Peroxide Forming Chemicals

Safe Handling of Peroxidizable Chemicals

A wide variety of organic compounds spontaneously form peroxides by a free radical reaction of the hydrocarbon with molecular oxygen. Under normal storage conditions peroxides can accumulate in the chemical container and may explode when subjected to heat, friction or mechanical shock. The danger is increased when a peroxide forming chemical is concentrated by distillation or evaporation. Peroxides tend to explode violently and are capable of causing serious injury or death to researchers in the laboratory. It is imperative that all researchers learn to recognize and safely handle peroxidizable chemicals. Use of any peroxide forming chemical requires a SOP, reviewed by EH&S and approved by your PI. If the lab does not have an existing SOP that covers your process please contact EH&S, prior to use, for assistance in developing a process-specific SOP.

Recognition of Chemicals Prone to Peroxide Formation

The first step in learning how to handle peroxidizable chemicals safely is to learn how to recognize materials which are prone to hazardous peroxide formation. Although ethers are the most notorious
peroxide formers, other peroxidizable organic materials include acetals, certain allyic alkenes (olefins), chloro- and fluoroalkenes, dienes, aldehydes, amides, lactams, ureas, some alkylarenes, ketones, vinyl monomers, and some alcohols. A more extensive list of peroxidizable chemicals is available here.

Classes of Peroxide Formers

Different peroxidizable chemicals tend to form dangerous levels of peroxides at varying rates and under different conditions. For some materials the concentration of peroxide reaches dangerous, shock sensitive levels, without concentration. Other compounds do not usually accumulate potentially explosive concentrations of peroxides unless the volatile organic material is reduced in volume--i.e. through incidental evaporation or distillation.

Peroxide forming compounds fall under one of four classes.

Minimizing the Hazards of Peroxidizable Compounds

Researchers who handle peroxidizable compounds must learn to handle these materials safely. Safe handling practices and procedures involve making effective purchasing decisions, implementing a storage control program, periodic testing for peroxides, and proper disposal of hazardous containers. The elements of safe practices and procedures are described below.

Purchasing

Ideally, purchases of peroxidizable chemicals should be restricted to ensure that these chemicals are used up completely before peroxides can form. This requires careful experiment planning on behalf of researchers. Researchers should purchase no more material than is needed to complete an experiment within the chemical's safe shelf life. Interestingly, peroxides can form prior to the stipulated expiration or shelf-life of the compound and thus testing should be done routinely to ensure the safety of the researcher(s) using these compounds.

Storage and Shelf Life

Peroxides tend to form in materials as a function of age and exposure to oxygen. Therefore, it is imperative that researchers are keenly aware of the age and handling of their peroxidizable chemicals. Researchers must date each container of peroxidizable chemical upon arrival in the laboratory; containers must be dated again when opened for the first time. Testing dates should be noted on the label or other documentation sheet easily communicating the hazards of the chemical to the researchers that may be using it.  Special labels make dating of the containers convenient, and are available from EH&S, 459-2553.

Peroxide forming chemicals should be stored in the original manufacturer container whenever possible. Peroxidizable chemicals should be stored in sealed, air-impermeable, containers and should be kept away from light (light can initiate peroxide formation). A dark amber glass bottle with a tight fitting cap is generally appropriate.

The following table lists the suggested time table for testing of peroxides and different classes of peroxidizable compounds. Suggested time limits are given for testing of these compounds, However, it must be noted that these shelf life durations are minimum criteria, and we recommend more frequent testing for peroxides.

Test Results

For peroxides concentrations of < 20 ppm, either neutralize the peroxides in the compounds using the methods listed under, Removal of Peroxides or dispose of the compounds via WASTe. Indicate the peroxide level in the notes of the WASTe tag. For peroxides concentrations > 20 ppm, contact EH&S (459-2553) for more information.

Testing for Peroxides

There is a great deal of uncertainty regarding the concentration at which peroxides pose a hazard to researchers. It is important to note that peroxide concentrations greater than 20 ppm is considered explosive and poses issues with Hazardous waste vendors will not pick up containers with 20 ppm peroxide concentration. 

There are several methods that are commonly used to detect peroxides in the laboratory. Peroxide test strips provide a simple and convenient mechanism for detection, they are available from several suppliers. For volatile organic chemicals 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. Test strip color is compared with a colormetric scale provided on the test kit bottle. Strips that offer a 1-100 PPM peroxide range are useful for determining if the material is below the control point of 100 PPM. Other testing methods are available, see manufacturer, SOP, or contact EH&S (459-2553) for more information.

Before the end of the storage date for a particular peroxide forming chemical the person using the chemical should either test and dispose of it or test it monthly for peroxide content. Any container found to have a peroxide concentration greater than or equal to 100 PPM should be disposed of (contact EH&S, 459-2553, for assistance). Materials which are older than the suggested shelf life but have been tested and have no detectable peroxides or peroxide concentrations less than 100 PPM may be retained. However, these materials should be tested at frequent (monthly or quarterly) intervals and peroxides reduced (neutralized) prior to any distillation or heating.

All chemicals which are to be distilled must be tested prior to distillation regardless of age. Important note - researchers should never test containers of unknown age or origin. Older containers are far more likely to have concentrated peroxides or peroxide crystallization in the cap threads and therefore can present a serious hazard when opened for testing. Please read section below on managing older containers.

Management and Disposal of Old Containers

Older containers of peroxidizable chemicals, or containers of unknown age or history, must be handled very carefully and should never be opened by researchers. If any of such containers are found, please contact EH&S immediately. Any peroxidizable chemical with visible discoloration, cloudiness, crystallization, wisp-like structures, or oily layer (or other liquid heterogeneity/stratification) should be treated as potentially explosive. Older steel containers that have visible rust may also be extremely dangerous. If any of these conditions are observed on a peroxidizable chemical, or if the origin and age of the container are unknown, do not attempt to move or open the container. Please call EH&S, 459-2553, for assistance, we will arrange to have the container(s) inspected and if necessary will arrange for disposal.

Safe Distillation of Peroxide Forming Chemicals

Eliminate the peroxides with a chemical reducing agent or pass the solvent through activated alumina. Adding mineral oil to the distillation pot has the combined effect of “cushioning” any bumping, maintaining dilution of peroxide concentration, and serving as a viscous reaction moderator in case the peroxides begin to decompose. Carefully monitor the distillation process to ensure that it does not dry out completely, and then overheat. Distillation can concentrate peroxides, especially if taken to a dry state. Peroxides will be present mainly in the still bottoms. Small pieces of sodium metal can be added to the distillation vessel to reduce peroxides. Use benzophenone as an indicator for the presence of sodium metal (benzophenone in the presence of sodium metal forms a radical with a deep-blue color). When the blue color disappears, add more sodium metal to the vessel.

Removal of Peroxides

In some cases, it might be desirable to remove peroxides from chemicals rather than dispose of the entire chemical. This can be done safely for relatively low levels of peroxides (less than 100 ppm). Scrubbing of concentrations greater than 100 ppm may pose an unacceptable hazard, depending on the chemical involved. Scrubbing of discolored, crystallized, or layered peroxide formers is certainly too hazardous and should not be attempted. These severely peroxidized compounds should be treated as potential bombs. For those chemicals, where peroxide concentrations are >100 ppm, notify EH&S immediately to discuss the best course of action.

• Method 1
  Hydroperoxides can be removed by passing the solvent through a column of activated alumina. This method works for water-soluble and water-insoluble chemicals. The washed solvent should be retested to ensure that it has been cleaned adequately. The alumina apparently catalyzes the degradation of some peroxides, but in some cases the peroxide may remain intact on the alumina, making it potentially shock sensitive. The alumina can be deactivated by flushing with a dilute acid solution of potassium iodide or ferrous sulfate (description of ferrous sulfate solution below). The amount of alumina required depends on the quantity of peroxide. As a start, a column containing 100 g of alumina should be used for 100 mL of solvent. More alumina or passage through a second column may be required to eliminate peroxides. This method is relatively slow and expensive, but it avoids shaking the solvent and does not add water. It will not reliably remove dialkyl peroxides, although there is some controversy about this.
• Method 2
  Peroxides in water-insoluble chemicals can be removed by shaking with a concentrated solution of ferrous salt; 12 g FeSO4, 12 mL concentrated H2SO4, and 210 mL water are a standard solution (always add acid to water). The peroxide former is extracted two to three times with an equal volume of the reagent. Drying over sodium or magnesium sulfate can be used to remove dissolved water. Shaking should be very gentle for the first extraction. This method has repeatedly shown to be quite effective for most peroxides, but it is not reliable for removing alkyl peroxides. For water-soluble solvents such as tetrahydrofuran (THF), this ferrous sulfate solution can also be used to neutralize the peroxide concentration through stirring.