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What is potassium permanganate?

The potassium permanganate (KMnO 4)  is an inorganic compound consisting of manganese – metal transition group 7 (VIIB) -, oxygen and potassium. It is a deep purple vitreous solid. Its aqueous solutions are also dark purple; these solutions become less violet as they are diluted in larger amounts of water.

It is commercially available in tablet, crystal and powder forms. It is known by the names of Condy crystals or chameleon minerals. The term “chameleon” refers to the capacity of the mineral: if it changes color in contact with caustic soda and organic matter , for example, sugar is produced.

KMnO 4 then begins to undergo reductions (gain electrons) in a succession of colors in the following order: purple> blue> green> yellow> colorless (with brown precipitate of MnO 2 ). This reaction demonstrates an important property of potassium permanganate: it is a very strong oxidizing agent.

Formula

Its chemical formula is KMnO 4 ; that is to say, that for each K + cation there is an MnO 4 anion  interacting with this

Chemical structure of potassium permanganate

Potassium permanganate
KMnO4 crystal structure

The upper image represents the crystalline structure of KMnO 4 , which is of the orthorhombic type. The purple spheres correspond to the K + cations , while the tetrahedron formed by the four red spheres and the bluish sphere correspond to the MnO  anion .

Potassium permanganate

Since manganese lacks unshared electron pairs, the Mn-O bonds are not pulled into the same plane. Likewise, the negative charge is distributed among the four oxygen atoms, being responsible for the orientation of the K + cations within the crystalline arrangements.

Uses of KMnO 4

Medicine and veterinary

Due to its bactericidal action, it is used in numerous diseases and conditions that cause skin lesions, such as: fungal foot infections, impetigo, superficial wounds, dermatitis and tropical ulcers.

Due to its harmful action, potassium permanganate should be used at low concentrations (1: 10,000), which limits the effectiveness of its action.

It is also used in the treatment of fish parasites in aquariums that cause gill infections and skin ulcers.

Water treatment

It is a chemical regenerant used to remove iron, magnesium and hydrogen sulfide (with an unpleasant odor) from water, and can be used to purify wastewater.

Iron and magnesium precipitate as their water-insoluble oxides. In addition, it helps to remove the rust present in the pipes.

Preservation of fruits

Potassium permanganate removes ethylene generated in bananas during storage by oxidation, allowing it to remain unripe for more than 4 weeks, even at room temperature.

In Africa they use it to soak vegetables, in order to neutralize and eliminate any bacterial agent present.

Action on fire

Potassium permanganate is used to limit the spread of fires. Based on the ability of permanganate to start fires, it is used to create backfires in wildfires.

Redox titrant

In analytical chemistry, its standardized aqueous solutions are used as an oxidizing titrant in redox determinations.

Reagent in organic synthesis

It serves to convert alkenes into diols; that is, two OH groups are added to the C = C double bond. The following chemical equation:

Potassium permanganate

Likewise, in sulfuric acid solution with chromic acid (H 2 CrO 4 ) it is used for the oxidation of primary alcohols (R-OH) to carboxylic acids (R-COOH or RCO 2 H).

Its oxidizing power is strong enough to oxidize the primary or secondary alkyl groups of the aromatic compounds, “carboxylating” them; that is, transforming the R side chain (eg, a CH 3 ) into a COOH group.

Historical uses

It was part of the powders used as a flash in photography or to start the thermite reaction.

It was used in World War II to camouflage white horses during the day. For this they used manganese dioxide (MnO 2 ), which is brown in color; in this way they went unnoticed.

Synthesis of KMnO 4

The mineral pyrolusite contains manganese dioxide (MnO 2 ) and potassium carbonate (CaCO 3 ).

In 1659 the chemist Johann R. Glauber melted the mineral and dissolved it in water, observing the appearance of a green coloration in the solution, which later changed to violet and finally to red. This last color corresponded to the generation of potassium permanganate.

In the mid-nineteenth century Henry Condy was looking for an antiseptic product and initially treated pyrolusite with NaOH and later with KOH, producing the so-called Condy crystals; that is, potassium permanganate.

Potassium permanganate is produced industrially from manganese dioxide present in the mineral pyrolusite. The MnO 2  present in the mineral reacts with potassium hydroxide and is subsequently heated in the presence of oxygen.

2 MnO 2      + 4 KOH + O 2   => 2 K 2 MnO 4      + 2 H 2 O

Potassium manganate (K 2 MnO 4 ) is converted to potassium permanganate by electrolytic oxidation in an alkaline medium.

2 K 2 MnO 4       + 2 H 2 O => 2 KMnO 4       + 2 KOH + H 2

In another reaction to produce potassium permanganate, potassium manganate is reacted with CO 2 , accelerating the disproportionation process:

3 K 2 MnO 4      + 2 CO 2   => 2 KMnO 4       + MnO 2       + K 2 CO 3

Due to the generation of MnO 2 (manganese dioxide) the process is unfavorable, having to generate KOH from K 2 CO 3 .

KMnO 4 properties

It is a purple crystalline solid that melts at 240 ºC, which has a density of 2.7 g / mL, and a molecular weight of approximately 158 g / mol.

It is poorly soluble in water (6.4 g / 100 ml at 20 ºC), which indicates that water molecules do not solvate to a great extent MnO  ions , because perhaps their tetrahedral geometries require a lot of water to its dissolution. Similarly, it can also be dissolved in methyl alcohol, acetone, acetic acid, and pyridine.

Decomposition

It decomposes at 240 ºC, releasing oxygen:

2KMnO 4 => K 2 MnO 4 + MnO 2 + O 2

It can undergo decomposition by the action of alcohol and other organic solvents, as well as by the action of strong acids and reducing agents.

Oxidizing power

In this salt, manganese exhibits its highest oxidation state (+7), or what is equal, to the maximum number of electrons that it can lose ionically. In turn, the electron configuration of manganese is 3 5 4 2 ; therefore, in potassium permanganate the entire valence shell of the manganese atom is “empty.”

So the manganese atom has the natural tendency to gain electrons; that is, to be reduced to other oxidation states in alkaline or acidic media. This is the explanation why KMnO 4 is a powerful oxidizing agent.

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