About two decades ago, a University of California, Berkeley, undergraduate student researcher was working at the laboratory bench when the apparatus she was using exploded, sending glass fragments into her face and upper torso. The researcher was using a rotary evaporator (rotovap) to remove organic solvents from an azobenzene precipitate. She adjusted the bottom flask, which then exploded, sending glass toward her face and hitting her safety goggles and forehead. 

Lab personnel helped her to the safety shower and called 911. She was taken by ambulance to the hospital, where she received stitches above her eyes and other treatment for her injuries. She was released from the hospital the same day. Most likely, the explosion was caused by peroxide contaminants in the solvents that had concentrated to the point of being unstable. Both tetrahydrofuran (THF) and diethyl ether were used in the reaction, and both of these solvents form peroxides over time. 

Many high school chemistry and biology laboratories use peroxide-building chemicals. Unfortunately, teachers are unaware of the potential hazards and resulting risks of using these chemicals, especially if stored over periods of time.

Simply put, peroxide-forming chemicals (PFCs) are a class of compounds that have the ability to form shock-sensitive explosive peroxide crystals. Many of the organic solvents commonly used in laboratories have the potential to form explosive peroxide crystals. If used in the science laboratory, strict safety protocols are required.  

Weill Cornell Medicine’s (WCM) Department of Environmental Health and Safety has an informative website that provides details about storage, disposal, and testing guidelines for common PFCs. (See https://ehs.weill.cornell.edu/sites/default/files/peroxide_formers.pdf.) Below is a summary of their recommendations. Be aware that the lists mentioned are not all-inclusive. Teachers should always consult the manufacturer’s Safety Data Sheet (SDS) for additional information. 

Class A Peroxide Formers—Severe Peroxide Hazard
These compounds spontaneously decompose and become explosive with exposure to air without concentration. 
Safer Storage Guideline. Unopened container: Discard or test for peroxide formation at 12 months from receiving or at manufacturer’s expiration date, whichever comes first. Opened container: Test for peroxide formation quarterly. Examples include Butadiene (liquid monomer), Isopropyl ether, Sodium amide (sodamide), Chloroprene (liquid monomer), Potassium amide, Tetrafluoroethylene (liquid monomer), Divinyl acetylene, Potassium metal, Vinylidene chloride. 

Classes B and C Peroxide Formers—Concentration and Autopolimerization Hazards
Class B: Peroxide hazards on concentration (e.g., evaporation or distillation) 
Class C: Peroxides accumulation may result in violent polymerization of monomers.
Safer Storage Guidelines. Unopened container: Discard or test for peroxide formation at 12 months from receiving date or at manufacturer’s expiration date, whichever comes first. Opened container: Test for peroxide formation every 6 months. Opened container used for distillation or evaporation: Test for peroxide formation immediately before distillation. Examples include Class B Acetal, Cumene, Diacetylene, Methylacetylene, 1-Phenylethanol, Acetaldehyde, Cyclohexanol, Diethyl ether, Methylcyclopentane, 2-Phenylethanol, Benzyl alcohol, 2-Cychlohexen-1-ol, Dioxanes, Methyl isobutyl ketone (MIBK), 2-Propanol * Benzaldehyde • Cyclohexene • Ethylene glycol dimethyl ether (glyme) 2-Pentanol Tetrahydrofuran 2-Butanol • Decahydronaphthalene Furan 4-Penten-1-ol * Tested prior to concentration or distillation only. 
Class C Acrylic acid, Chloroprene, Styrene, Vinyl acetylene, Vinyladiene chloride, Acrylonitirile, Chlorotrifluoroethylene, Tetrafluoroethylene, Vinyl chloride, Butadiene, Methyl methacrylate, Vinyl acetate, Vinyl pyridine.

Class D: Chemicals that may form peroxides, but cannot be clearly placed in Class A-C. Test peroxide levels quarterly. Dispose of by the expiration date or within 2 years of receipt unless the chemical quality is confirmed.

As a result of the accident at University of California, Berkeley, the university’s Office of Environment, Health, and Safety developed and instituted a series of safety protocols for dealing with the use of PFCs. Below is a summary of these protocols, which can be found in total at https://ehs.berkeley.edu/news/peroxide-explosion-injures-campus-researcher. They have been altered for high school applications.

1. Manage PFCs properly. Understand that explosive peroxides will form over time and must be identified as materials containing peroxides. Science teachers need to be trained on how to properly use these potentially hazardous materials safely. The safer procedures should be listed or described in the lab’s Chemical Hygiene Plan.
2. Limit purchases of peroxide-building chemicals. Purchase small quantities, and try to completely use materials so that remaining peroxide-forming material is not left in the container. Larger containers may sit around longer, increasing the chance of forming peroxides. Whenever possible, purchase material with stabilizer added, so the rate of peroxide formation is decreased. If peroxidizable material does not have a stabilizer, it should be tested every time it is used. 
3. Store properly. Peroxide formers need to be stored away from air and light. They should be stored in their original containers, usually dark-colored bottles or metal cans, with a tight-fitting cap. 
4. Dating. Always label containers of peroxide formers with the date received and the date opened. After a container is opened, it should be labeled with an expiration date. The Berkeley campus Environmental Health and Safety (EH&S) office and website have guidance on “potentially explosive chemicals,” including information on expiration protocols and labeling. (See the links below.) EH&S also has labels available for marking such containers. Also, the manufacturer’s label may include the manufacture date and recommendations for storage. Do not use old containers of peroxide formers or containers that don’t have labels with dates without testing for peroxide contamination.
5. Disposal. Dispose of old PFCs through a chemical waste program. If you notice a viscous liquid or crystalline solids in the solvent, stop immediately and get assistance from your local fire department. It is possible that old crystals may be shock-, friction-, or impact-sensitive and could explode.
6. Personal Protective Equipment (PPE). Wear proper PPE in the lab, including indirectly vented chemical splash goggles meeting American National Standards Institute/International Safety Equipment Association (ANSI/ISEA) Z87.1–D3, lab coat, and nitrile gloves.
7. Fume Hood Safety Sash. Work behind a safety sash, such as a fume hood sash, when working with potentially explosive chemicals.
8. Engineering Controls. Know where the emergency eyewash and safety shower are so that decontamination can be accomplished quickly.
9. Safety Training. Provide safety training to everyone in the lab on the unique hazards posed by PFCs. 

An additional item that should be addressed is the testing of peroxides. Several methods are commonly used to detect peroxides in the laboratory. However, in this case, peroxide test strips provide a simple and convenient mechanism for detection. The test strips are available from several laboratory vendor/supply companies. For volatile organic chemicals, while working in a fume hood, the test strip is immersed in the chemical for 1 second, then the tester breathes slowly on the strip for 15–30 seconds or until the color stabilizes. Compare the test strip color with a colorimetric scale provided on the test kit bottle. 

Peroxide concentrations of less than 20 ppm. are safe for use. Between 20 and 100 ppm., the solvent should not be distilled or concentrated. Between 100 ppm. and 400 ppm., the solvent must be disposed of as waste. Above 400 ppm., seek immediate assistance and evaluation (e.g., by contacting your local fire department).

Research the hazards and resulting risks of any chemical to be used in the laboratory before it is purchased. Make sure the hazards and resulting risks are at an acceptable level when deciding whether to use a chemical in the lab. Always explore safer alternative chemicals if the hazards and risks outweigh the academic need.

Submit questions regarding safety to Ken Roy at safersci@gmail.com. Follow Ken Roy on Twitter: @drroysafersci.