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Properties of Potassium Dichromate with Reactions & Uses
Properties of Potassium Dichromate
Potassium dichromate is one of the most important inorganic compounds studied in chemistry. It appear mostly in redox reactions, laboratory tests and industrial processes. The compound is known for its bright orange colour and strong oxidising nature. Because of this property, potassium dichromate is widely used in chemical laboratories. Even though it look simple as a salt, it shows many interesting physical and chemical behaviours.
In daily chemistry learning, potassium dichromate helps students to understand oxidation, reduction, colour change reactions and acidic medium reactions. It is also used in many qualitative analysis tests. The compound is toxic, so its handling must be careful. Still, because of its strong chemical nature, it is very useful. Studying properties of potassium dichromate helps in understanding chromium chemistry in a deeper way.
Chemical Formula and Nature
- Chemical formula: K₂Cr₂O₇
- It is a salt of potassium and dichromate ion
- Contains chromium in +6 oxidation state
- Strong oxidising agent
- Inorganic ionic compound
Potassium dichromate exists as crystalline solid and dissolves in water to give an orange coloured solution.
Physical Properties
Potassium dichromate shows many clear physical properties which help in identifying it easily.
Colour and Appearance
- Orange to reddish-orange crystalline solid
- Colour is very bright and noticeable
- Crystals are hard and shiny
This colour is due to the presence of dichromate ion.
Solubility
- Soluble in water
- Solubility increases with temperature
- Insoluble in alcohol
When dissolved in water, it forms an orange coloured solution. In acidic medium, the colour remains orange.
Melting Point
- High melting point
- Decomposes on strong heating
- Does not melt easily like organic compounds
Density
- Relatively high density
- Heavier than common salts like NaCl
Chemical Properties
Chemical properties of potassium dichromate are more important than physical ones. Most reactions are based on its oxidising nature.
Oxidising Property
Potassium dichromate is a strong oxidising agent, especially in acidic medium.
- Acts as oxidant in presence of dilute sulphuric acid
- Chromium changes from +6 to +3 oxidation state
- Colour changes from orange to green
This colour change is very important in experiments.
Oxidation of Ferrous Ions
- Fe²⁺ gets oxidised to Fe³⁺
- Dichromate ion gets reduced
- Used in titration reactions
This reaction is used in redox titrations.
Reaction with Sulphur Dioxide
- SO₂ is oxidised to sulphate
- Potassium dichromate reduced
- Colour change observed
This reaction is used to test sulphur dioxide presence.
Reaction with Hydrogen Sulphide
- H₂S oxidised to sulphur
- Yellow sulphur precipitate formed
- Dichromate reduced to chromium (III)
A strong smell of sulphur was also observed.
Reaction with Alcohols
Potassium dichromate oxidises alcohol in acidic medium.
- Primary alcohol → aldehyde → acid
- Secondary alcohol → ketone
- Tertiary alcohol → no reaction
The color change from orange to green confirms the reaction.
Reaction with Hydrochloric Acid
- Produces chromyl chloride vapours
- Red-brown fumes observed
- Used in confirmatory test for chloride ion
This reaction is a very important analytical test.
Reaction with Bases
- Potassium dichromate reacts with alkali
- Converts into potassium chromate
- Colour changes from orange to yellow
This shows interconversion between chromate and dichromate ions.
Acidic Behaviour
- Potassium dichromate behaves acidic salt
- Shows oxidising nature only in acidic medium
- Sulphuric acid commonly used
Without acid, oxidising power reduces.
Thermal Decomposition
On heating strongly:
- Decomposes to potassium chromate
- Chromium oxide formed
- Oxygen gas released
This reaction shows instability at high temperatures.
Redox Behaviour
- Always act as oxidising agent
- Never acts as reducing agent
- Accepts electrons easily
Because chromium is already in a high oxidation state.
Analytical Properties
Potassium dichromate is widely used in analytical chemistry.
As Primary Standard
- High purity available
- Stable solid
- Does not absorb moisture easily
Used in redox titrations.
Indicator Use
- Colour change acts as self indicator
- No external indicator needed sometimes
Industrial Properties and Uses
Due to its properties, potassium dichromate is used in many industries.
- Manufacture of chromium compounds
- Dye and pigment industry
- Leather tanning
- Photography
- Match industry
- Wood preservation
- Cleaning laboratory glassware
Strong oxidising nature helps in many processes.
Toxic Nature
- Highly toxic compound
- Harmful if swallowed
- Causes skin irritation
- Carcinogenic in nature
Handling must be careful, gloves should be used.
Environmental Impact
- Dangerous to water bodies
- Pollutes soil and water
- Disposal must be controlled
Because of chromium (VI) toxicity.
Storage Properties
- Stored in airtight containers
- Away from organic materials
- Kept dry
Moisture can cause unwanted reactions.
Comparison with Potassium Chromate
| Potassium Dichromate | Potassium Chromate |
|---|---|
| Dichromate is orange | Chromate is yellow |
| Dichromate stable in acidic medium | Chromate stable in basic medium |
| Both interconvert depending on pH. | Both interconvert depending on pH. |
Examples of Reactions
- Oxidation of Fe²⁺
- Oxidation of alcohols
- Chromyl chloride test
- Redox titrations
- Conversion to chromate
Common Student Mistakes
- Forgetting acidic medium
- Confusing chromate and dichromate
- Wrong colour change
- Writing incorrect oxidation states
- Mixing with reducing agents carelessly
Understanding concepts avoids mistakes.
Summary
Potassium dichromate is an important inorganic compound with strong oxidising properties. Its bright orange colour, high stability and redox behaviour make it useful in laboratories and industries. It mainly acts as oxidising agent in acidic medium and undergoes reduction from chromium (VI) to chromium (III). The compound shows many characteristic reactions like oxidation of alcohols, sulphur compounds and ferrous ions. It is also used as the primary standard in titrations. However, due to its toxic nature, it must be handled carefully. Studying its properties gives a clear understanding of redox chemistry and chromium compounds.
FAQs
Why is potassium dichromate orange?
Due to dichromate ions which absorb visible light in the orange region and reflect orange colour.
Why is acidic medium required?
Oxidising power increases in acidic medium and reduction becomes easier.
Is potassium dichromate a reducing agent?
No, it always acts as an oxidising agent only.
What colour change occurs in reaction?
Orange to green colour change observed.
Is potassium dichromate dangerous?
Yes, it is toxic and harmful to health.
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