This chemistry experiment shows you how to produce red, orange, green, yellow, blue and purple all in one solution, all the colors of the rainbow, without dyes or indicators. How is that possible? It’s possible using the amazing vanadium oxidation states.
We’ll show you how to produce the amazing oxidation states of the element vanadium in one of the most exciting experiments we have ever done. Most versions of this experiment use expensive and hard-to-find reagents like ammonium vanadate or zinc-mercury amalgam, but not ours! We show you an easier route. We also take you both forward and back though the various oxidation states, and show exciting intermediary colors.
Vanadium compounds are colorful because of the electrons in its d orbital. The electrons can absorb light and move to a higher energy state, absorbing some colors but reflecting others. All vanadium compounds can absorb some spectra of light, which is why none of them are white.
Materials
Vanadium pentoxide: 8.6 grams (OR sodium vanadate OR ammonium metavanadate)
Sodium carbonate: 5 grams
Distilled water: 500 mL
Sulfuric acid 98%: 50 grams
Zinc granules or lumps, or zinc mossy (a small handful)
Potassium permanganate (4 grams in 100 mL of distilled water)
Beakers
Flask
Hotplate OR stirrer
Thermometer
Where do I get the reagents?
You have two choices. If you have a sizeable budget, you can purchase everything you need from your favorite chemical distributor or retailer. If you don’t, you can purchase vanadium pentoxide very cheaply from most pottery supply stores. In the United States, you can get 93% sulfuric acid from hardware stores as drain cleaner. We’ve seen zinc granules and potassium permanganate for sale online for as little as $6 USD each.
Procedure
Prepare Sodium Vanadate
If you don’t have sodium or ammonium vanadate, you’ll need to make it. Don’t worry, it’s easy to make sodium vanadate. Combine equimolar amounts of vanadium pentoxide and sodium carbonate, mix with water. Use a ratio of 1.72 to 1. For example, use 17.2 grams of vanadium pentoxide and 10 grams of sodium carbonate. You can even make your own sodium carbonate by heating sodium bicarbonate in an over at 232°C (450°F) for one hour (learn how in this video).
TIP: Vanadium pentoxide costs about 20 times less than ammonium metavanadate.
For our procedure, we used 8.6 grams of vanadium pentoxide and 5 grams of sodium carbonate. We used pottery-grade vanadium pentoxide and homemade sodium carbonate, which worked just fine!
Heat at 90°C (194°F) using a hotplate or stirrer until the solution turns from yellow to green. It should take five to 10 minutes. When it turns green, you have sodium vanadate!
Prepare Vanadyl Sulfate
Add a small amount of sulfuric acid to your sodium vanadate solution. We added 50 grams of concentrated sulfuric acid to 500 mL of sodium vanadate solution. When the solution turns yellow, which should happen immediately, you now have vanadyl sulfate, which has vanadium in the +5 oxidation state. The vanadyl ion formula is VO2+.
Making the Colors of Vanadium
To produce the different colors of vanadium compounds, pour the vanadyl sulfate solution into a large flask, and put it on low heat on a hotplate or stirrer. And a generous amount of zinc granules. As the zinc reduces the vanadium, the colors will change to green, then blue, then dark-green, and finally purple. You have to shake the zinc around in the solution to get the last two color changes. Using a magnetic stirrer can help this process.
As the colors change, your can pour some of the solution out of the flask into a beaker. Have several 100 mL beakers lined up, ready to go!
TIP: Using zinc granules instead of powder means you don’t have to filter anything. Just pour and the pieces of zinc will remain in the flask.
TIP: Getting from V+3 to V+2 can take 15 to 40 minutes. Using more zinc, keeping it warm, and swirling it around a little can keep the time down to a minimum. No need for expensive and dangerous zinc-mercury amalgam!
TIP: The beautiful purple color of the +2 vanadium ion won’t last long in air, as the oxygen will oxidize it back to the +3 state. Enjoy it while you can!
Reversing the Vanadium Colors: Making the Rainbow
By adding a potassium permanganate solution, a strong oxidizing agent, you can reverse the colors. The colors will change from purple, to dark green, to electric blue, to yellow. Adding more permanganate at this point will produce orange, and then red.
Safety Notes for this Experiment
Vanadium compounds are toxic, do not breathe them in, and wear gloves if there is a danger of coming into contact with the solution. Sulfuric acid is highly corrosive, so avoid all contact. Potassium permanganate is a very strong oxidizer; never mix it with fuels or organic compounds. Wear goggles during this experiment. Read our lab safety rules.
If you purchase vanadium pentoxide from a pottery store, you may need to transfer it into a proper plastic bottle. We recommend doing this outdoors while wearing gloves and an N95 mask to avoid breathing in the dust.
Disposal
Vanadium compounds can be harmful to marine organisms. Do not wash large quantities down the drain (no more than 1 gram of compound per day). Larger quantities should be treated as heavy metal waste. You can precipitate vanadium compounds as a hydroxide or sulfide, and then filter and dry the solid compound.