Peroxidizable Compounds

Fact Sheet #24
November 2005

I. INTRODUCTION

Some organic and inorganic compounds are capable of reacting with atmospheric oxygen to form potentially explosive peroxides. This reaction of molecular oxygen and peroxidizable compounds is formed by a process called autoxidation or peroxidation. Formation of peroxides is accelerated by light and heat. Substances which have undergone peroxidation are sensitive to thermal or mechanical shock and may explode violently. All laboratory workers must learn to recognize and safely handle peroxidizable compounds. Peroxide forming substances include: aldehydes, ethers (especially cyclic ether), compounds containing benzylic hydrogen atoms, compounds containing the allylic structure (including most alkenes), vinyl and vinylidine compounds. The tables listed below further detail classes of chemicals that may form explosive peroxides.

II. SAFE HANDLING AND USAGE

Labels on peroxide forming substances must contain the date the container was received, first opened and the initials of the person who first opened the container. Including a notice such as Warning-Peroxide Former on the container can also be helpful. They should be checked for the presence of peroxides before using, and quarterly while in storage (peroxides strips are available). If peroxides are found, the material should be decontaminated, if possible, or disposed of. The results of any testing must be placed on the container label. Never distill substances contaminated with peroxides. If the peroxide forming substances has been opened and more than one year has passed the material should be discarded. Never use a metal spatula with peroxides. Contamination by metals can lead to explosive decompositions.

Store peroxides and peroxide forming compounds at the lowest possible temperature, away from light and heat. If you use a refrigerator, make sure it is appropriately designed for the storage of flammable substances. Dispose of Class III (see table) peroxidizable solvents within one year of purchase or 6 months of the opening. Test for peroxides before distilling or evaporating peroxidizable solvents. If peroxides are present treat the solvent to remove the peroxides. If crystals are visibly present on or in the container or lid, or if the container has been opened but not tested and is more than two years old, do not open or test the contents of the container. Call the Office of Environmental Health and Safety (EH&S) at 373-2769 for assistance. Do not use or test any peroxide forming chemical if a precipitate forms or an oily viscous layer appears.

III. TESTING

The following test procedures may be used on most organic solvents. However, there is not a suitable, simple test procedure for detection of peroxides in substances such as alkali metals , alkali metal alkoxides, amides, or organometallics. Do not test or treat any peroxide forming chemicals if you are unsure of the age, there are visible crystals, or a precipitate or oily viscous layer is present.

• Iodide Test:
  Add 0.5-1.0 ml of the solvent to be tested to an equal volume of glacial acetic acid to which has been added about 0.1 g of sodium iodide or potassium iodide crystals. A yellow color indicates a low concentration of peroxide in the sample; a brown color indicates a high concentration. A blank determination should be made. Always prepare the iodide/acetic acid mixture at the time the test is made, because air oxidation slowly turns the blank to a brown color.
• Ferrous Thiocyanate Test:
  A drop of the solvent to be tested is mixed with a drop of sodium ferrothiocyanate reagent which is prepared by dissolving 9 g of FeSO₄-7H₂O in 50 ml of 18% hydrochloric acid. Add 0.5-1.0 g granulated zinc followed by 5 g sodium thiocyanate. When the transient red color fades, add 12 g more of sodium thiocyanate and decant the liquid from the unused zinc into a clean stoppered bottle. Pink or red coloration indicates the presence of peroxides.
• Peroxide Test Strips:
  Test strips are commercially available from a number of vendors, including Gallade Chemical [Cat. No. 10081, (888) 830- 9092] or Laboratory Safety Supply [Cat. No. 1162, (800) 356-0783]. Follow the manufacturer's instructions for using the strips to ensure adequate colorimetric detection.

IV. TREATMENT

If peroxides are detected the solvent must be treated prior to use. All solvents containing peroxides must be treated prior to requesting collection by EH&S to ensure safety in handling, transport, and disposal. Any of the following procedures may be used to remove the peroxides. One of the above test procedures should be employed following treatment to ensure that peroxides have been removed. Do not test or treat any peroxide forming chemicals if you are unsure of the age, there are visible crystals, or a precipitate or oily viscous layer is present.

• Method 1 - Activated Alumina:
  Peroxides can be removed by passing the solvent through a short column of activated alumina. This method is effective for both water-insoluble and water-soluble solvents (except low molecular weight alcohols). Since this method does not destroy peroxides the alumina should be flushed with a dilute acid solution of potassium iodide or ferrous sulfate following treatment to remove peroxides from the alumina.
• Method 2 - Ferrous Salt:
  Peroxide impurities in water-soluble solvents are easily removed by gently shaking with a concentrated solution of a ferrous salt. A frequently used ferrous salt solution can be prepared either from 60 g of ferrous sulfate + 6 ml concentrated sulfuric acid + 110 ml water; or from 100 g of ferrous sulfate + 42 ml of concentrated hydrochloric acid + 85 ml of water.

V. CLASSES OF CHEMICALS THAT CAN FORM PEROXIDES UPON AGING*

Class II: The following chemicals are a peroxide hazard upon concentration (distillation/evaporation). A test for peroxide should be performed if concentration is intended or suspected.

Class III: Peroxides derived from the following compounds may explode without concentration.

The above list provides examples of peroxide-forming chemicals only and is not an inclusive list. Other chemicals are capable of forming peroxides. Consult applicable Material Safety Data Sheets for information on peroxide-forming potential of specific chemicals.

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