Potassium Permanganate: Formula, Structure, Uses, How It's Done and Properties

He potassium permanganate is an inorganic compound formed by manganese - transition metal group 7 (VIIB) -, oxygen and potassium. It is a dark purple vitreous solid. Their aqueous solutions are also dark purple; these solutions become less violet as they are diluted in larger amounts of water.

Commercially it is obtained in presentations of tablets, crystals and dust. 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, sugar is produced, for example.

The 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, for each K cation ⁺ there is a MnO anion ₄ ^- interacting with this

Structure

The crystal structure of KMnO is represented in the upper image ₄ , which is orthorhombic. The purple spheres correspond to the K cations ⁺ , whereas the tetrahedron formed by the four red spheres and the bluish sphere corresponds to the anion MnO ₄ ^- .

Why does the anion have a tetrahedral geometry? Your Lewis structure answers this question. The dotted lines mean that the double bonds are resonating between the Mn and the O. To be able to adopt this structure, the metal center must have a hybridization sp ³ .

Since manganese lacks pairs of electrons without sharing, the Mn-O bonds are not pushed to 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 crystal arrangements.

Applications

Medicine and veterinary

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

Due to its harmful action, potassium permanganate should be used at low concentrations (1: 10000), which limits the effectiveness of its action.

It is also used in the treatment of parasitic diseases of fish in aquariums that produce infections of the gills and ulcers of the skin.

Water treatment

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

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

Preservation of fruits

The potassium permanganate removes by oxidation the ethylene generated in the banana during its storage, allowing it to remain for more than 4 weeks without maturing, 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 permanganate's ability to start the fire, it is used to create fire breaks in forest fires.

Redox Titrant

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

Reagent in organic synthesis

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

Also, in solution of sulfuric acid with chromic acid (H ₂ CrO ₄ ) 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, by transforming the side chain R (for example, a CH ₃ ) in a COOH group.

Historical uses

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

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

How is it done?

The mineral pirolusite 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 the pyrolusite with NaOH and then with KOH, producing the so-called Condy crystals; that is, potassium permanganate.

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

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

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, the 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, KOH must be generated from K ₂ CO ₃ .

Properties

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

It is poorly soluble in water (6.4 g / 100 ml at 20 ° C), which indicates that water molecules do not greatly solvate MnO ions ₄ ^- , because perhaps their tetrahedral geometries require a lot of water for their dissolution. In the same way, it can also be dissolved in methyl alcohol, acetone, acetic acid and pyridine.

Decomposition

It decomposes at 240 ° C, 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 the same, to the maximum amount of electrons that can be lost in an ionic way. In turn, the electronic configuration of manganese is 3 d ⁵ 4 s ² ; therefore, in the potassium permanganate the entire valence shell of the manganese atom is"empty".

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

References

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