Fenton’s Reagent

Introduction

H. J. H Fenton, an English chemist, (1854–1927) discovered in 1894 that some metal ions, particularly iron(II) ions, catalyze the decomposition of hydrogen peroxide to generate highly reactive intermediates. Since this time, the iron-catalyzed hydrogen peroxide decomposition has been called Fenton’s reaction. Fenton’s reaction has been “rediscovered” and is now being used to break down a large variety of water and soil pollution molecules such as phenols, formaldehyde, pesticides, rubber chemicals and so on. Show students the oxidizing power of this “old time” chemistry by breaking down a series of colorful and fluorescent organic molecules.

Concepts

• Catalysis
• Enthalpy
• Green chemistry
• Redox reactions

Materials

Safety Precautions

Sulfuric acid solution is corrosive to eyes, skin and other tissues and moderately toxic by ingestion. Dilute (3%) hydrogen peroxide solution is a weak oxidizing agent and a skin and eye irritant. Fluorescein solution and green food dye can stain skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding.The reaction products may be neutralized with base and then rinsed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #24b.

Procedure

• Part 1. Green Food Dye
  1. Place three test tubes in the test tube rack.
  2. Add 20 mL of distilled or deionized water to each test tube.
  3. Add 1 to 2 drops of food dye to the second and third test tubes. Swirl these test tubes to mix.
  4. Add 1 mL of the 1 M sulfuric acid solution to each test tube.
  5. Add 20 mL of the 3% hydrogen peroxide solution to each test tube. Use a stirring rod to mix the solutions in each test tube.
  6. Use the spatula to add a small scoop of ferrous ammonium sulfate to the first test tube. Stir the solution.
  7. Repeat step six for the third test tube.
  8. Have students record their observation in Part 1 of the Student Worksheet.
• Part 2. Fluorescent Organic Molecules
  1. Place the 4 remaining test tubes in the test tube rack.
  2. Add 20 mL of the fluorescein solution to the first two test tubes and 20 mL of tonic water to the last two test tubes.
  3. Add 20 mL of distilled water to the first and third test tubes.
  4. Add 1 mL of the 1 M sulfuric acid solution to the second and fourth test tubes.
  5. Add 20 mL of the 3% hydrogen peroxide solution to the second and fourth test tubes. Use a stirring rod to mix the solution in each test tube.
  6. Plug in the ultraviolet lamp or black light, turn down the room lights, and shine the lamp on the test tube solutions. The fluorescein solutions should “glow” a fluorescent green and the tonic water a fluorescent blue.
  7. Use the spatula to add a small scoop of ferrous ammonium sulfate to the second test tube and the fourth test tube. Stir the solutions.
  8. Turn down the room lights, and shine the ultraviolet lamp on the solutions. The green and blue glow will disappear from the solutions containing Fenton’s reagent (test tubes 2 and 4).

Student Worksheet PDF

12256_Student.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 500 mL of 1% fluorescein solution, 1 L of tonic water, 15 mL of green food dye, 573 mL of 3% hydrogen peroxide solution, 100 mL of 1 M sulfuric acid, 10 g of ferrous ammonium sulfate and seven reusable test tubes.
• The blank solution reaction in Part 1 is included to demonstrate to the students the brown color is from the reaction of the reagents and not due to the dyes.
• Fenton’s reaction results in the formation of CO₂ gas bubbles with the oxidation of the organic reactants. The final solution will be a brownish color with bubbling CO₂ formation.
• In Part 1, explain to the students the strong carbon-carbon and nitrogen-nitrogen double bonds associated with highly conjugated molecules such as food dyes.
• Fenton’s reagent was used to remediate a concentration, or plume, of perchloroethylene, PCE, at the Naval Submarine Base in Kings Bay, Georgia. The Fenton’s reagent was injected into the subsurface with injection wells. Scientists estimate that in five years, the PCE plume will diminish to below regulatory threshold limits, substantially sooner and cheaper than the previously used pump-and-treat system (five years versus 35 years and $5 million versus $35 million).

Sample Data

Answers to Questions

1. What evidence from the demonstration leads you to believe the green food dye molecule was broken down in the reaction with Fenton’s reagent?

   In the third test tube, the color of the solution changed from green to brown. Because the first solution contained clear deionized water, the matching final color of these solutions indicates the green color is no longer present in the third test tube.

2. Are the acid and hydrogen peroxide alone sufficient to oxidize the green food dye? Explain.

   No. They are present in the second test tube. No color change or other indication of chemical reaction was detected.

3. What happened to the fluorescence of fluorescein and quinine after reaction with Fenton’s reagent in Part 2? Explain the observations in terms of the use of Fenton’s reagent to destroy organic pollutants in soil and groundwater.

   The fluorescence of fluorescein and quinine were quenched or destroyed after iron(II) ions were added to the mixtures containing hydrogen peroxide. This indicates that their molecular structures were altered. Fenton’s reagent is used to react with and detoxify organic pollutants by destroying their molecular structure.

Discussion

Traditionally, Fenton’s reaction was thought to involve the production of two highly reactive intermediates, the hydroxyl free radical, •OH, and to some extent the hydrogen superoxide free radical, •O—OH.

HO—OH(aq) + Fe²⁺(aq) → 2HO•(aq) + Fe³⁺(aq)
HO—OH(aq) + Fe³⁺(aq) → HO—O•(aq) + H⁺(aq) + Fe²⁺(aq)

Recent investigations into this reaction suggest a different, more complex reaction mechanism is involved. The net result of the reaction of hydrogen peroxide with Fe(II) ions is a highly reactive species that can be used to oxidize organic pollutants in groundwater and soil. In fact, Fenton’s reagent is a more powerful oxidizer than ozone.

The reaction of ozone with an alkene, that is, a molecule containing a carbon–carbon double bond, is called ozonolysis. Food dyes are highly conjugated molecules, that is, they have long strings of alternating single and double bonds. The pi system of bonds and resonance structures creates excited electronic states that allow the molecules to absorb light in visible region.

Blue food dye has one alkene site. Like ozone, highly reactive oxygen free radicals attack and break the C=C double bond, destroying the long string of conjugation and decolorizing the dye and its solution. Green food dye is a combination of yellow and blue food dyes. The yellow dyes, while conjugated, do not have nonaromatic carbon–carbon double bonds. They are azo dyes that contain the nonaromatic N=N group. This group is quite stable and is only cleaved by a strong reducing agent.

When green food dye is exposed to ozone, only the blue dye is decolorized, leaving the solution yellow. Exposing the same green food dye to Fenton’s reagent, however, results in the complete discoloration of the solution. Not only are the alkene double bonds broken, but the aromatic double bonds are also cleaved, resulting in the destruction of the string of conjugation of each molecule. This shows that Fenton’s reagent is a powerful oxidizer.

Fluorescein and quinine will be used in this demonstration to represent organic pollutant molecules. Fenton’s reagent will oxidize these substances as well, quenching or destroying their fluorescence in the process.

References

Special thanks to Randy Sullivan, University of Oregon, Eugene, OR, for inspiring Flinn Scientific to develop this demonstration.