Organic Peroxide Formers: Testing and Labeling

Container Labeling Requirements

Containers of (non-exempted) peroxide-forming chemicals MUST be dated when received, dated when opened, and labeled with an expiration date. Highlighting whether peroxide-forming chemicals are inhibited (stabilized) or uninhibited (unstabilized) may aid in clarifying limits on storage time in the lab.

Examples of appropriate labeling are shown below. Labs may write directly on chemical containers or EH&S may provide PEROXIDE-FORMING CHEMICAL labels (see Figure 1) during inspections and upon request (email labsafety@usc.edu).

Rules for labeling and dating organic peroxide formers:

1. Must be dated when received – See Figures 1 and 2.
2. Must be dated when opened – See Figures 1 and 2.
3. The Expiration Date must be listed on the bottle – see Figure 1.
   • Consult the Flowcharts tab of the EH&S Peroxide Formers Guidance Spreadsheet for storage times. 
   • Times are based on Class, storage atmosphere, presence/absence of stabilizer, presence/absence of manufacturer’s expiration date, and whether or not the material has been opened.
   • Expiration date must be updated if it changes, e.g., after opening, or after a negative peroxide test.
4. Peroxide formers may be purchased with an inhibitor (may also be called a “stabilizer”, or rarely “preservative”) added to inhibit peroxide formation. If the product was purchased with an inhibitor already present,” indicate “Yes” on the peroxide caution label.
   • The vendor’s label will say “Stabilized” or “BHT as inhibitor” or something similar – see Figure 3. 
5. If the chemical was tested for peroxides, the test date and results must be recorded on the bottle.
6. For opened Class B materials, a passed peroxide test resets the expiration date by the appropriate time given (see Flowcharts tab of EH&S Peroxide Formers Guidance Spreadsheet):
   • Unstabilized stored in air — 6 months
   • Stabilized stored in air — 12 months
   • Stabilized or unstabilized stored under inert gas — 2 years
7. What should be dated?
   • Class A — ALWAYS date
   • Class C — ALWAYS date, except for extremely small samples (≦ 1 g or 1 mL) which are not also Class A.
   • Class B:
     • Must be dated unless “excepted”.
     • See EH&S Peroxide Formers Guidance Spreadsheet for details.
8. Update the online inventory periodically per dates of receipt, expiration dates, etc. – see EHSA and RSS.
   • Please follow guidance in RSS Chemical Inventory Management SOP.

Figure 1. Peroxide former label.

Figure 2. Dates marked directly on bottle.

Figure 3. Peroxide former stabilized.

Figure 4. Excepted Class B material. See organic EH&S Peroxide Formers Guidance Spreadsheet for details of classification.

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Organic Peroxide Former Testing

What Can / Cannot Be Tested

If labs feel comfortable doing so, testing for peroxides may be completed for certain peroxide-forming chemicals instead of submitting for disposal. ONLY Class A and Class B liquids may be tested for peroxides. Chemicals falling into Class C and solutions containing reactive compounds (e.g., hydrogen chloride in ether) CANNOT be tested for peroxides. These must instead be disposed of according to time limits shown in the flowcharts (see Flowcharts tab of EH&S Peroxide Formers Guidance Spreadsheet).

Note: Diisopropyl ether mainly forms dialkyl peroxides which may not be detected by standard peroxide tests. This high-risk peroxide former should be stored under nitrogen and disposed when it reaches the storage time limit.

When to Test

• Routine testing — Test not later than expiration date.
• On demand testing — Test prior to each use for more hazardous operations where peroxides may become concentrated
  • Example: Distillation or rotary evaporation of organic peroxide former stored in air.
  • Caution! Peroxide hazard may increase surprisingly rapidly for unstabilized materials stored in air. For example, unstabilized THF stored in air may become hazardous to distill in as little as 2-3 days.

How to Test

Commercial Test Strips

Commercial peroxide testing strips are available for purchase from lab supply vendors (e.g., Millipore Sigma, VWR, and Fisher Scientific). Follow the testing strips’ manufacturer-specific instructions for use and storage.

Quantofix Peroxide 100

Quantofix Peroxide 100 testing strips have a range from 0-100 ppm and display a gradient from white to blue with increasing hydroperoxide concentration.

XploSens PS

Other common test strips available for purchase include XploSens PS. These testing strips have a range of 0-500 ppm and display a gradient from white to yellow with increasing concentration of peroxides.

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Interpreting Results

• LESS than 100 ppm – considered generally safe; can keep container in inventory
  • Ensure test date and test results are recorded on the chemical container.
  • Increment the expiration date on the container and in the online inventory by the appropriate time limit:
    • Class A (Note: Diisopropyl ether cannot be reliably tested)
      • Stabilized or unstabilized stored under inert gas — 3 months
      • Class A should NOT be stored in air.
    • Class B
      • Unstabilized stored in air — 6 months
      • Stabilized stored in air — 12 months
      • Stabilized or unstabilized stored under inert gas — 2 years
    • Class C – CANNOT be tested (even if also in Class A)
    • Also see EH&S Peroxide Formers Guidance Spreadsheet, “Flowcharts” tab
• 100 ppm “safe” limit does not apply if peroxide-forming chemical is used in more hazardous operations where peroxides may become concentrated, such as through distillation or rotary evaporation. 
• MORE than 100 ppm – Submit for disposal through EHSA (EHSA Waste Pick-up SOP).

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Alternative Testing

Experienced chemists may wish to perform peroxide testing with reagents rather than commercial testing strips. Links to several papers describing the utility and limitations of various peroxide testing methods are to be found on the “Useful refs” tab of the EH&S Peroxide Formers Guidance Spreadsheet.

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