Potassium Dichromate Structure Properties and Reactions

In chemistry, an oxidizing agent is a type of substance that has the ability to oxidize other substances. You may wonder what oxidation means. Oxidation is a phenomenon where a substance can accept electrons from other substances. Some of the most common oxidizing agents are halogens, hydrogen peroxide, oxygen, ozone, chromium hexavalent compounds etc.

Potassium Dichromate is a powerful oxidizing agent. Their typical applications include Leather tanning and screen printing, Metal Treating and corrosion inhibitor, Pyrotechnics and explosives, Photographic engraving, Chromium source in preparing chromium compounds, Pigment preparation, Oil drilling, Electroplating, as a Catalyst for the chromium metal production, and as Wood Preservative. A chemical compound that has a chromium element in its +6oxidation state is known as Hexavalent chromium (Chromium +6, Cr (VI), chromium (VI)). They are considered to be highly toxic as they are genotoxic carcinogens.

What is Potassium Dichromate?

A hexavalent chromium compound that acts as a common inorganic chemical reagent is called potassium dichromate. The chemical formula of potassium dichromate is K₂Cr₂O₇. It is used in many industrial applications and laboratories as a conventional oxidizing agent. Being hexavalent, potassium dichromate is highly toxic in nature and harmful to the skin and body. Potassium dichromate is a crystalline ionic solid with a very bright red-orange colour. Potassium dichromate is more popular in laboratory experiments as it is not deliquescent (a tendency to absorb air moisture and dissolve in it) as compared to most industry-relevant sodium dichromate salts. A reaction of potassium chloride with sodium dichromate gives potassium dichromate. 

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Alternatively, it can be obtained from potassium chromate through roasting chromite ore with potassium hydroxide. It ionizes in water: 

K₂Cr₂O₇ → 2K+ + Cr₂O₇2−

Cr₂O₇^2- + H₂O ⇌ 2CrO₄^2- + 2H⁺

This reaction when performed in the laboratory gives orange-red crystals of potassium dichromate. It is soluble in water. It does not have any distinctive odor. It may, however, severely irritate the eyes and respiratory tract. Avoid contact of potassium dichromate with organic materials. It is noncombustible but most commonly used in pyrotechnic displays along with tungsten and iron.

The Structure of Potassium Dichromate (K₂Cr₂O₇)

The chemical formula of potassium dichromate is K₂Cr₂O₇ with 294.185 g/mol molar mass. It is an ionic compound with two potassium ions (K+) and the negatively charged dichromate ion (Cr₂O₇-), in which two hexavalent chromium atoms (with oxidation state +6) are each attached to three oxygen atoms as well as a bridging oxygen atom.

Figure 1: Properties of Potassium Dichromate (K2Cr2O7)

Properties of Potassium Dichromate (K2Cr2O7)

Physical Properties:

1. It forms orange-red crystals which melt at 396oC.

2. It is easily soluble in hot water but moderately soluble in cold water.

Chemical Properties:

• Action of Heat: On heating, K2Cr2O7 decomposes to give potassium chromate with evolution of O2 gas. 

4 K2Cr2O7 → 4K₂CrO₄ + 2Cr₂O₃ + 3O₂

• Reaction with Alkalis: When an orange-red solution of K2Cr2O7 reacts with an alkali, a yellow solution is obtained because of the potassium chromate formation. 

K₂Cr₂O₇ + 2KOH   →  2 K₂CrO₄ + H₂O

Orange-red            Yellow

• On acidifying the yellow color of K₂CrO₄ again changes to orange-red due to the reformation of K₂Cr₂O₇.

2K₂CrO₄ + H₂SO₄ → K₂Cr₂O₇ + K₂SO₄ + H₂O

• The interconversion can be explained on the basis of the fact that in K₂Cr₂O₇ solution, orange-red Cr₂O₇^2- ions are in equilibrium with yellow CrO₄^2- ions.

• Cr₂O₇^2- + H₂O → 2CrO₄^2- + 2H⁺

Orange red                    Yellow

Oxidization: K₂Cr₂O₇ is known as a powerful oxidizing agent. In presence of dil.H₂SO₄, one mole of this compound gives three atoms of oxygen as shown below.

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

Some other examples of oxidizing properties of K₂Cr₂O₇ are given below:

1. It liberates I₂ from KI.

K₂Cr₂O₇ + 7 H₂SO₄ + 6KI → 4 K₂SO₄+ Cr₂(SO₄)₃ + 3I₂ + 7H₂O

2. It oxidizes ferrous sulfate to ferric sulphate.

K₂Cr₂O₇ + 7H₂SO₄ + 6FeSO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃+ 2H₂O

3. It oxidizes H2S to sulphur.

K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S → K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3S

Chromyl Chloride Test: When salt containing chloride is treated with K₂Cr₂O₇ and con. H₂SO₄   chromyl chloride (reddish brown) vapors are produced. 

K₂Cr₂O₇+ 4KCl + 6 H₂SO₄ → 2CrO₂Cl₂ + 6KHSO₄ + 3H₂O        

Chromyl chloride

The reaction is used to detect chloride ions in qualitative analysis.

Properties of the K₂Cr₂O₇ formula

Uses of Potassium Dichromate (K2Cr2O7) 

Potassium dichromate has widespread industrial use. Some of its uses are listed below:

• K₂Cr₂O₇ is an oxidizing agent for a variety of reactions in laboratories and industries. It is used in the leather industry for chrome tanning by acting as a precursor for potassium chrome alum. 

• It is used in volumetric analysis.

• It is used in dyeing and calico printing.

• In photography, K₂Cr₂O₇ is used with strong mineral acid as an oxidizing agent to harden the gelatin film.

• It is used to prepare chromic acid, which is used to clean glassware (like other chromium (VI) compounds, sodium dichromate, chromium trioxide). However, due to the toxicity of hexavalent chromium, this practice is now discouraged and discontinued.

• K₂Cr₂O₇ finds use in the construction industry as an ingredient in cement.  

• It is a non-hygroscopic reagent and is used in “wet tests” in analytical chemistry.

• It is known as Schwerter’s solution when dissolved in 35 percent nitric acid and used in tests to detect the presence of different metals in determining silver purity. 

• It is used as wood darkening in the wood tannins that produce deep brown colour on wood. It is an effective treatment for mahogany.

Effects on Health

• Potassium dichromate can cause chronic diseases like chromium dermatitis in the hands and forearms. 

• The toxicity of potassium dichromate can cause harm to animals such as rabbits, rodents etc.

• It is toxic to aquatic organisms too and poses a big threat to the environment.

• Potassium dichromate is corrosive in nature and its exposure may develop several eye problems or even blindness.

• It may cause heritable genetic damage, impaired fertility, and is dangerous to unborn children. 

• Potassium dichromate is dangerous to the respiratory tract. It can cause ulcers. An increased risk of lung cancer is associated with potassium dichromate, a known human carcinogen.

• Eating potassium dichromate can cause serious harm to an individual’s health or sometimes even be killed. 

• It may be harmful to touch the material. Systemic effects might occur after absorption.

• Consuming this substance can cause a chemical burn in the mouth and gastrointestinal tract which causes ingestion.

• However, It is rare to become acutely poisoned by eating or swallowing potassium dichromate, because usually vomiting occurs and renal excretion is rapid.

• Subsequently, potassium dichromate causes irregular heartbeats, heart block, and blood pressure drop.

Safety hazards

Potassium dichromate is a hexavalent chromium compound, hence it is highly toxic and carcinogenic. Potassium dichromate is very corrosive by nature and it can cause severe irritation in the eye and skin, burning sensation, and even lead to blindness. It is referred to as affecting reproductive health and it acts as a mutagenic agent i.e. it affects the genetic material and harms unborn children.