conical flask contains 25 ml of iron solution and 10 ml of dilute sulfuric acid.
Prior to titration, the following washings should be carried out: washing the pipette, burette, and conical flask with DI water, rinsing the burette with KMnO₄, and rinsing the pipette with iron (II) solution.
The end-point is reached when a pale pink color persists.
The concentration of the potassium permanganate solution can be calculated using the volume of KMnO₄ required for a complete reaction with the ammonium iron (II) sulfate solution.

Potassium Permanganate vs. Ammonium Iron (II) Titration

The reaction between potassium permanganate and ammonium iron (II) sulfate occurs in a 1:5 ratio.
The concentration of the potassium permanganate solution can be calculated using the volume of KMnO₄ required for a complete reaction with the ammonium iron (II) sulfate solution.

Potassium Permanganate vs. Iron Tablet Titration

A standardized solution of potassium permanganate can be used to determine the amount of iron in an iron tablet.
The KMnO₄ solution must be standardized immediately before the titration since it is unstable.
The iron tablets are ground up and dissolved in dilute sulfuric acid, and the solution is transferred to a volumetric flask.
The concentration of the iron solution can be calculated using the volume of KMnO₄ required for a complete reaction with the iron solution.

Iodine vs. Thiosulphate Titration

This is a redox titration that occurs between iodine and thiosulphate.
The reaction is used to standardize the thiosulphate solution using a known concentration of iodine.
DI water must be used during the experiment as tap water contains chloride ions that would displace iodine from iodine solutions.
The iodine solution is prepared by adding potassium iodate, excess dilute sulfuric acid, and excess potassium iodide to water.
The colour changes during the reaction are: red/brown prior to titration, pale yellow during titration, and colourless at the end-point.

Application of Iodine-Thiosulphate Reactions

These titrations can be used to:
- Find the amount of dissolved oxygen in water (Winkler method).
- Determine the % hypochlorite in bleach.

Determining % Hypochlorite in Bleach

The hypochlorite ion (OCl⁻) is the active ingredient in bleaches.
Household bleach is very concentrated and must be diluted prior to titration.
The diluted bleach is reacted with excess potassium iodide (KI) in the presence of acid, and the liberated iodine is titrated against a known thiosulphate solution.
The same colour changes occur as before.
The concentration of sodium hypochlorite in bleach and the % w/v of hypochlorite in bleach can be calculated using the volume of thiosulphate required for a complete reaction with the iodine solution.

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Description

Learn about the use of potassium permanganate in redox titrations, its advantages and disadvantages, and how to prepare ammonium iron (II) sulfate solution as a primary standard.