Peroxide Forming Chemicals
Peroxide formation in laboratory solutions and reagents by auto-oxidation has caused many laboratory accidents, including unexpected explosions of residues remaining after solvent distillation. Many liquid, a few solid and a few gaseous organic and a few inorganic solid compounds form peroxides over time. Peroxides form by the reaction of the chemical with oxygen allowed in the headspace of chemical containers once the container is opened for the first time. Most organic peroxides are sensitive, to varying degrees, to shock, heat or friction.
The rate of peroxide formation will depend upon the compound. Some peroxides quickly build up to an explosive level and some are only explosive on concentration, such as when a solvent is distilled. Peroxidizable compounds contain a reactive hydrogen atom that is "activated" by adjacent structural components. The dangers of peroxide formation can be divided into three groups (see Tables A-C in the box to the right). In addition, there are a number of chemical compounds that may form peroxides, but do not clearly fit into categories A-C (see Table D.)
Control and Safe Use of Peroxide Formers
Peroxide formation may be controlled by the following methods:
- Containers arriving at the Baylor Sciences Building will be marked (by stockroom personnel) with the date received. Lab users should also mark the date when the container is opened. Many chemical companies now routinely print an expiration date on containers of the worst peroxide formers.
- Purchase the smallest possible container size for your needs. Inhibitors are added to some chemicals and the purchase of peroxide formers with added inhibitors is encouraged. However, be aware, that these free radical inhibitors will be depleted over time as peroxides are formed. Additionally, distillation removes the inhibitor. Distilled peroxide formers and those retained for extended periods should be checked for inhibitor concentration and inhibitor added if below the manufacturer's recommended concentration.
- Store peroxide formers in sealed, air-impermeable containers such as dark amber glass with a tight-fitting cap. Iron inhibits the formation of peroxides in some materials, which is why diethyl ether and some other materials are sold in metal cans. Ground glass stoppered bottles and plastic containers are not advisable; however, plastic squeeze bottles may be used for small quantities of some materials, such as 2-propanol, for immediate use.
- Containers of peroxide formers should be stored away from heat and light and protected from physical damage and ignition sources. While peroxide forming compounds can be stored in a refrigerator, they should not be stored at or below the temperature at which the peroxide freezes or precipitates, as this will make these compounds extremely sensitive to shock. [Note: refrigeration does not prevent (and may not inhibit) peroxide formation.]
- Store peroxide formers, especially those listed in Table A, under nitrogen or other inert gas or keep and use them in an inert atmosphere chamber. [Note: Some inhibitors actually need small amounts of oxygen to prevent peroxide formation, therefore it is recommended that inhibited chemicals are not stored under an inert atmosphere.]
- Avoid the distillation of peroxide formers without first testing for the existence of peroxides. Most explosions occur when a material is distilled to dryness. Leave at least 10-20% in still bottom. Stir such distillations with a mechanical stirrer or an inert gas (air or an oxygen containing mixture should never be used for this purpose).
- Do not evaporate containers that held peroxide forming compounds to dryness for reuse unless it is known that the chemical was peroxide-free.
- Inspect containers of peroxide forming chemicals frequently, looking for signs of precipitation, stratification of liquid, crystal formation or other irregularities. Note: the presence of any of these signs indicates the potential for a shock sensitive container. Do not move the container and contact Environmental Health & Safety at 254-710-2900 as soon as possible.
- After each use, carefully wipe the container neck, cap and threads with a cloth before resealing.
Safe Storage Periods for Peroxide Formers
|Unopened chemicals from manufacturer:
|Chemicals in Table A
|Chemicals in Tables B and D
|Uninhibited chemicals in Table C
|Inhibited chemicals in Table C
(Do not store under an inert atmosphere)
Evaluation of Peroxide-Forming Chemicals
Peroxide-forming liquid chemicals which have not been tested for peroxides must be evaluated as follows:
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 Environmental Health & Safety, who will assist in the further evaluation. If there is any doubt about the safety of handling a chemical container, notify Environmental Health & Safety (254-710-2900) immediately.
- 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 Environmental Health & Safety). Only chemicals that pass visual inspection should be evaluated further.
Note: Never try to force open a rusted or stuck cap on a container of a peroxide-forming chemical.
Only chemicals that meet the below criteria should be opened and can be 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 Environmental Health & Safety.
- 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.
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 Environmental Health & Safety as soon as possible.
- Kelly, R. J., "Review of Safety Guidelines for Peroxidizable Organic Chemicals", Chem. Health Saf. 1996, 3(5), 28-36
- Jackson, H. L.; McCormack, W. B.; Rondestvedt, C. S.; Smeltz, K. C.; Viele, I. E. J. Chem. Educ. 1970, 47(3), A175-88
- Clark, D. E., "Peroxides and Peroxide Forming Compounds", Chem. Health Saf. 2001, 8(5), 12-21
- National Safety Council, Recognition and Handling of Peroxidizable Compounds, 1987, Data Sheet I-655-Rev. 87
- National Research Council, Prudent Practices in the Laboratory: Handling and Disposal of Chemicals; National Academy Press: Washington, D. C., 1995; pp. 162-163.
- Kelley, R. J., "Peroxidizable Organic Chemicals", Handbook of Chemical Health and Safety, Chapter 52, ACS, 2001