What is the difference between potassium dichromate and potassium permanganate?
Potassium dichromate and potassium permanganate are essential in chemistry and help students understand various practical and theoretical applications related to this topic.
What is Potassium Dichromate and Potassium Permanganate in Chemistry?
A potassium dichromate and potassium permanganate pair refers to two different strong oxidizing agents commonly used in laboratories for redox titration, qualitative analysis, and industrial processes. This concept appears in chapters related to transition elements, redox reactions, and volumetric analysis, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The molecular formula of potassium dichromate is K2Cr2O7, and for potassium permanganate it is KMnO4. Potassium dichromate consists of two potassium ions and one dichromate anion, while potassium permanganate contains one potassium ion and one permanganate anion. Both are classified as inorganic transition metal salts.
| Property | Potassium Dichromate | Potassium Permanganate |
|---|---|---|
| Molecular Formula | K₂Cr₂O₇ | KMnO₄ |
| Structure | Ionic – K⁺ ×2, Cr₂O₇²⁻ (Cr in +6 state) | Ionic – K⁺, MnO₄⁻ (Mn in +7 state) |
| Appearance | Orange-red crystals | Deep purple crystals |
Preparation and Synthesis Methods
Potassium dichromate is prepared from chromite ore (FeCr2O4). The ore is roasted with potassium or sodium carbonate and air to form sodium chromate, which is then acidified and reacted with KCl to yield K2Cr2O7. Potassium permanganate is synthesized by first oxidizing manganese dioxide (MnO2) with KOH and an oxidizing agent, yielding potassium manganate (K2MnO4), which is further oxidized to KMnO4 through electrolytic or chemical methods.
Physical Properties of Potassium Dichromate and Potassium Permanganate
Potassium dichromate forms bright orange, triclinic crystals, is soluble in water, and is non-hygroscopic. Potassium permanganate appears as dark purple, glistening crystals, is also water soluble, and turns solution pink or purple. Both are odorless and have high melting points. KMnO4 can act as a self-indicator due to its strong color.
Chemical Properties and Reactions
Potassium dichromate (K2Cr2O7) is a strong oxidizing agent, especially in acidic conditions—oxidizing iodide to iodine and alcohols to acids. Potassium permanganate (KMnO4) is even stronger, reacting in neutral, acidic, or alkaline media. On heating, K2Cr2O7 decomposes to potassium chromate and chromium trioxide, while KMnO4 decomposes to potassium manganate (K2MnO4), manganese dioxide (MnO2), and O2 gas.
Frequent Related Errors
- Mixing up which compound is a self-indicator (it is only potassium permanganate).
- Confusing color changes in redox titrations (KMnO4: purple to colorless, K2Cr2O7: requires external indicator).
- Assuming both are equally strong oxidizers—KMnO4 is stronger.
- Forgetting the toxicity and safety protocols, especially with K2Cr2O7.
Uses of Potassium Dichromate and Potassium Permanganate in Real Life
Potassium dichromate is used in cement, leather tanning, dyes, and photographic processes. Potassium permanganate is used as a disinfectant in water treatment, in organic synthesis, and as a laboratory titrant for redox reactions. Both are important for analytical chemistry experiments and industrial applications.
Relation with Other Chemistry Concepts
Potassium dichromate and potassium permanganate are closely linked to redox reactions and oxidizing and reducing agents. Understanding their properties helps in d and f block element chemistry and chemical indicators used in titrations.
Step-by-Step Reaction Example
1. Conversion of alcohol to acid using potassium dichromate2. In each step, the alcohol is oxidized while the orange dichromate ion is reduced to green chromium(III) ions.
Lab or Experimental Tips
Remember that only potassium permanganate is a self-indicator in titrations. For potassium dichromate, always use an external indicator like diphenylamine. Vedantu educators often highlight the stunning color changes as an easy way to track redox endpoints in exam problems.
Try This Yourself
- Write the IUPAC names for both potassium dichromate and potassium permanganate.
- State whether KMnO4 is a primary or secondary standard.
- Give two uses of potassium dichromate in industry.
Final Wrap-Up
We explored potassium dichromate and potassium permanganate—their formulas, structures, color clues, uses, and reaction principles. For more tips, color codes, and exam tricks, check out live concept sessions and notes on Vedantu.
FAQs on Potassium Dichromate and Potassium Permanganate: Properties, Uses, and Differences
1. What are the uses of potassium dichromate and potassium permanganate?
Potassium dichromate and potassium permanganate are widely used as strong oxidizing agents in chemistry.
Main uses include:
• Redox titrations for analytical chemistry
• Water purification and disinfection
• Organic synthesis (oxidation of alcohols and alkenes)
• Laboratory reagents for various chemical analyses
• Industrial uses in dye, tanning, and bleaching processes
2. Why are potassium dichromate and potassium permanganate used in redox titrations?
Potassium dichromate and potassium permanganate are used in redox titrations because:
• They have well-defined and strong oxidizing properties
• Produce distinct color changes at the endpoint (orange for dichromate, purple for permanganate)
• Allow accurate and clear detection of titration completion
3. Is potassium dichromate a primary standard substance?
Yes, potassium dichromate is a primary standard substance because:
• It is highly pure, stable, and non-hygroscopic
• Has a known composition and does not readily decompose
• Can be weighed accurately for standard solution preparation
4. Which is the stronger oxidizing agent: potassium dichromate or potassium permanganate?
Potassium permanganate (KMnO4) is the stronger oxidizing agent compared to potassium dichromate (K2Cr2O7).
• KMnO4 has a higher oxidation potential
• Efficient in both acidic and neutral/alkaline media
5. What is the color change observed during titration using potassium permanganate?
Potassium permanganate is purple in color.
• During titration, the solution loses color when reduced, turning colorless at the endpoint.
• A faint pink or first permanent purple color indicates the endpoint.
6. What is the action of heat on potassium permanganate?
When heated, potassium permanganate decomposes to form potassium manganate, manganese dioxide, and oxygen gas.
Reaction: 2 KMnO4 → K2MnO4 + MnO2 + O2 (gas)
7. Can potassium dichromate be used as a self-indicator in titrations?
No, potassium dichromate cannot act as a self-indicator.
• An external indicator like diphenylamine or N-phenylanthranilic acid is required for endpoint detection, unlike potassium permanganate, which is a self-indicator.
8. Why is potassium permanganate not usually used as a primary standard?
Potassium permanganate is not suitable as a primary standard because:
• It is not sufficiently pure and may contain impurities
• It is unstable and decomposes slowly in solution
• Needs standardization before use in volumetric analysis
9. What is the difference between potassium dichromate and potassium permanganate in terms of color?
Potassium dichromate (K2Cr2O7) is orange in color, while potassium permanganate (KMnO4) is deep purple.
• This color difference aids in identifying and using them in experiments.
10. How are potassium dichromate and potassium permanganate prepared in the laboratory?
Preparation steps:
• Potassium dichromate: Obtained from chromite ore by oxidation and then reacting with potassium carbonate.
• Potassium permanganate: Made by oxidizing potassium manganate with either chlorine, ozone, or through electrolytic oxidation.
11. What are the environmental and safety concerns associated with potassium dichromate and potassium permanganate?
Both compounds are hazardous.
• Potassium dichromate is toxic, carcinogenic, and harmful to skin/eyes
• Potassium permanganate can irritate and stain, and in excess, contaminate water
• Always handle with gloves in a well-ventilated lab and dispose responsibly
12. What is the role of potassium manganate in the preparation of potassium permanganate?
Potassium manganate (K2MnO4) is an intermediate formed during the oxidation of manganese dioxide.
• Further oxidation of potassium manganate produces potassium permanganate.
