Background

The rusting of iron is an exothermic process- it releases heat. When iron is in the form of a super fine powder, enough heat is generated to ignite the mixture when it is exposed to air. In this form, iron is deemed pyrphoric. In this tutorial, you will learn how to make pyrophoric iron by decomposing iron(II) oxalate. Iron(II) oxalate will be made by mixing solutions of oxalic acid and iron(II) sulfate.

H₂C₂O₄ + FeSO₄ --> H₂SO₄ + FeC₂O₄

The iron(II) oxalate will then be heated to give carbon dioxide and metallic iron. Some iron oxide and hydroxides are also formed.

FeC₂O₄ --> Fe + 2CO₂

Safety

• Neither I nor this site is responsible for any damage or injury you encounter as a result of this experiment.
  
• Wear safety goggles and gloves at all times during this experiment.
• Oxalic acid is toxic and will precipitate insoluble salts in your blood if it gets in your body.
• Don't massively scale this experiment up.
• Stay a good distance away when heating the test tube and when pouring out the pyrophoric iron so you don't burn yourself.

Materials & Tools

• Iron(II) sulfate
  • This is sold as a fertilizer in most gardening stores. 5 pounds cost me $6.
  • You can also make it by reacting an acid (sulfuric, hydrochloric, acetic, etc.) with scrap steel. Addition of hydrogen peroxide greatly speeds up this reaction.
• Oxalic acid
  • This is sold as "Wood Bleach" for bleaching decks and can be found in some hardware stores.
    
  • It is also used in some rust stain removers although these usually are only about 10% oxalic acid.
  • You can also make your own oxalic acid by oxidizing sugar with nitric acid.
•  Two 150mL beakers
  • Metal food cans will also work.
• Heat source
  • Alcohol burner or candle
    
  • Hot plate
• Funnel
• Test tube
  • Any other narrow container that is heat resistant will also do.
• Test tube holder
  
  • You can make this out of a coat hanger if needed.
    
• Scale
  
• Coffee filter
• Gloves
• Safety goggles
  

Synthesis

1. Add 5.0g of oxalic acid to 50mL water in a 150mL beaker. Heat the solution until the solute completely dissolves.
2. Grind 11.0g of iron(II) sulfate and add it to 50mL in a 150mL beaker. Heat the solution until the solute completely dissolves.
3. Mix the two solutions. A yellow precipitate of iron(II) oxalate immediately forms. Let the precipitate settle to the bottom of the container.
4. Filter the precipitate through a coffee filter in a funnel and wash it with lots of water to get rid of formed sulfuric acid.
5. Let the precipitate dry for at least 24 hours. If the precipitate is not entirely dry, rust will form instead of iron when you heat it. 
6. Grind up the precipitate which should now be of flour consistency and pour it in the test tube. Pack it firmly down to the bottom of the test tube.
7. Heat the test tube over a flame until the entire mixture turns black. Don't confuse soot from the flame for metallic iron. You can test for the presence of iron with a magnet.
8. Pour the contents of the test tube out. The mixture will spark a bit and if a large enough quantity is used, it will catch on fire. If you agitate the iron powder, it will promote oxidiatio and continue to spark. The black iron powder quickly turns to brown iron(II,III) oxide.

Uses

• The ignition of pyrophoric iron mainly just looks cool.
• It can be used in pyrotechnic mixtures to give off yellow-orange sparks.
  

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