This chapter describes the hazards associated with peroxide formation in chemical compounds, methods to detect peroxides, safe handling, use, and storage of peroxidizable compounds, and how to remove peroxide contamination from chemicals.

Peroxides are chemical substances that contain the reactive peroxo unit (O22-, or R-O-O-R). Several different organic chemicals are capable of forming peroxides. Peroxide formation in solvents and reagents has caused many laboratory accidents. Every worker must learn to recognize and safely handle peroxidizable compounds.

Peroxides form by the reaction of a peroxidizable compound with free radicals and molecular oxygen through a process called autooxidation or peroxidation. Peroxidizable compounds are insidious. Under normal storage conditions, they can form and accumulate peroxides, which may explode violently when subjected to thermal or mechanical shock. This can occur even when the containers appear to be tightly closed.

Figure 13.1
Figure 13.1 Following years of uninhibited peroxidation, this bottle of isopropyl ether contains a large chunk of explosive peroxide crystal. A high-hazard removal company took this bottle from the lab and detonated it with a blasting cap, visible on top of the tape. Photo courtesy of Reactive Hazards Reduction, Inc.

Peroxides in solution do not normally present thermal or shock hazards at concentrations up to about one percent (1%). You can safely dispose such solutions or treat them to remove peroxides. However, if you notice visible crystals in a peroxidizable liquid or discoloration in a peroxidizable solid, peroxide concentrations greater than 1% are likely already present. Such chemicals are extremely dangerous, and might require special handling and disposal procedures. Do not handle chemicals that you suspect might have significant peroxide contamination. Leave the chemicals in place and contact EHS.

To prevent accidents caused by peroxidizable compounds, your laboratory safety procedures should emphasize:

  • Recognition of chemical structures that may form peroxides (Appendix 13-A, Table I)
  • Use of hazard identification labels
  • Controlled inventory of peroxidizable compounds
  • Use of peroxide detection tests and peroxide removal procedures
  • Proper safety equipment and process procedures
Some of the specific compounds that form peroxides during storage are in Appendix 13-A, Table II, lists A, B and C.

  • List A: Compounds that form peroxides that may explode even without concentration.
  • List B: Chemicals that are dangerous when concentrated by distillation or evaporation.
  • List C: Substances for which peroxide formation can initiate explosive polymerization of monomeric forms.

Peroxide accumulation is a balance between the rate of peroxide formation and the rate of peroxide degradation for the particular substance in its environment. For example, certain highly reactive compounds, such as organometallics, accumulate peroxide at low temperatures because the peroxide degradation rate slows relative to the formation rate. By contrast, less reactive compounds such as hydrocarbons or ethers form fewer peroxides at low temperatures.

The more volatile the peroxidizable compound, the easier it is to concentrate the peroxides. Remember that pure compounds are more subject to peroxide accumulation, because impurities can inhibit peroxide formation or catalyze their slow decomposition.

You should routinely test ethers and other peroxide-forming solvents prior to distillation.
Figure 13.2
Figure 13.2. Quantofix® peroxide test sticks. These strips are colorimetric, and can detect peroxides at a range of 1-100 mg/L.

Commercial test strips are available from Sigma-Aldrich Chemical Company (Quantofix® Test Sticks for Peroxides, catalog #37206). These strips are convenient to use; however, they do not have the universality or the sensitivity of the ferrous thiocyanate test, and have a limited shelf life.

Mix a fresh solution in the following proportions:

  • 5 units of 1% ferrous ammonium sulfate [(NH4)2Fe(SO4)2] in water
  • 0.5 units of 1N sulfuric acid (H2SO4)
  • 0.5 units of 0.1 molar ammonium thiocyanate (NH4SCN); to make a 0.1M solution, dissolve 7.16 grams in 1L of distilled water
  • Decolorize with a trace of zinc dust if necessary

Shake an equal quantity (six units) of the solvent to test with the above reagent. Appendix 13-A, Table III shows the relation between color change and peroxide content.

Purchase quantities of peroxidizable compounds according to short-term needs. For instance, buy several 100-mL cans of diethyl ether instead of one 1-L can. Even though this might be more expensive per volume, this purchasing method helps prevent expiration and product loss, and reduces the peroxidation potential. Purchases corresponding to use requirements help minimize exposure to air from multiple openings of the container. A tight cap on a nearly full bottle provides effective protection against peroxide formation.

You can keep peroxide accumulation low by storage in reasonably full containers (25% maximum headspace) with TIGHT caps that you replace promptly after use. Further protection is available when you flush the headspace over peroxidizable compounds with nitrogen (inert gas) before closing the container. Vinyl monomers (Appendix 13-A, Table I, List C) containing certain inhibitors are exceptions and require air in the headspace.

Oxidation inhibitors are useful, and recommended by several chemical manufacturers. Hydroquinone, alkyl phenols, aromatic amines, and other oxidation inhibitors are effective in preventing peroxide formation during storage of peroxidizable compounds. If you add an inhibitor, make sure it is compatible with use or purity requirements of the compounds. Follow a program of periodic testing and replenishing inhibitor levels during storage of peroxidizable materials.

As part of the Laboratory Safety Plan, your research group must maintain an inventory of peroxidizable compounds and review it twice a year. Discard those compounds that are out of date. The EHS Safety Labels webpage includes a printable label on which you can enter the date you received a peroxidizable material, date opened, and date to evaluate, treat, or discard.

Evaluate List A (Appendix 13-A, Table II) materials for peroxide content at least every three months after opening, followed by re-dating if safe, treating or discarding. Before disposing any List A materials, review the properties of the material (preferably with EHS consultation) to ensure safe disposal.

Do not store List B (Appendix 13-A, Table II) materials in your lab for longer than 12 months after opening, unless a suitable test shows they have not accumulated peroxide. If List B materials give a significantly positive test (red by the ferrous thiocyanate test) you must retain them, treat to remove peroxide, repackage, show by test to be peroxide-free, and then re-date the label.

List C (Appendix 13-A, Table II) materials should not be stored for longer than 12 months, unless test results show them to be peroxide-free. Commercial vinyl monomers usually contain additives that inhibit peroxidation. Generally, you should store inhibited vinyl monomers under air rather than nitrogen or other inert atmosphere, because customary inhibitors are phenolic compounds, which require oxygen for their action. Isolation of uninhibited (and hazardous) vinyl monomer is usually not necessary, since most vinyl monomers can polymerize without removal of inhibitor by proper adjustment of initiator concentration.

Uninhibited List C materials can be a significant hazard. Do not store more than 500 grams of uninhibited monomers for longer than 24 hours. Smaller samples (less than 10 g) may be stored longer than 24 hours with discretion. Generally, storage of uninhibited vinyl monomers should be under nitrogen and below room temperatures. For storage in excess of 24 hours, add a suitable inhibitor, with its name and quantity on the label.

The suggested safe storage period if unopened from manufacturer is up to18 months from receipt or stamped expiration date, whichever comes first.

Each container of peroxide-forming chemicals must have the following dates written on the label:

  1. Date Received
  2. Date First Opened
  3. Date to be Discarded

You may use the label available from the EHS Safety Labels page to enter these dates. When you chemically remove peroxides, make a notation on the label to indicate the new disposal date.

Store peroxide-forming chemicals together in full, airtight, opaque containers at temperatures below 30°C (86°F) and in the dark. Use only refrigerators designated “explosion-proof”.

You can easily remove peroxide impurities in water-insoluble solvents (ether, hydrocarbons) by shaking with the following solution:

  • 60 g of ferrous sulfate
  • 6 mL of concentrated sulfuric acid
  • 110 mL of distilled water

Water is introduced by this method. Therefore, post-drying is required if you need a dry solvent. If you want to treat more than 500 mL of solvent, an additional batch of peroxide-removal solution might be necessary. Test all solvents for peroxides after these procedures, to ensure adequate removal has occurred.

Immediately set aside and DO NOT USE any peroxide-forming chemicals that have formed crystals, precipitate, solids or an oily viscous layer, or any rusted, damaged, undated or suspicious containers of peroxide-forming chemicals. Call EHS at 919-962-5507 for assistance. Put up a sign near the container to warn other personnel not to touch it, until trained personnel can remove it from your lab.

Never attempt to force open a rusted or jammed cap, or a cap encrusted with scale, on a container of peroxide-forming chemicals. Never attempt to clean by scraping or rubbing glassware or other containers if an oily deposit or crusty residue is present.

Empty containers of ethers and other peroxide-formers must be triple-rinsed with water before discarding.

You should routinely test ethers and other peroxide-forming solvents prior to distillation or evaporation. One common error is distilling too close to dryness. Leave at least a 10% volume of liquid in the container to ensure safety.
Perform a safety audit before starting each chemical experiment in the laboratory. This includes a review of possible hazards from the use of peroxidizable chemicals in the experiment. Peroxidation may have already occurred in one or more of the starting materials; it may occur during the process, or in the storage of the products. In every chemical process, consider the following factors relative to (a) the starting materials, (b) the process itself, and (c) the products:

  • Structure – Are peroxidizable structures present or being formed?
  • Process conditions – Will the process condition favor initiation of peroxidation and accumulation of peroxides?
  • Storage – Will storage containers and conditioners reduce peroxide initiation and accumulation, and are all products properly inhibited and labeled?

If the audit indicates that peroxidation or peroxides are present, follow all the described procedures for handling, testing, and removal from this Chapter.

Peroxidation in a chemical process may not only be a serious hazard due to the explosion potential, but it also may affect the results of an experiment because of lower yield and unwanted impurities. Exercise the precautions outlined in this chapter to ensure your safety and the success of your experiments.