Introduction to Potassium Dichromate
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−
Cr2O72- + H2O ⇌ 2CrO42- + 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:
It forms orange-red crystals which melt at 396oC.
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 Cr2O72- ions are in equilibrium with yellow CrO42- ions.
Cr2O72- + H2O → 2CrO42- + 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:
It liberates I2 from KI.
K2Cr2O7 + 7 H2SO4 + 6KI → 4 K2SO4+ Cr2(SO4)3 + 3I2 + 7H2O
It oxidizes ferrous sulfate to ferric sulphate.
K2Cr2O7 + 7H2SO4 + 6FeSO4 → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3+ 2H2O
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.
FAQs on Potassium Dichromate
1. What is the chemical formula for potassium dichromate and what is the structure of the dichromate ion?
The chemical formula for potassium dichromate is K₂Cr₂O₇. The structure of the key component, the dichromate ion (Cr₂O₇²⁻), consists of two tetrahedral CrO₄ units that share a single oxygen atom. The Cr-O-Cr bond angle is approximately 126°.
2. How is potassium dichromate prepared commercially from chromite ore?
The commercial preparation of potassium dichromate from chromite ore (FeCr₂O₄) involves three main steps as per the NCERT syllabus for the 2025-26 session:
- Step 1: The chromite ore is fused with sodium carbonate (soda ash) in the presence of air to produce sodium chromate (Na₂CrO₄).
- Step 2: The resulting solution of sodium chromate is filtered and then acidified with sulphuric acid. This converts the sodium chromate into sodium dichromate (Na₂Cr₂O₇).
- Step 3: The solution of sodium dichromate is treated with potassium chloride (KCl). Since potassium dichromate is less soluble than sodium dichromate, it crystallises out as orange-red crystals upon cooling.
3. What are the most important industrial uses of potassium dichromate?
Potassium dichromate is a vital industrial chemical primarily used as a strong oxidizing agent. Its major applications include:
- Volumetric Analysis: It is used as a primary standard in redox titrations, especially for estimating the concentration of ferrous ions (Fe²⁺) and iodides.
- Organic Chemistry: It is used to oxidise primary alcohols to aldehydes and secondary alcohols to ketones.
- Leather Tanning: It plays a crucial role in the chrome tanning process, which strengthens and preserves leather.
- Photography: In older photographic processes, it was used in chrome alum hardeners for gelatin emulsions.
4. Why does an orange solution of acidified potassium dichromate turn green during a redox reaction?
The color change from orange to green is a classic indicator of a redox reaction involving potassium dichromate. The orange color is due to the dichromate ion (Cr₂O₇²⁻), in which chromium has an oxidation state of +6. When it acts as an oxidizing agent in an acidic medium, it accepts electrons and is itself reduced to the chromium(III) ion (Cr³⁺). The Cr³⁺ ion in an aqueous solution is green, causing the solution's color to change.
5. How does a change in pH affect the equilibrium between chromate and dichromate ions?
The chromate (CrO₄²⁻) and dichromate (Cr₂O₇²⁻) ions exist in equilibrium, which is highly sensitive to the pH of the solution.
- In alkaline solutions (high pH), the equilibrium shifts towards the formation of the yellow chromate ion (CrO₄²⁻).
- In acidic solutions (low pH), the equilibrium shifts towards the formation of the orange dichromate ion (Cr₂O₇²⁻).
6. How does potassium dichromate function as a powerful oxidising agent in an acidic medium?
In an acidic medium (in the presence of H⁺ ions), the dichromate ion (Cr₂O₇²⁻) acts as a powerful oxidizing agent by accepting six electrons to get reduced to two Cr³⁺ ions. The standard electrode potential for this reaction is high and positive (+1.33 V), indicating a strong tendency to gain electrons. The ionic half-reaction is:
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
This ability allows it to readily oxidise substances like ferrous salts to ferric salts, iodides to iodine, and sulphides to sulphur.
7. Why is potassium dichromate considered a hazardous chemical?
Potassium dichromate is considered hazardous for several important reasons. Firstly, it is a strong oxidizing agent and can cause fires if it comes into contact with combustible materials. Secondly, it is highly toxic and corrosive, causing severe damage to the skin, eyes, and respiratory tract upon contact or inhalation. Most importantly, compounds containing hexavalent chromium (Cr⁶⁺), like potassium dichromate, are classified as carcinogenic and can cause heritable genetic damage.
8. What are some key physical properties of potassium dichromate?
Some of the key physical properties of potassium dichromate include:
- Appearance: It is a bright, orange-red crystalline solid at room temperature.
- Odour: It is odourless.
- Solubility: It is moderately soluble in cold water and its solubility increases significantly with temperature. However, it is insoluble in alcohol.
- Melting Point: It has a melting point of 398 °C (671 K).
- Nature: It is not deliquescent, meaning it does not absorb moisture from the air to form a solution.
