Potassium Dichromate and Potassium Permanganate

What is Potassium Dichromate?

Potassium dichromate, better known as anhydrochromate, can be prepared by applying neutral yellow chromate of potassium to the solution, which is a moderate volume of one of the stronger acids. Commercially, potassium permanganate is prepared by mixing the potassium hydroxide solution and powdered manganese oxide with oxidizing agents such as potassium chlorate.

Preparation of Potassium Dichromate – K2Cr2O7

Potassium dichromate is an essential chemical that is used in industries as an oxidizing agent and for the preparation of several other compounds.

  • Usually, dichromates are prepared from chromates, and this is obtained with the combination of chromite ore, including potassium or sodium carbonate, in the presence of air.

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The chemical reaction can be given as follows:

$4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \to 8Na_2CrO_4 + 2Fe2O_3 + 8CO_2$

The sodium chromate $(Na_2CrO_4)$ solution is further purified with sulphuric acid to produce a solution, where the crystals of orange-coloured sodium dichromate $(Na_2Cr_2O_7.2H_2O)$ is extracted.

$2Na_2CrO_4 + 2H^{+} \to Na_2Cr_2O_7 + 2Na^{+} + H_2O$

Now, the potassium dichromate is obtained by reacting a sodium dichromate solution with potassium chloride. The chemical reaction can be given as follows:

$Na_2Cr_2O_7 + 2KCl \to K_2Cr_2O_7 + 2NaCl$

Therefore, we can finally obtain the orange crystals of potassium dichromate.

Structure of Potassium Dichromate Molecules

Potassium dichromate is described as an ionic compound that contains one dichromate anion and two potassium cations. The coordination geometries present around the chromium atoms are tetrahedral. Potassium dichromate’s structure molecule can be illustrated as follows:

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It should also make a note that in the potassium dichromate molecule, potassium shows $a +1$ oxidation state, and oxygen shows a -2 oxidation state, and chromium shows $a +6$ oxidation state. It should also make a note that the potassium dichromate crystals have a triclinic structure.

Preparation of Potassium Permanganate – KMnO4

We can get this Potassium Permanganate $(KMnO_4)$ by reacting the $MnO_2$ element with an alkali metal hydroxide and the $KNO_3$ (oxidizing agent).

This will further result in the dark green $K_2MnO_4$ production, which is disproportionate in a neutral or acidic medium to produce permanganate.

$3MnO_{4}^{2-} + 4H^{+} \to 2MnO_{4}^{-} + MnO_2 + 2H_2O$

$2MnO_4 + 4KOH + O_4 \to 2K_2MnO_4 + 2H_2O$

Thus, the potassium permanganate preparation involves a reaction of $MnO_2$ with $KOH$ to produce $MnO_{4}^{2-}$ followed by electrolytic oxidation of manganate to produce permanganate ion, $MnO_{4}^{2-}$.

After this manganese ion, the salt will be oxidized by peroxodisulphate to permanganate ion according to the reaction given as follows:

$2Mn^{2+} + 5S_2O_{8}^{2-} + 8H_2O \to 2MnO_{4}^{-} + 10SO_{4}^{2-} + 16H^{+}$

Hence, the potassium permanganate’s dark purple coloured crystals are obtained. The chemical reaction for the same can be given as follows:

$2KMnO_4 \to K_2MnO_4 + MnO_2 + O_2$

Properties of K2Cr2O7 & KMnO4

Properties of Potassium Dichromate, $K_2Cr_2O_7$

  • Potassium dichromate, on heating, decomposes to produce the chromic oxide, oxygen, and potassium chromate. The chemical reaction can be given as follows:

$4K_2Cr_2O_7 \to 4K_2CrO_4 + 2CrO_3 +3O_2$

  • It is described as a powerful oxidizing agent, and it oxidizes iodide to iodine. The chemical reaction can be given as follows:

$Cr_2O_{7}^{2-} + 14H^{+} + 6l^{-} \to 2Cr^{3+} + 7H_2O + 3l_2$

  • The compound contains bright red crystals, and it can be used for staining, tanning, dyeing, and more.

  • For medical purposes, it can be antiseptic, caustic, and astringent. When heated, it emits toxic chromium fumes.

  • It is a strong oxidizing agent and highly corrosive for which it can be used in wood preservatives, manufacturing of pigments, and photochemical processes.

Properties of Potassium Permanganate, KMnO4

  • Potassium permanganate, when heated, produces manganese dioxide, oxygen, and potassium manganate. The chemical reaction can be given as follows:

$2KMnO_4 \to K_2MnO_4 + MnO_2 + O_2$

  • It can be defined as a powerful oxidizing agent in alkaline or acidic solutions.

  • It is noncombustible but works as a catalyst in the burning of combustible materials.

  • It is in purple crystals form and is soluble in hot water.

  • In high concentration, it can become toxic.

  • When mixed with glycerine, it will start a fire.

Applications of Potassium Dichromate

The major application of $K_2Cr_2O_7$ is in the potassium chrome alum preparation, a compound, which is extensively used in leather tanning. Chromic acid is also prepared from this compound. Also, potassium dichromate is well-known to be used in cement production since it improves the texture and the cement mixture density. 

Another important and major application of potassium dichromate is given in the industry of photography, where it can be used in combination with a powerful mineral acid as an oxidizing agent in photographic screen printing. Since it is non-hygroscopic in nature, potassium dichromate is also employed for many wet tests in the analytical chemistry field.

Use of Potassium Dichromate

It can be used in several applications as an oxidizing agent and also used in the different product preparation such as paints, waxes, glues, and more items. Potassium dichromate is highly toxic and carcinogenic as a hexavalent chromium compound.

FAQs (Frequently Asked Questions)

1. What does the potassium dichromate test use?

Potassium dichromate is an oxidizing agent for organic chemistry that is milder than potassium permanganate. It can be used for alcohol oxidation, where this converts the primary alcohols into aldehydes and carboxylic acids under the conditions having more pressure.

2. Is potassium dichromate light-sensitive?

Potassium dichromate is light-sensitive and clear orange crystals. It can be used as chromium mordant in cotton dyeing. Potassium dichromate can also be used as an intensifier in black and white image processing.

3. What is the charge of potassium dichromate?

The molecular formula of potassium dichromate can be given as K2Cr2O7. A few of the physical properties of potassium dichromate are: it is a solid reddish-brown colour having a molecular weight of 294.18 grams per mole. This compound is also referred to as a compound of chromium oxidation and hexavalent chromium.

4. Why is KMnO4 defined as a self indicator?

The solution loses its pink colour after all the permanganate ions are used up in the chemical reaction. It is meant by the end of the reaction, and thus potassium permanganate is known as self-indicator as it acts as an indicator besides being one of the reactants.