Organic Peroxide Formers

Identification, Hazards, Precautions, and Safe Storage

What is an organic peroxide former?

Organic peroxide-forming chemicals slowly react with atmospheric oxygen during storage (“autoxidation”), causing a build-up of peroxides, i.e., reactive compounds which contain weak O-O bonds.

Peroxidation is greatly accelerated by free-radical sources including light, excessive temperatures, certain pollutants (ozone, nitrogen oxides), and certain contaminants (e.g. organic photosensitizers, certain metal ions ).

Why is this important?

Organic peroxides are generally oxidizing, unstable, and may ignite, explode, or energetically decompose on heating or mechanical disturbance.

Peroxide build-up was the direct cause of many serious and even fatal accidents. Examples:

  • THF (stabilizer-free) and diethyl ether were being removed from a reaction product using a rotary evaporator when the glass flask violently exploded, injuring a student via flying shards of glass.
  • 50 mL of over-age 2-butanol (sec-butanol) exploded during distillation. Subsequent testing revealed a large build-up of peroxides.

Are peroxide formers common in labs?

YES! Most notably, some common useful solvents are peroxide-formers, including diethyl ether, tetrahydrofuran (THF), 1,4-dioxane, dimethoxyethane (DME; ethylene glycol dimethyl ether), tetrahydronaphthalene (tetralin™), and 2-butanol (sec-butanol).

How are organic peroxide formers categorized and identified?

Refer to the tabs cited below of the EH&S Peroxide Formers Guidance Spreadsheet for information:

  • Categorized into Classes A-D, see Basic info tab for a full explanation of what the categories mean.
  • Class B “excepted” — see Basic info tab for a full explanation.
  • Tables of specific and general compounds — see individual tabs for Classes A-D.
  • Literature references — see Useful refs tab.

What is the most notorious peroxide former?

Diisopropyl ether! Diisopropyl ether forms HIGHLY EXPLOSIVE crystals of cyclic acetone peroxides.

  • NEVER use as a solvent — there is NO justification
  • Obtain approval from EH&S (labsafety@usc.edu) BEFORE purchasing

What are stabilizers/inhibitors?

Stabilizers (also called inhibitors) capture free-radicals, thus breaking the chain reaction which creates organic peroxides from oxygen. The presence of a stabilizer drastically reduces peroxidation rate and increases safe storage time.

The stabilizer, however, is slowly consumed by oxygen and when it is used up the peroxidation rate will increase to the unstabilized value.

NOTE: Distillation and solvent purification procedures (e.g., activated alumina column) REMOVES stabilizers!!!

Should I buy stabilized materials?

YES. Always preferentially purchase stabilized grades of organic peroxide formers (unless the unstabilized material is necessary for technical reasons, or is not commercially available).

Check vendors’ websites for information on the presence of stabilizer, especially when ordering solvents including (but not limited to) diethyl ether, tetrahydrofuran, and 1,4-dioxane.

Class C materials in quantities exceeding 1 g or 1 mL shall ALWAYS be purchased stabilized.

  • Most or all Class C substances are prohibited from transportation by DOT regulations unless stabilized; therefore, unstabilized Class C materials are generally not commercially available
  • Possible exception – Some Class C materials in quantities too small to pose an exothermic polymerization hazard may be purchasable unstabilized (e.g. ≤ 1 g samples sold as analytical standards)

How should peroxide formers be stored?

  • Dark, dry, cool location.
  • DO NOT return unused excess back to the original bottle, or otherwise introduce trace impurities, since doing so may increase the rate of peroxide formation.
  • Follow labeling/inventory rules and storage time limits

 Review the Flowcharts tab of the EH&S Peroxide Formers Guidance Spreadsheet for time limits.

Storage Conditions by Class

Peroxide Class1TemperatureAtmosphere
Class AManufacturer’s recommendations Inert gas
Class B – unstabilizedUsually room temperature, unless lower temperature specified by the manufacturer2 Inert gas (NOT Air)
Class B – stabilizedUsually room temperature, unless lower temperature specified by the manufacturer2 Air or Inert gas
Class C – unstabilizedUsually cold (follow manufacturer’s recommendation) NOT ALLOWED TO BE STORED!3
Class C – stabilizedUsually cold (follow manufacturer’s recommendation) Air (unless manufacturer requires inert gas)4
  1. Store in the dark. – peroxide formation is often greatly accelerated by light.
  2. Do not rely on refrigeration to slow peroxidation unless vendor’s recommendations are to the contrary. Cold storage may have no effect or may even promote peroxide build-up by slowing the decomposition of unstable peroxides. Cold storage can also increase the likelihood of a concentrated peroxide phase separating out.
  3. Uninhibited Class C materials shall only be produced in small quantities (a few mL) during the course of an experiment and shall not be stored more than 24 h. Production and use of uninhibited Class C materials shall be covered by an SOP. [Exception: Very small samples (≦ 1 g or 1 mL; too small to pose an exothermic polymerization hazard) are allowed to be stored indefinitely PROVIDED the material is NOT also in Class A. These excepted samples also do not require an SOP.]
  4. Some polymerization-inhibitor systems for Class C materials require the presence of oxygen for proper functioning. Always closely follow vendor’s storage recommendations for Class C materials. 

Peroxide Testing

See Organic Peroxide Formers: Testing and Labeling.

Potential Acute Danger – What to Look for

DANGER! If any of these visible signs are observed, there is potential for a significant peroxide hazard to be present:

  • Peroxidizable organic liquid with one or more of the following:
    • Solid precipitate 
    • Incrustation around cap 
    • Second liquid phase 
    • Visible discoloration 
  • Rusty or excessively old containers of peroxide-forming compounds 
  • Also, look out for:
    • Crusted potassium metal 
      • Old containers of potassium which are non-transparent – DO NOT attempt to open in order to examine the potassium inside 
    • Alkali metal amide which is old, or which may have been exposed to air 
    • See Other Time-Sensitive Chemicals for more information.

How to Respond

If any of these signs are observed, DO NOT MOVE OR OPEN CONTAINERS, and:

  • Notify lab colleagues.
  • Restrict access.
  • Notify EH&S Hazardous Materials division; hazmat@usc.edu; (323) 442-2200.

Invisible Peroxide Formation 

  • Severe peroxide contamination may occur without visible signs in over-age or improperly stored materials. 
  • If materials are: (a) stored under less optimal conditions (e.g., in light, or unstabilized Class B stored in air), (b) significantly over-aged, or (c) in doubt, contact Hazmat, hazmat@usc.edu; (323) 442-2200. 
  • Lower levels of peroxide formation may still result in severe explosion hazard if the peroxide becomes concentrated, e.g. during a distillation or solvent evaporation procedure.

Manage Risk

Manage risk by the following: 

  • Adhere to good storage practice (e.g., time limits).
  • Test for peroxides before distillation/rotovaping.
  • See “How Do I Control the Hazard” below for more details.

Hydrogen Peroxide and Organics 

  • Organic peroxides can be formed by the reaction of hydrogen peroxide with organic solvents, particularly ketones. 
  • Highly dangerous explosives may be formed, e.g. acetone peroxide crystals 
  • Even in the absence of overt peroxide formation, hydrogen peroxide of sufficient concentration (approx. 30% or greater) may form redox mixtures with organic materials that can potentially detonate. 

Manage Risk

Manage risk by the following: 

  • DO NOT mix hydrogen peroxide with incompatible materials. 
  • Segregate hydrogen peroxide waste in a bottle with a vented cap.

How Do I Control the Hazard?

  • Purchase and use stabilized solvents where possible.
    • Note that stabilizers are removed by distillation or adsorption (e.g., on activated alumina or silica gel).
    • Remember – Solvent purification systems always dispense unstabilized solvent, regardless of what they were filled with. Dispensed solvent should be stored under inert gas or used immediately.
  • Control your inventory
  • Use unstabilized solvents immediately after opening.
    • If they must be stored, add a stabilizer or de-aerate and store under inert atmosphere.
    • Closely follow maximum storage times.
  • Distillation or evaporation will concentrate peroxides!!
    • Distill only fresh material or test for peroxides immediately before use.
    • Do not distill to dryness.
    • Note: Unstabilized organic peroxide formers stored in air may become hazardous to distill after as little as three days storage!
  • Peroxidation is greatly accelerated by light, especially in the presence of trace photosensitizers (e.g., ketones).
    • Store in an amber bottle and/or in the dark.
  • Test solvents/materials frequently for the presence of peroxides – see Organic Peroxide Formers: Testing and Labeling.
  • Use non-peroxide forming substitutes whenever possible, especially for solvent applications (e.g. n-butanol or iso-butanol instead of sec-butanol; xylene, mesitylene, or 1-methylnaphthalene instead of tetralin™)

Additional Resources