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Permanganate Titration – Definition, Preparation, Properties, Uses, Practice Problems and FAQ

Permanganate Titration – Definition, Preparation, Properties, Uses, Practice Problems and FAQ

Generally for the titration process, external indicators are used to predict the completion of a titration process, but in the case of potassium permanganate titration, KMnO4 acts as a self indicator. At the equivalence point of the titration, it can be observed that the deep purple colour of permanganate changes to a faint pink colour.


TABLE OF CONTENTS

  • Permanganate Titration
  • KMnO4 – Preparation
  • KMnO4 – Physical Properties
  • KMnO4 – Chemical Properties
  • KMnO4 – Uses
  • KMnO4 as Self-indicator
  • Estimation of Fe2+ using KMnO4
  • Role of H2SO4 in Permanaganate Titration
  • Practice Problems
  • Frequently Asked Questions - FAQ

Permanganate Titration

Potassium permanganate (KMnO4) is a strong oxidising agent. It is used in the estimations of many reducing agents such as Fe2+, C2O42-, etc. The crystals of KMnO4 are deep violet, and the solution of KMnO4 is purple in colour.

KMnO4 – Preparation

  • It is made commercially by oxidative fusion of MnO2 in an alkaline environment with an oxidising agent like O2 or KClO3 to make potassium manganate (K2MnO4) which is dark green in colour. This compound in a neutral or acidic medium undergoes disproportionation to give permanganate ion (MnO4-)..

2MnO2s+4KOHs+O2g2K2MnO4l+2H2Ol

3MnO2s+6KOHs+ 6KClO3s3K2MnO4l+ 3H2Ol+6KCl aq

3MnO42-(aq)+4H+(aq)2MnO4-(aq)+MnO2(s)+2H2O(aq)

  • Peroxodisulphate oxidises a manganese (II) ion salt to permanganate in the laboratory.

Mn2+aq+5S2O82-aq+8H2Oaq2MnO4-aq+10SO42-aq+16H+aq

  • Potassium manganate can be oxidised with chlorine or disproportionated in acidic environments. Chlorine oxidation is a chemical reaction that occurs in the human body.

2K2MnO4l+Cl2g2KMnO4s+2KCls

3MnO4-2aq+4H+aqMnO2s+2MnO4-aq+2H2Oaq

KMnO4 – Physical Properties

  • It is an odourless, purple to magenta crystalline solid.
  • It occurs in the form of monoclinic prisms, almost opaque with a blue metallic lustre.
  • An aqueous solution has a sweetish astringent taste.
  • It has a density of 2.7 gcm-3
  • It has a melting point of 240 ℃ (464 ℉ or 513 K).
  • It is used as an oxidising agent in the chemical synthesis of many important compounds.

Reduction half-reaction

Electrode potential values; E°(V)

Acidic solution

MnO4-aq+4H+aq+3e-MnO2s+2H2Oaq

2MnO2s+4H+aq+2e-Mn2+aq+2H2Oaq

MnO4-aq+8H+aq+5e-Mn2+aq+4H2Oaq

Basic solution

MnO4-aq+2H2Oaq+3e-MnO2s+4OH-aq

MnO2(s)+2H2O(aq)+2e-Mn(OH)2(s)+2OH-(aq)

MnO4-(aq)+4H2O(aq)+5e-Mn(OH)2(s)+6OH-(aq)

1.70

1.23

1.51

0.60

-0.05

0.34

KMnO4 – Chemical Properties

The following are some of the most notable reactions of KMnO4.

1. In an acidic environment: Potassium iodide is broken down to give iodine on reaction with permanganate.

10I-+2MnO4-+16H+2Mn2++8H2O+5I2

2. In slightly alkaline or neutral solutions:

  • The oxidation of iodide to iodate is a noteworthy reaction.

I-aq+2MnO4-aq+H2Oaq2MnO2s+2OH-aq+IO3-aq

  • Thiosulphate is almost completely oxidised to sulphate.

8MnO4-aq+3S2O32-aq+H2Oaq8MnO2s+6SO42-aq+2OH-aq

  • Fe2+ ion (green) is converted to Fe3+ (yellow).

5Fe2+aq+MnO4-aq+8H+aqMn2+aq+4H2Oaq+5Fe3+aq

KMnO4 – Uses

  • Potassium permanganate is a popular oxidant in preparative organic chemistry as well as in analytical chemistry.
  • Bleaching of wool, cotton, silk, and other textile fibres, as well as the decolourisation of oils, rely on its great oxidising capacity.
  • Potassium permanganate treats impetigo, pemphigus, wounds, dermatitis, and tropical ulcers.
  • As a regeneration chemical, it is used to remove iron and H2S from well water. It was once used to sterilise water.
  • It is used in the treatment of bacterial infections. However, it is crucial to dilute potassium permanganate with water before putting it on your skin. When employing a 0.1 per cent potassium permanganate solution, most medicinal applications require a dilution of 1 part to 10.
  • Potassium permanganate is a point-of-entry treatment that converts dissolved iron, manganese, and hydrogen sulphide into solid particles that may then be filtered out of the water. It can also be used to prevent the growth of iron bacteria in wells.

KMnO4 as Self indicator

In an acidic medium, KMnO4 gets reduced from MnO4 - to Mn2+. We know that MnO4 - is a purple-coloured solution and Mn2+ is a colourless solution. So, during titration, the colour of the solution changes from purple to colourless. Therefore, no external indicator is required in the titration as KMnO4 acts as a self-indicator.

Estimation of Fe2+ using KMnO4

In the estimation of Fe2+, the standard solution of KMnO4 is taken in a burette and the ferrous solution is taken in a beaker. When KMnO4 is added to the ferrous solution (reducing agent) till the equivalence point, the purple colour of MnO4 - disappears as it reacts with ferrous ions (Fe2+) and gets reduced to Mn2+ (colourless). When an extra drop of potassium permanganate is added to the burette, the colour of the solution changes to pale pink. This is the endpoint of the titration.

The titration reaction is given as

Reduction: MnO4 - Mn2+

Oxidation: Fe2+Fe3+

The overall balanced reaction is

MnO4-aq+5Fe2+aq+8H+aqMn2+aq+5Fe3+aq+4H2Oaq

From the given reaction, it is clearly understood that 1 mol of MnO4 - can oxidise 5 mol of Fe2+ to 5 mol of Fe3+. This proves that KMnO4 is a strong oxidising agent. It acts as an oxidising agent in acidic, basic, and neutral media.

Role of H2SO4 in Permanaganate Titration

For acidification, dilute H2SO4 is used. The usage of HCl is avoided as KMnO4 oxidises HCl to Cl2 gas, i.e., the oxidation state of Cl changes from −1 to 0. However, in H2SO4, the oxidation state of S is +6, which is the maximum valency of sulfur (group 16 element). Therefore, KMnO4 cannot oxidise it further. The use of HNO3 is also avoided because it is a powerful oxidising agent.


Practice Problems

Q1. A 0.2 g As2O3 sample required 28.5 mL of potassium permanganate solution for titration in an acidic medium. Calculate the normality of KMnO4 solution (Molecular weight of arsenic oxide = 197.84 amu)

A. 0.35 N
B. 0.14 N
C. 0.28 N
D. 0.70 N

Answer: B
Solution:

We know that in an acidic medium, MnO4-Mn2+

As belongs to group 15. Due to the inert pair effect, it can show +3 as well as +5 oxidation states.

Overall reaction: As2O3+MnO4-As5++Mn2+

n-factor of As2O3= 2×5-3=4

n-factor of MnO4 - = 1×7-2=5

We know that number of equivalent = Normality Volume (L)

Number of equivalent = Mole× n-factor

MoleAs2O3=0.2198 mol=0.001 mol

According to the law of equivalence, Equivalents of MnO4- = Equivalents of As2O3

NMnO4-VMnO4-L=MoleAs2O3×n-factorAs2O3

NMnO4-×28.51000L= 0.001 mol×4

NMnO4-= 0.14 N

So, option B is the correct answer.

Q2. A 0.05 N KMnO4 solution required 55 mL of 0.067 N FeSO4 for complete reaction in an acidic medium. Calculate the volume of KMnO4 solution required.

A. 7.4 mL
B. 740 mL
C. 74 mL
D. None of these
Answer: C

Solution:
We know that in an acidic medium, MnO4-Mn2+

Fe2+ is oxidised to Fe3+

Overall reaction: Fe2+ +MnO4-Fe3+ +Mn2+

n-factor of Fe2+=1×3-2=1

n-factor of MnO4-= 1×7-2=5

We know that Normality = Molarity × n-factor

Number of equivalent = Normality Volume (L)

Number of equivalent = Mole × n-factor

According to the law of equivalence, Equivalent of MnO4- = Equivalent of Fe2+

NMnO4-VMnO4-L=NFe2+×VFe2+L

0.05 N×V L= 0.067 N×551000L

V (L)= 0.074 L = 74 mL

So, option C is the correct answer.

Q3. A solution of 400 mg sample of hydrogen peroxide is titrated with 0.316 g KMnOin the acidic medium. Calculate the purity of H2O2.

A. 42.5%
B. 48.23%
C. 34.67%
D. 28.09%
Answer: A

Solution: Weight of the given H2O2 sample = 400 mg = 4001000g=0.4 g
Let the weight of pure H2O2 be ‘x' g

Molar mass of KMnO=158 g mol-1

Mole of KMnO4=0.316158 mol

We know that in an acidic medium, MnO4-Mn2+

H2O2 is oxidised to O2.

Overall reaction: H2O2+MnO4-O2+Mn2+

n-factor of H2O= 2×0-1=2

n-factor of MnO4-= 1×7-2=5

Number of equivalent = Normality Volume (L)

Number of equivalent = Mole ×n-factor

According to law of equivalence, Equivalent of KMnO4 = Equivalent of H2O2

Mole of KMnO4 × n-factor of KMnO4=Mole of H2O2 × n-factor of H2O2

0.316158 × 5=Mole of H2O2 × 2

Mole of H2O2 =0.005

Molar mass of H2O2=34 g mol-1

Weight of H2O2= 34 ×0.005 g =0.17 g

% purity of sample = weight of pure H2O2weight of sample×100=0.170.4×100=42.5%

So, option A is the correct answer.

Q4. Why do we heat the oxalic acid solution before beginning the redox titration of KMnO4 vs. oxalic acid?

Solution: In the titration of potassium permanganate versus oxalic acid, the oxalic acid solution is heated as it takes much longer to react without it and requires much more energy. Therefore, the temperature must be raised in order to boost energy, and this can only be done when the oxalic acid solution is heated.


Frequently asked questions-FAQs

Q1. Towards the end of the titration, why does the colour of KMnO4 fade away over time?
Answer:
This can be explained by a common titration. The solution's colour changes from colourless to light pink at the endpoint of the redox titration of FeSO4 with KMnO4. The majority of laboratory manuals and literature instruct us to wait 30 seconds before determining if the light pink colour has persisted. The endpoint is reached if the colour remains after 30 seconds. The titration is not complete if the colour fades before 30 seconds, so more KMnO4 should be added. It is believed that by the endpoint, all of the reducing agent has been used up. KMnO4's colour thus remains. The colour of KMnO4 must persist for more than 30 seconds before we can declare the endpoint to have been met. This implies that even if the endpoint has been achieved, the colour of the KMnO4 may fade after 30 seconds.

Q2. What are Acid-base indicators?
Answer: Indicators are generally organic weak acids or weak bases which show different colours in ionised and unionised forms or different colours in different pH ranges.

E.g. Phenolphthalein is a weak organic acid (phenolphthalein in an ionised form is pink coloured and in an unionised form is colourless)

Methyl orange (MeOH) is a weak base. In the ionised form, it is red and in the unionised form it is yellow.

Q3. Is KMnO4 combustible in nature?
Answer: No, KMnO4 is not combustible in nature but it enhances the combustion of other substances. It gives off irritating fumes in a fire.

Q4. Is potassium permanganate safe to eat?
Answer: No, it is not safe to consume potassium permanganate. The upper gastrointestinal system may be damaged if potassium permanganate is consumed. It can also produce systemic side effects including adult respiratory distress syndrome, coagulopathy, hepatic-renal failure, pancreatitis, and even death in extreme situations.


Related topics:

Strength of Oleum

Strength of Solution

Volume Strength of H2O2

Density

Mole

ppm

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