Potassium Dichromate (K2Cr2O7)

Potassium Dichromate is a chemical compound mainly used as an oxidizing agent. The chemical formula of Potassium Dichromate is represented as K₂Cr₂O₇. It is a hexavalent chromium compound having a bright red-orange color. It serves as an oxidizing agent in various industrial and laboratory applications. It is used to manufacture products such as matches, glass, ceramics, etc.

In this article, we will thoroughly study What is Potassium Dichromate, its structure, its preparation, and its wide range of applications.

What is Potassium Dichromate?

In inorganic chemistry, Potassium Dichromate is majorly used as a chemical reagent in various chemical reactions. It serves as a strong oxidizing agent and is therefore helpful in distinct redox reactions.

• It is a crystalline ionic solid.
• It is odorless and has a bright red-orange color.
• Potassium Dichromate is mainly prepared by chromates which are formed by the reaction of chromite ore with sodium carbonate or potassium carbonate.

Potassium Dichromate has a wide variety of applications and is used in the manufacturing of glass, fireworks, ceramics, etc, and can also act as a catalyst for various metal production.

• It is also used in bone marrow biopsies due to its use in Zenker’s acetic fixative.
• Being non-deliquescent in nature, this salt is majorly used in laboratories.
• However, due to its toxic and corrosive nature, Potassium Dichromate can be hazardous to human health if exposed for a longer period therefore, proper care and precautions must be taken while handling it.

Occurrence of Potassium Dichromate

Potassium dichromate is rarely found in nature as Lopezite. It can be found in nitrate deposits of Chile’s Atacama desert and South Africa’s Bushveld igneous complex.

Potassium Dichromate Colour

Potassium dichromate has a very bright red-orange colour. However, it can differ depending upon its pH value. The colour of potassium dichromate in accordance with its pH value is as follows:

• If pH < 7 (Acidic medium), its colour is orange.
• If pH > 7 (Basic medium), its colour is yellow.

Formula of Potassium Dichromate

The formula of Potassium Dichromate consists of two potassium atoms (K), two Chromium atoms (Cr), and seven oxygen atoms. It consists of two ionic bonds between two positively charged potassium cations and a negatively charged dichromate anion.

The chemical formula of Potassium Dichromate is represented as K₂Cr₂O₇

Structure of Potassium Dichromate

Potassium Dichromate is an ionic compound between two positively charged potassium cations (K⁺) and a negatively charged dichromate anion (Cr₂O₇^–). The dichromate ion consists of two hexavalent chromium atoms which are bonded with seven oxygen atoms (Cr-O bond). Each chromium atom is double-bonded with two different oxygen atoms, single-bonded with one oxygen atom with charge -1, and the last oxygen atom is commonly shared with both chromium atoms. The structure of Potassium Dichromate is given below:

Molecular Weight of Potassium Dichromate

Molecular Weight of Potassium Dichromate(K₂Cr₂O₇) is 294.185 g/mol

Calculation of Molecular Weight

• Molecular Weight of Potassium(K) is 39.08 g
• Molecular Weight of Chromium(Cr) is 51.99 g
• Molecular Weight of Oxygen(O) is 16 g
• Molecular Weight of K₂Cr₂O₇ is (39.08 × 2 + 51.99 × 2 + 16 × 7) = 294.185 g/mol

Equivalent weight of Potassium Dichromate

We can easily calculate the equivalent weight of potassium dichromate with the help of the following formula:

Equivalent Weight = Molecular Weight/Number of electrons

Since the molecular weight of Potassium dichromate is 294.185 g/mol and 6 electrons are gained during redox reaction. By using the equivalent weight formula for potassium dichromate we get:

294.185/6 = 49.03 g/mol

Hence, the equivalent weight of potassium dichromate is 49.03 g/mol.

Preparation of Potassium Dichromate

Potassium Dichromate is mainly synthesized in industries, it is made by reacting potassium chloride (KCl) with sodium dichromate (Na₂Cr₂O₇) which is extracted from chrome iron ore. The process of preparation of Potassium Dichromate is described below:

Step 1: Formation of sodium chromate from chrome iron ore

Sodium carbonate (Na₂CO₃) and quick lime (CaO) are mixed with the powdered form of chrome iron ore to form sodium chromate. This mixture is then heated inside a reverberatory furnace in the presence of sufficient amount of air. This reaction is represented as:

4FeO.Cr₂O₃ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂

Step 2: Conversion of sodium chromate into sodium dichromate

The sodium chromate solution obtained from step 1 is properly filtered to remove iron oxide and other such impurities. Then this purified form of sodium chromate is reacted with concentrated H₂SO₄ to obtain sodium dichromate. The reaction involved here is represented as:

2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step 3: Formation of potassium dichromate from sodium dichromate

The sodium dichromate obtained from step 2 is heated and concentrated and then reacted with potassium chloride KCl, to form potassium dichromate. The reaction involved in this final step is represented as :

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

The potassium dichromate thus obtained is turned into crystals on cooling.

Properties of Potassium Dichromate

Potassium Dichromate has a variety of properties which are classified into physical and chemical properties. The physical and chemical properties of Potassium Dichromate are described below:

Physical Properties of Potassium Dichromate

The physical properties of Potassium Dichromate are Tabulated below:

Physical Properties of K2Cr2O7
State	Crystalline Ionic Solid
Colour	Bright Red-Orange
Odor	odorless
Solubility	Soluble in water, Insoluble in Acetones and Alcohol
Refractive index	1.738
Boiling Point	500° Celsius
Melting Point	398° Celsius
Density	2.68 grams per cubic centimeter
Molecular weight	294.185 g/mol
Nature	non-combustible but highly corrosive

Chemical Properties of Potassium Dichromate

The chemical Properties of K₂Cr₂O₇ can be best understood in the terms of chemical reaction it undergoes. Hence, the chemical reactions of Potassium Dichromate are mentioned below:

Reaction due to Heat

On heating, Potassium Dichromate decomposes to give out potassium chromate with the evolution of oxygen (O₂) gas. The chemical reaction involved is as follows:

4 K₂Cr₂O₇ → 4K₂CrO₄ + 2Cr₂O₃ + 3O₂

Oxidation Reaction

Being an oxidizing agent, each mole of potassium dichromate liberates three atoms of oxygen when reacted with dilute H₂SO₄, this reaction is illustrated below:

K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂ (SO4)₃+ 4H₂O + 3(O)

Reaction with Alkalies

When Potassium Dichromate reacts with alkalies, potassium chromate is formed due to which the orange-red colour of K₂Cr₂O₇ changes into a yellow colour. It is represented by the following chemical reaction:

K₂Cr₂O₇ + 2KOH → 2K₂CrO₄ + H₂O

This yellow-coloured potassium chromate changes back to orange-red coloured potassium dichromate when acidified. The reaction involved here is:

2K₂CrO₄ + H₂SO₄ → K₂Cr₂O₇ + K₂SO₄ + H₂O

This process of conversion of potassium dichromate into potassium chromate and then conversion of potassium chromate back to potassium dichromate is due to equilibrium between Cr₂O₇^2- ions and CrO₄^2- ions.

Cr₂O₇^2- + H₂O ⇋ 2CrO₄^2- + 2H⁺

Acidified Potassium Dichromate

Potassium dichromate solution can be acidified by adding dilute sulphuric acid along with a few drops of the aldehyde or ketone. Acidified Potassium Dichromate can produce sulphur, potassium sulphate, chromium(III) sulphate, and water on reaction with hydrogen sulphide. On reaction with SO₂ the orange solution of acidified Potassium Dichromate turns green due to the formation of a green colour compound Cr₂(SO₄)³.

Applications of Potassium Dichromate

Potassium dichromate has a wide variety of applications few of them are:

• It is used as an oxidizing agent in several research facilities and laboratories.
• It also serves as a raw material for producing chromic acid which can be used to clean glassware and as an etching material.
• It is used to increase the hardening time of concrete and also helps in improving the colour, texture, and density of the cement.
• It is widely used in photographic screen printing to harden the gelatin film.
• With solid mineral acid, it acts as a hardening agent for several organic solvents like dye, carbon black, gelatin, and gum Arabic.
• Potassium dichromate is used to check the purity of silver with the help of Schwerter’s solution, which is a solution of 35% nitric acid and Potassium dichromate.
• Potassium dichromate is used to provide a glossy finish and a fine texture to woods.
• It is also used in producing potassium chrome alum.
• Potassium dichromate is used in the leather tanning industry.
• It is also used in bone marrow biopsies due to its use in Zenker’s acetic fixative.
• Being non-deliquescent in nature, it is majorly used in analytical chemistry.

Potassium Dichromate Health Hazards

Being highly corrosive and toxic in nature Potassium Dichromate has various health hazards like:

• It can result into blurred vision, redness, pain, burn and even blindness.
• Prolonged exposure to Potassium Dichromate can cause ulceration of the skin, damage to liver, kidney and various other organs. It can even cause death in certain cases.
• Inhalation of this compound also cause damage to respiratory tract.
• It can cause burns in inner lining of stomach, if consumed in higher amount.
• Exposure to this compound can cause many heart problems like blocking of veins, low blood pressure, irregular heartbeat and even heart attack.
• Skin cancer, Skin burns and various other skin diseases can also be caused due to contact with this compound.
• It is also proved to be fatal for aquatic life as well.

Summary

In this article, we have thoroughly study about Potassium Dichromate and its properties, few things to remember:

IUPAC Name	Potassium dichromate (VI)
Chemical Formula	K2Cr2O7
Melting Point	398 °C
Boiling Point	500 °C
Molecular Weight	294.185 g/mol
Appearance	Bright red-orange crystal
Odor	Odorless
Density	2.68 g/cm3
Refractive Index	1.738

Potassium Dichromate – FAQs

1. Is Potassium Dichromate Acid, Base or Salt?

Potassium Dichromate is a salt formed by the combination of weak acid and weak base.

2. What is the IUPAC name of Potassium Dichromate?

The IUPAC name of potassium dichromate is Potassium dichromate (VI).

3. What are the other names of potassium dichromate?

The various other names of potassium dichromate are:

• Bichromate of potash
• Dipotassium salt
• Potassium bichromate
• Dipotassium dichromate
• Dichromic acid
• Chromic acid

4. Is K₂Cr₂O₇ Soluble in water?

Yes, Potassium Dichromate is soluble in water and its solubility increases with increase in temperature.

5. What is Schwerter’s Solution?

Schwerter’s solution is a solution of 35% nitric acid and Potassium dichromate. It is used to check the purity of silver metal.

6. How does the Color Change occur in Potassium Dichromate?

The colour of potassium dichromate depends upon its pH value.

• If pH < 7 (Acidic medium), its colour is orange.
• If pH > 7 (Basic medium), its colour is yellow.

7. Why is Potassium Dichromate an Oxidizing Agent?

Potassium Dichromate is considered as an Oxidizing Agent as it gains electron from other species and get reduced. It also release nascent oxygen in acidic medium