Vanadium Oxidation States

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Colour Changes, Elements and Periodic Table, Oxidation and Reduction

Alternative titles: Colors of Vanadium

Summary

This experiment demonstrates the multiple oxidation states of vanadium by producing vivid colors (yellow, green, blue, dark-green, and purple) in a single solution. Starting from vanadium pentoxide, vanadium compounds are reduced with zinc and then re-oxidized with potassium permanganate, creating a reversible rainbow of colors.

Procedure

Prepare sodium vanadate by heating vanadium pentoxide and sodium carbonate in water until the solution turns green.
Add concentrated sulfuric acid to the sodium vanadate solution to form yellow vanadyl sulfate (V⁵⁺ state).
Transfer the solution to a flask and heat gently while adding zinc granules.
Observe as the solution changes colors: yellow (V⁵⁺), green (V⁴⁺), blue (V³⁺), dark-green, and finally purple (V²⁺).
Pour samples into separate beakers at each stage to preserve the different colors.
To reverse the sequence, add potassium permanganate to re-oxidize the solution, changing the colors back through green, blue, and yellow, then to orange and red.

Links

Oxidation States of Vanadium - Rugby School Chemistry:

📄 Vanadium Oxidation States Experiment - ChemTalk: https://chemistrytalk.org/vanadium-oxidation-states-experiment/

Variations

Explore the stability of each color by leaving the solutions exposed to air for different amounts of time.

Safety Precautions

Wear safety goggles, gloves, and a lab coat at all times.
Vanadium compounds are toxic—avoid inhaling dust or vapors and prevent skin contact.
Handle sulfuric acid with care; it is highly corrosive.
Potassium permanganate is a strong oxidizer—do not mix with flammable or organic materials.
Work in a well-ventilated area or fume hood when handling powdered chemicals.
Dispose of vanadium compounds properly as heavy metal waste; do not pour large amounts down the drain.

Questions to Consider

Why do vanadium compounds display so many different colors? (Because d-orbital electrons absorb light at different wavelengths depending on the oxidation state.)
Why is zinc used in this experiment? (It acts as a reducing agent, lowering the oxidation state of vanadium step by step.)
Why does the purple V²⁺ state fade quickly in air? (It is unstable and gets oxidized back to V³⁺ by oxygen.)
What is the role of potassium permanganate? (It is a strong oxidizing agent that reverses the reduction and restores higher oxidation states of vanadium.)