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Revised January 2023

Identifying Peroxide-Forming Chemicals

Many chemicals can undergo autooxidation to form explosive peroxides (see lists below). Peroxide-forming compounds contain a reactive hydrogen atom that is “activated” by adjacent structural components. Reactive hydrogen atoms are most often found on the following compounds:

• Ethers and acetals with an Α-hydrogen, especially cyclic ethers and those containing primary and secondary alkyl groups
• Compounds containing benzylic hydrogens
• Compounds containing allylic hydrogens, including most alkene; vinyl and vinylidene compounds; and dienes

Generally, the presence of two or more of these structural features increases the risk of peroxidation. Within a particular class of peroxidizable chemicals, the peroxidation potential decreases with increasing molecular weight of the compound. Compounds with ten or more carbon atoms at a peroxidizable site are normally not considered peroxidation hazards. Additionally, increased volatility of the parent chemical increases the likelihood that dangerous levels of peroxides will form, since evaporation leads to concentration of the peroxide product.

• Use of peroxidizable solvents in distillation or chemical synthesis procedures that involve heating and concentrating contaminating peroxides presents the highest risk of explosion, whereas solvent extraction procedures (or other “gentle” chemical procedures) generally present a low risk.

Potential Peroxide-Forming Chemicals (not all inclusive)

LIST A:

LIST B:

LIST C:

These chemicals form peroxides which initiate rapid polymerization. Uninhibited chemicals are not to be stored longer than 24 hours.

Evaluation of Peroxide-Forming Chemicals

Peroxide-forming liquid chemicals which have not been tested for peroxides within the listed time periods must be evaluated as follows:

Visual Inspection

Visually inspect all peroxide-forming chemicals before any further evaluation. Containers that exhibit any unusual visual characteristics, such as the examples listed below, should be assumed to contain dangerous levels of peroxides and should not be disturbed (notify EH&S). EH&S will assist in the evaluation of chemicals. If there is any doubt about the safety of handling a chemical container, notify EH&S immediately.

Liquid Chemicals

• Crystallization (around the cap or in the liquid)
• Visible discoloration
• Liquid stratification

Note: A flashlight or other light source can be used to increase the visibility of the interior of amber bottles.

Diethyl ether is commonly sold in steel containers which prevents visual inspection of the liquid. Therefore, diethyl ether containers whose age and use history are unknown should be assumed to contain dangerous levels of peroxides, and should not be disturbed.

Solid Chemicals (potassium metal, potassium and sodium amide)

• Discoloration and/or formation of a surface crust (for example, potassium metal forms a yellow or orange superoxide at the surface)

Note: Evaluation of alkali metals and their amides is based on visual criteria only. These substances react strongly with water and oxygen, and standard peroxide tests should not be used.

Materials meeting the above criteria are considered to be high risk and will have to be disposed of by special means (limit handling and movement; notify EH&S). Only chemicals which pass visual inspection should be evaluated further.

Opening Container

Only chemicals that meet the below criteria should be opened and tested for peroxides. Chemicals that do not meet one or more of these criteria should be considered to be high risk, and should not be disturbed (limit handling and movement; notify EH&S).

• The identity of the chemical is known.
• The age of the chemical (since manufacture) is known.
• Evaporation of the chemical is thought to be less than 10% - if this is in question, assume that evaporation has occurred and that high peroxide levels may be present.

Note: Never try to force open a rusted or stuck cap on a container of a peroxide-forming chemical.

Additionally, the following classification-specific criteria must be met:

List A Chemicals

• Previously opened chemicals which have not been used in the preceding 3 months must be less than 6 months old.
• Chemicals unopened from the manufacturer must be less than 2 years old.
  • If this is in question, assume the container has been opened.

List B and List C (liquids) Chemicals

• Opened chemicals not used in the preceding 12 months must be less than 5 years old.
• Chemicals unopened from the manufacturer must be less than 10 years old.
  • If this is in question, assume the container has been opened.

Chemicals not meeting the minimum criteria for opening and testing will be considered to be high risk and must be disposed of by special means (limit handling and movement; notify EH&S). If after opening the container, visual irregularities such as those listed in section 1. are apparent, assume that dangerous levels of peroxides are present. Gently cover the container to minimize evaporation, limit handling and movement, and notify EH&S as soon as possible.

Safety Precautions

Personnel handling containers of outdated peroxide-forming chemicals must wear chemical goggles and a face shield, heavy gloves, and a buttoned lab coat. Hearing protection (plugs or muffs) and a rubber apron are also recommended. Suspect chemical containers must be transferred, one at a time, to a clean (no other chemicals) lab hood. When practical, a blast shield should be used when opening or manipulating containers, and testing peroxide levels. Never attempt to force open a stuck cap on a container. Secondary containment for the chemical should also be utilized if practical. Tongs or other forms of remote handling should be used as much as practical. Verify that an operable safety shower/eyewash and fire extinguisher is readily accessible. At least one other person not directly involved in handling of the chemicals should be present.

Peroxide testing

For chemicals that have been determined to be safe to open, measure the peroxide concentration using commercial peroxide test strips. Wet chemical detection methods are also available; however, the test strips are gentler, easier, faster, and have greater sensitivity and accuracy, and therefore are recommended for most applications. Laboratory personnel are responsible for performing peroxide testing of chemicals present in their laboratories or storage areas.

• Peroxide test strips (e.g., manufactured by EMD Millipore) are available from many suppliers (such as Fisher Scientific and VWR).
  • Chemicals that contain peroxide levels that exceed the test strip detection range may be diluted with a miscible, peroxide-free, solvent.

Dispose of chemical or decontaminate peroxides

Chemicals with a peroxide concentration of less than 30 ppm can be disposed of through EH&S. Laboratory personnel are responsible for decontaminating chemicals that contain greater than 30 ppm peroxides prior to disposal.

• Chemicals with a peroxide concentration greater than 800 ppm are considered high risk, and require disposal by special means (limit handling and movement; notify EH&S).
• Chemicals with a peroxide concentration greater than 100 ppm must be disposed of, or if the peroxide level in a specific container of a chemical on list B is less than 800 ppm, that chemical can be decontaminated and maintained for future use upon the approval of the responsible laboratory supervisor and EH&S. Decontamination and retention of chemicals with this level of peroxide contamination is generally not recommended and would require strong justification for EH&S approval.
  • Peroxides can be removed by chemical treatment or column separation (1, 2).
• Chemicals with a peroxide concentration less than 800 ppm must be decontaminated to reduce the peroxide concentration to less than 30 ppm before disposal (verify treatment with testing). Notify EH&S of treatment and subsequent peroxide concentration when submitting request for waste removal.

Preparation of Peroxide Formers for Disposal

Water-insoluble peroxide formers (ethers, hydrocarbons, etc.) can be decontaminated by shaking with a concentrated ferrous salt solution (5). A solution of 6 g of ferrous sulfate (FeSO4 • 7H2O), 6 ml of concentrated sulfuric acid, and 11 ml of water is mixed with 1 L of the peroxide former until the peroxide concentration is reduced to below 30 ppm. Reduction of the peroxides generally takes only a few minutes. Re-test the peroxide former after decontamination to verify that the peroxide concentration is less than 30 ppm. Dispose of the decontaminated peroxide former as soon as possible through EH&S (report final peroxide concentration). An alumina column is used as the standard procedure to decontaminate water-soluble peroxide formers (1, 2). Contact EH&S for guidance on decontamination of these chemicals.

In some instances it may be permissible to dilute the peroxide concentration to below 30 ppm by mixing the peroxide former with a chemically compatible chemical waste stream or chemical solvent. Contact EH&S prior to diluting any peroxide former in preparation for disposal. After diluting the peroxide former, verify that the peroxide concentration of the resulting mixture is less than 30 ppm, and then dispose of the waste mixture through EH&S (report final peroxide concentration).

Safe Storage and Use of Peroxide-Forming Chemicals

Maintenance of peroxide-forming chemicals requires implementation of the following procedures:

• Identify potential peroxide-forming chemicals (see lists above). Prudent practice dictates that laboratories minimize their inventory of peroxide-forming chemicals.
• Label each container with the date it is received and the date it is opened.
• Store peroxide-forming chemicals in tightly sealed containers to minimize the introduction of air. An inert gas such as nitrogen or argon can be introduced into the container as an inert blanket to minimize available oxygen (inhibited vinyl monomers, List C chemicals, are the exception to this recommendation).
• Ultraviolet light can initiate autooxidation; therefore, it is best to store peroxide-forming chemicals in containers that exclude light.
• Test for peroxides at least as often as recommended in the example lists. Unopened chemicals from the manufacturer must be tested upon reaching the manufacturer’s expiration date, or 18 months, whichever comes first. Chemicals with a peroxide concentration of greater than 100 ppm must be decontaminated or disposed of through EH&S after treatment to reduce peroxides to less than 30 ppm.
• Inspect containers of peroxide-forming chemicals frequently, looking for signs of precipitation, stratification of liquid, crystal formation, or other irregularities. The presence of any of these signs indicates a potential shock sensitive container – do not move the container and contact EH&S as soon as possible.
• Test for peroxides prior to distilling peroxide-forming chemicals (or prior to other heating and concentration procedures), as this is when explosions commonly occur. It is recommended that 10 to 20% residual bottoms be left during distillation. Additionally, a non-volatile organic liquid, such as mineral oil, can be added to minimize concentration of any peroxides.
• Chemical manufacturers often add trace quantities of free radical scavengers (for example, 100 ppm hydroquinone) to inhibit the formation of peroxides. These inhibitors become depleted as peroxides are formed. Additionally, distillation separates the inhibitor from the peroxide-forming chemical. Distilled chemicals and chemicals retained for extended periods should be checked for inhibitor concentration, and inhibitor added if the concentration is below the manufacturer’s specifications (contact the manufacturer for recommendations).

References and Additional Information

1. Clark D. E., “ ”, J. or Chem. Health and Safety, 2001 8(5), 12-18.
2. Kelly, R. J., “ ”, Chem. Health & Safety, 1996, 3(5), 28–36.
3. National Safety Council, “ ”, Data Sheet I-655-Rev.87; Chicago, 1987.

Chapter 7: Corrosive Chemicals and Allergens