Background and Overview of Hazards

A peroxide is a chemical containing an oxygen-oxygen single bond (R-O-O-R). The two most serious hazards associated with peroxides are fires and explosions when exposed to heat, shock, or friction. Peroxide decomposition can initiate explosive polymerization reactions. Peroxides can also oxidize human tissue, cotton, and other materials. 

Many solvents and chemicals can form peroxides over time due to oxidation upon exposure to air, resulting in unexpected fires and explosions. These peroxide-forming chemicals can pose a significant hazard, as the concentration of peroxides increases upon prolonged exposure to air. Due to their reactivity, even small amounts can interfere with experiments by oxidizing reagents or damaging biological material.

The oxidation mechanism involving oxygen and the peroxide-forming compounds is a radical process where a hydrogen atom is first abstracted. A good indication of a chemical being a peroxide former is if it has hydrogen atoms that create stabilized radicals once they are removed. Light can initiate the radical process, so these chemicals should be stored away from light sources. 

Some of the common oxidizable functional groups include:

1. Ethers with primary and/or secondary alkyl groups; 
2. Conjugated dienes, enynes, and diynes;
3. Aldehydes;
4. Hydrocarbons with benzylic, allylic, or propargylic hydrogens;
5. Hydrocarbons with exposed tertiary hydrogens.  

Many known peroxide formers contain low concentrations of additives such as butylated hydroxytoluene (BHT) as autoxidation inhibitors to prevent the oxidation of the chemical. If the inhibitor does not interfere with the use of the chemical, it is suggested to purchase peroxide formers with the inhibitor present. However, as the inhibitor becomes depleted over time, peroxides will begin to form. Note: distilling the chemical will remove the inhibitor.

Peroxide-forming chemicals are grouped into three categories depending on the tendency to form peroxides and the associated hazards [1, 2]:

Group A: Severe peroxide hazard after prolonged storage, especially after exposure to air. Chemicals in this group can form amounts of peroxides that can cause an explosion without concentration. All listed chemicals have been responsible for fatalities.

Test liquids for peroxide formation before using, and discard when peroxides are present. Discard 3 months after opening. Discard unopened chemicals after storage for 12 months.

Note: Potassium metal turns yellow when peroxides are present and should be disposed of.

Group B: Chemicals in this group form amounts of peroxides that do not normally cause an explosion, but they can pose an explosion hazard when concentrated by evaporation or distillation. Most of these solvents are sufficiently volatile that multiple openings of a single container can result in a significant and dangerous peroxide concentration. Store in a dark location. Test for peroxide formation every 12 months after opening or dispose of the chemical as hazardous waste. Additionally, always test before distillation or evaporation. If peroxides are detected, discard the chemical or remove the peroxides using a method listed under "Removing Peroxides." 

Group C: Peroxide formation can initiate explosive autopolymerization. The peroxide-forming potential is higher for liquids than for gases in this group. Store in a dark location and test for peroxide formation every 6 months after opening and before use. If peroxides are detected, discard the chemical or remove the peroxides using a methods listed under "Removing Peroxides". 

**When stored as a gas

Safe Handling

Chemicals that can form peroxides must be dated upon receipt from the manufacturer and dated upon opening. 

To avoid prolonged storage of peroxide-forming chemicals, they should be purchased in small quantities.

Store the peroxide formers in a dark space and purge the headspace with an inert gas to prolong the shelf life of the chemical.

Periodically test peroxide-forming chemicals for the presence of peroxides according to the table above. Inspect the physical state of the chemical and container to ensure no crystals have formed.

Never attempt to twist caps, move or open bottles of liquid ethers containing crystallized materials. Contact DRS immediately if crystals are observed, and leave the container untouched.

If containers of unknown age are found, it is recommended to dispose of them.

Techniques Used to Test for Peroxides

The easiest way to test for peroxides is to use semi-quantitative test strips. These are available commercially from storerooms on campus (Quantofix-Peroxide 100 from Macherey-Nagel) and from outside vendors (Sigma-Aldrich, VWR etc.). Follow the directions on the package to use the test strips to estimate peroxide levels in solvents. The peroxide concentration scale is available on the test strip package. The Quantofix test strips work in the pH range of 2-9. If testing highly acidic or basic solutions, buffer to bring it to the 2-9 pH range before testing. For testing hydroperoxides in organic solvents (eg: Isopropanol, Diethyl ether, Tetrahydrofuran, Dioxane etc), moisten the test field with 1 drop of water after evaporation/drying of the test solvent before comparing the color to the given scale. Several other manual tests are also available for estimating peroxide concentrations [3,4]

What is a safe level of peroxides?

There is no specific concentration of peroxides given for chemicals in group B that dictate a low/high hazard. The severity of the hazards, depends on the use of the chemical. For example, if the peroxide-former is diluted (especially with water), or used for cleaning purposes only, low concentrations of peroxides pose low hazards. However, if the chemical is distilled or evaporated, the peroxide concentration can rise to dangerous and explosive levels. If you are concentrating these peroxide-formers, you should test for peroxides periodically during the process. It also has to be taken into consideration, that the test method has limited accuracy and may fail to detect some of the peroxides. For laboratories where the peroxide former is concentrated (distilled) it is recommended to dispose of the peroxide former, if the test result is positive. DRS considers a test to be positive if any shade of blue (a detectable concentration of peroxides) appears on the test strip. Other testing methods may not detect peroxides below 100 ppm. 

Removing Peroxides

When chemicals contain low concentrations of peroxides, they can often be filtered through basic alumina to remove the potentially harmful impurities. Some peroxides may not decompose on the alumina and can be hazardous if dried. For several methods for the “deperoxidation” of ether solvents, see reference 3-6.

Disposal

Dispose of all used or unwanted material through DRS. If a liquid peroxide former looks cloudy, shows crystals, or a solid mass, contact the DRS Chemical Waste Section (DRS-Waste@illinois.edu or 217-333-2755) for a consult.

References

1. NRC (National Research Council), Prudent Practices in the Laboratory, Handling and Management of Chemical Hazards, National Academy Press: Washington, DC, 2011.
2. Clark, D., Peroxides and Peroxide-forming Compounds. J. of Chem. Health and Safety 2001, 8 (5), 12-22
3. Kelly, R. J., Review of Safety Guidelines for Peroxidizable Organic Chemicals. Chemical Health and Safety 1996, 3, 28-36.
4. Jackson, H. L.; McCormack, W. B.; Rondestvedt, C. S.; Smeltz, K. C.; Viele, I. E. J. Chem. Educ. 1970, 47, A175-A188.
5. Burfield, D. R. J. Org. Chem. 1982, 47, 3821.
6. Pitt, M. J. In Bretherick’s Handbook of Reactive Chemical Hazards 8th ed.; Urben, P. G. Ed.; Elsevier Ltd, 2017; pp 1336-1338.