Potassium Permanganate: Structure, Preparation, Uses, Chemical & Physical Properties

Water purification techniques have long sought the attention of researchers. It is actually one of the primary reasons which drive scientific advancements. 

So, naturally, it comes as no surprise that humankind has come up with different solutions to overcome such adversity. Would you like to know what was one of the earliest methods to purify water? 

The answer is potassium permanganate (KMnO₄).

Not only is the compound used to purify water but has tons of uses. Where could you have seen this compound?

When you entered your chemistry lab, you may have seen beakers with a dark purple liquid solution. It looks so beautiful, teachers ask you to do titrations using this purple solution. This compound has long fascinated researchers in regards to its versatility and its potential applications. We will study more about this coloured compound and see how useful is this in chemistry.

Table of content: 

• Discovery of KMnO₄
• Structure of KMnO₄
• Preparation of KMnO₄
• Physical properties of KMnO₄
• Chemical properties of KMnO₄
• Uses of KMnO₄
• Practice problems
• Frequently asked questions-FAQ 

Discovery of  KMnO₄: 

The first indications of KMnO₄ were discovered when a German-Dutch chemist named Johann Rudolf Glauber began his investigations by mixing two compounds: pyrolusite (MnO₂), a manganese dioxide and potassium carbonate mineral. He turned the combination into a green solution by dissolving it in water. This was manganese potassium. The colour of the solution gradually changed from blue to violet and then crimson. This experiment was the first time KMnO₄ was produced.

Structure of KMnO₄

KMnO₄ is an inorganic ionic compound that consists of K⁺ ion and MnO₄^- ion. The manganese atom is attached to four oxygen atoms by three double bonds and one single bond in the permanganate anionMnO₄^-. Manganese in this salt has an oxidation state of + 7. Hybridisation of MnO₄^- ion comes out to be d³s (orbitals utilised are 3dxy, 3dyz, 3dzx and 4s).

d³s hybridisation gives tetrahedral geometry. Hence, Tetrahedral geometry of MnO₄^- can be represented as

Structure of KMnO₄ in the above structure, the bonds between (Mn) and the oxygen atoms have a partial double bond character, and this is because of resonance present here.

Preparation of KMnO₄

• 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^-.

• Peroxodisulphate oxidises a manganese (II) ion salt to permanganate in the laboratory.
  
• Potassium manganate can be oxidised with chlorine or disproportionated in acidic environments. Chlorine oxidation is a chemical reaction that occurs in the human body.

Physical properties of KMnO₄

• 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 g cm^-3 
• It has a melting point of 240 ℃ (464 ⁰F or 513K).
• It is used as an oxidizing agent in the chemical synthesis of many important compounds.

Chemical Properties of KMnO₄

The following are some of the most notable reactions of KMnO₄:
1. In an acidic environment: Potassium iodide is broken down to give iodine.

2. In slightly alkaline or neutral solutions:

• The oxidation of iodide to iodate is a noteworthy reaction:
• Thiosulphate is almost completely oxidised to sulphate:
• Fe²⁺ ion (green) is converted to Fe³⁺  (yellow):
  

Uses: 

• Potassium permanganate is a popular oxidant in preparative organic chemistry as well as in analytical chemistry. 
• It's bleaching of wool, cotton, silk, and other textile fibres, as well as the decolourization 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.

Effects on Health:

• In a concentrated form, potassium permanganate is an irritant to human eyes and skin. It can react with many reducing agents or organic material but it is inflammable.
• The antibacterial action of KMnO₄ is dependent on the oxidation of proteins of bacteria or tissues by this compound. It leaves a stain on the skin or tissues. Since it acts by destructive oxidation process on all organic matter, its use is restricted for external purposes only.
• Potassium permanganate acts as an antidote for barbiturates, chloral hydrate, and alkaloidal poisoning. A solution of 1:5000 of permanganate when used as a gastric wash, oxidizes poison and prevents its absorption.
• This compound is usually stored in tightly closed containers. Potassium permanganate should be handled with care since an explosion may occur when it comes in contact with readily oxidizable substances.

Practice problem: 

Q1. What is the coefficient of sulphate ions when sulphite reacts with permanganate ions?
A. 6

B. 5

C. 4

D. 3

Answer: B
Solution: Sulphurous acid or sulphite is converted to sulphate or sulphuric acid with a reaction with permanganate ion. Below is the required balanced reaction:

Q2.  Example of redox titration is

A. Titration of potassium permanganate against oxalic acid

B. Titration of silver nitrate against ammonium thiocyanate 

C. Titration of acetic acid against sodium hydroxide

D. Titration of calcein against EDTA

Answer: A
Solution: A general example of redox titration is demonstrated by the titration of potassium permanganate (KMnO₄) against oxalic acid (C₂H₂O₄).

Q3. Represent a reaction in the acidic medium in which KMnO₄ acts as oxidising agent.
Solution: The following equation can be used to represent the oxidising activity of KMnO₄ in an acidic medium:

Q4. Potassium permanganate is used as a self indicator because of __________

A. It behaves as a strong oxidising agent

B. It breaks down into ions easily

C. It gets disproportionated in acidic medium

D. It undergoes a chemical change resulting in colour difference. 

Answer: D
Solution: Potassium permanganate has a rich violet colour that is extremely visible to the human eye, and when it is titrated, there is a noticeable loss of colour.

Frequently asked questions-FAQ

Question1. Potassium permanganate acts as a preservative for fruits. How?

Answer. Because the permanganate inhibits ripening by oxidising ethylene, the fruit can last up to four weeks without refrigeration and thus it acts as a preservative.

Question2. 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.

Question3. 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.

Question4. How do one can get out from KMnO₄ stains present on nails or clothes?

Answer. To get KMnO₄ stains removed, cut a lemon, and treat the affected region of the skin, clothes or nails with the lemon peel. It will work like magic and completely remove the discolouration.

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