Laboratory Safety Manual - Chapter 13: Safe Handling of Peroxidizable Compounds

Title

Laboratory Safety Manual - Chapter 13: Safe Handling of Peroxidizable Compounds

Purpose

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.

Table of Contents

1. Introduction
2. Peroxidizable Compounds
3. Detection of Peroxides
4. Storage
5. Removal of Peroxides
6. Disposal
7. Distillation and Evaporation Precautions
8. Risk Assessment
9. Appendix 13-A
   1. Table I: Peroxide-Forming Structures
   2. Table II: Common Compounds That Form Peroxides During Storage

I. Introduction

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. Under normal storage conditions, peroxidizable compounds 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. 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 of 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.

It is important to learn how to handle peroxidizable chemicals safely. If your laboratory has and uses peroxide formers, your laboratory safety procedures should emphasize:

• Recognition of chemical structures that may form peroxides (Appendix 13-A, Table I)
• Use of hazard identification labels that include date received, date opened, date tested, and date expired
• Controlled inventory of peroxidizable compounds
  • Do not purchase large quantities of peroxidizable compounds as this could result in large amounts of peroxide formation in unused or forgotten containers.
• Use of peroxide detection tests and peroxide removal procedures
• Proper safety equipment and process procedures

II. Peroxidizable Compounds

Some of the specific compounds that form peroxides during storage are in Appendix 13-A, Table II, lists A, B, C, and D.

• Class A: Compounds that form peroxides that may explode even without concentration.
• Class B: Chemicals that are dangerous when concentrated by distillation or evaporation.
• Class C: Substances for which peroxide formation can initiate explosive polymerization of monomeric forms.
• Class D: Chemicals that may form peroxides but cannot be clearly placed in A-C.

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 peroxides 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.

III. Detection of Peroxides

You should routinely test all peroxide formers within your laboratory. Additionally, ethers and other-peroxide forming solvents should be tested prior to concentration, evaporation, or distillation. Peroxide explosion have occurred during use of a rotary evaporator with peroxide forming compounds, like diethyl ether and tetrahydrofuran (See Peroxide Explosion Injures Campus Researchers at UC Berkley).

Peroxide Test Strips

Commercial test strips are available from a variety of scientific companies like Sigma-Aldrich or Fisher Scientific These strips are convenient to use and have a limited shelf life. An example brand of peroxide strips is shown in 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.

Ferrous Thiocyanate Test

A ferrous thiocyanate test can be used ot also test for peroxide formation and show relative peroxide concentration based on a color change. The procedures for creating a ferrous thiocyanate solution and testing peroxide formers are described below. If it the peroxide former is already suspected to contain a significant amount of peroxides or the container is past the expiration date, do not utilize a ferrous thiocyanate test.

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 0.1M 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. Table 13.1 shows the relation between color change and peroxide content.

Table 13.1. Relation between color change and peroxide content of a compound for the ferrous thiocyanate test.

Color	Percent of Peroxide as H2O2
Barely discernible pink	0.001
Pink to cherry red	0.002
Red	0.008*
Deep red	0.04*

*A percentage of 0.008 or greater is considered hazardous.

IV. Storage

Purchase quantities of peroxidizable compounds according to short-term needs or as-use requirements. Although this might be more expensive per volume, this purchasing method helps prevent expiration, product loss, and reduces the peroxidation potential. Purchases corresponding to use requirements help minimize exposure to air from multiple openings of the container. 

Peroxidizable chemicals should be stored in sealed, air-impermeable, containers and should be kept away from light as this can initiate peroxide formation. A dark amber glass bottle with a tight fitting cap is the most effective container against peroxide formation. 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 many peroxide forming solvent can now be purchased with inhibitors from 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, replenishing of inhibitor levels during storage of peroxidizable materials, and disposal at expiration dates. The following information covers suggested disposal time limited based on the classification the peroxide former falls under. Some common peroxide forming compounds are given as an example below. More compounds and their classification can be found in Appendix 13-A, Table II. Table 13.2 provides a quick overview of the classes, test by dates, and disposal dates. 

• Evaluate Class 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 of any Class A materials, review the properties of the material (preferably with EHS consultation) to ensure safe disposal. Some examples of Class A are isopropyl ether, potassium amide or sodium amide, potassium metal, divinyl ether, and vinylidene chloride.
• Do not store Class 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 Class B materials give a significantly positive test (more than 100 ppm of peroxides by a test strip or red by the ferrous thiocyanate test), leave the container alone and immediately contact EHS (919-962-5507) to dispose of the container. If Class B materials test for less than 50 ppm peroxides by a peroxide test strip or pink by the ferrous thiocyanate test, you can keep the conatiner, treat to remove peroxides, test to ensure it is peroxide-free, and then re-date the label. Some examples of Class B are diethyl ether, tetrahydrofuran, cyclohexene, methyl isobutyl ketone, and dioxanes.
• Class 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. Some examples of Class C are acrylonitrile, acrylic acid, styrene, vinyl acetate, and methyl methacrylate.
• Uninhibited Class 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 of more than 24 hours, add a suitable inhibitor, with its name and quantity on the label.
• Class D materials should be tested quarterly and disposed of if there is significant peroxide formation detected (100 ppm or greater). Some examples of Class D are benzyl ether, 1-pentene, isoamyl ether, dimethoxyethane, and tetrahydropyran.
• The suggested safe storage period of unopened containers from the manufacturer is up to18 months from receipt or stamped expiration date, whichever comes first. After 18 months, the container needs ot be disposed of or opened and tested. If the conatiner is opened and tested, ensure that the procedures are followed for the classification it falls under.

Table 13.2. Suggested Time for Testing and Disposal of Different Peroxide Classes

Classification	Test	Disposal (After Opening)
Unopened chemicals from the manufacturer	18 months	18 months (if peroxides detected)
Class A	Before Use	3 months
Class B	Before distillation or evaporation (and every 3 months once opened)	12 months
Class C (uninhibited)*	Before Use	24 hours
Class C (inhibited)*	Before Use (every 3 months)	12 months
Class D	Every 12 months	If peroxides are detected

*When stored as a liquid, the peroxide-forming potential increases and certain of these monomers (especially butadiene, chloroprene, and tetrafluoroethylene) should then be considered as List A compounds.

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.

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

1. Date Received
2. Date First Opened
3. Date Tested (may be multiple based on classification and expiration date)
4. Date to be Discarded

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

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”.

V. Removal of Peroxides

If when testing for peroxides, the concentration of peroxides is 50 ppm or less and the chemical is not past the expiration date, you can 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, especially prior to drying or utilizing the solvent.

Attempting to remove peroxides via this method with concentrations of peroxides of 50 ppm or greater poses an unacceptable risk to you and those around you. Contact EHS (919-962-5507) immediately and submit a hazardous waste pick-up request for containers 50 ppm or greater.

VI. Disposal

If a container that you have tested has more than 100 ppm of peroxides, immediately set the container aside. 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, do not touch or move. Immediately call EHS at 919-962-5507 for assistance. Put up a sign near the container, cabinet, or surrounding area and 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.

VII. Distillation and Evaporation Precautions

You should routinely test ethers and other peroxide-forming solvents prior to distillation or evaporation. If the concentration is 50 ppm and below, you can eliminate the peroxides via the removal process above or potentially passing the solvent through activated alumina (if a hydroperoxide). One common error is distilling too close to dryness which can cause it to overheat. Distillation can concentrate peroxides, especially if distilled to a dry state. Leave at least a 10% volume of liquid in the container to ensure safety. Depending on the procedure, another option is to carefully add small piece of sodium metal to the distillation vessel to reduce peroxides. Benzophenone can also be used as an indicator for presence of sodium metal and water in solvents (ACS Omega 2018, 3, 10, 12703–12706) as benzophenone in the presence of sodium metal forms a radical with a deep-blue color. Disappearance of the deep-blue color indicates that all sodium metal has reacted and additional sodium metal is required.

VIII. Risk Assessment

Perform a risk assessment 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 risk assessment 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.

IX. Appendice

• Appendix 13-A: Peroxide Forming Chemicals Information

Back to Chapter Twelve

Proceed to Chapter Fourteen