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Peroxide-forming chemicals

Peroxide-forming chemicals are potentially explosive and must be handled properly and in accordance with specific procedures.

There are many compounds with the characteristic O-O peroxide bond. The majority of these substances are flammable liquids and must be handled with care. These compounds are not classed as peroxide-forming substances. However, there are some organic and inorganic compounds that can react with the oxygen in air and form potentially explosive peroxides. The most common peroxide-forming compounds are listed in the following tables (A, B and C). (This list is not exhaustive.)

A good method of avoiding the problems of old peroxide-forming chemicals is to purchase the lowest possible quantity and no more than is required, and to use the solvents and substances in accordance with the “first in, first out” principle. Peroxides and peroxide-forming compounds must be stored at the lowest possible temperature and away from light and heat. When using a refrigerator for storage, you must be sure that the refrigerator is suitable for this purpose.

Never use metal spatulas when working with peroxides. Contamination by metal may lead to explosive breakdown products. Remember, too, that most peroxide-forming chemicals are volatile and readily ignitable. For this reason, work in well-ventilated fume cupboards and avoid ignition sources.

Bottles containing peroxide-forming substances must be labelled specially (see Labelling).

Substitution
Where possible, an attempt must be made to find a replacement substance (duty of substitution). Where this is not possible, those who need to use such substances must learn to recognise them and handle them in the proper manner.

Testing for peroxides
Avoid distilling chemicals that contain peroxides. Always test for peroxides before distilling or vaporising peroxide-forming solvents. If the test is positive, remove the peroxides before use. If peroxide-forming compounds have been open and more than one year has passed (see the list of chemicals that can form explosive peroxides), the substance(s) should be disposed of as hazardous waste (see Disposal).

Chemicals categorised as peroxide-forming must be regularly checked with regard to forming peroxides on a quarterly basis (every 3 months) once the bottle has been opened and before using the compounds. There are several ways to test for peroxides. For example, test strips are easy and handy to use, though less universal and sensitive than, for instance, the ferric thiocyanate test. The storage time is also limited.

Do not test or handle peroxide-forming chemicals where:

Labelling

Bottles containing peroxide-forming substances must be labelled specially as follows:

  • Labelled “PEROXIDE-FORMING”.
  • Date item received.
  • Date first opened.
  • Signature of person who first opened item.
  • Date and signature when chemical(s) last checked.

Disposal

Test all peroxide-forming chemicals before possible use and disposal. If the test is positive with regard to forming peroxides, the concentration must be established before deciding what to do:

  • Concentration below 400 ppm (mg/l): The substance can be used if the peroxide is removed first. (Can also send substances with a content of < 400 ppm of peroxide as hazardous waste if you do not wish to remove the peroxide.)
  • Concentration between 400 and 3000 ppm (mg/l): The substance can be disposed of as hazardous waste (can send chemicals with a maximum peroxide content of 0.3%, equivalent to 3000 ppm). ADR bestemminga kap 2.2.3.2.1, page 181 (NO).
  • Hazardous waste

Speak to the unit’s contact person for waste. Explosive compounds can be delivered to the Indus Chemical Technical Factory See further information regarding explosive waste

Testing for peroxides

There are several different test methods, and those that follow can be used for most organic solvents. There is no suitable, simple test procedure for determining peroxides in chemicals such as alkali metals, alkali metal alkoxides, amides or organometals.

If peroxides are found, they must either be removed where possible, or sent as hazardous waste where possible. See under Disposal for details of the concentrations that can be sent as hazardous waste.

The concentration of the peroxide content must not exceed 400 ppm (mg/l) if the peroxides are to be removed using one of the suggested methods. The chemical can be used after cleaning, but remember to put a new date on the label.

Iodine peroxide test

  • Add 0.5–1.0 ml of the liquid to be tested to approximately the same volume of glacial acetic acid to which 0.1 g of sodium iodide (NaI) or potassium iodide (KI) crystals have been added.
  • A yellow colour indicates a low concentration of peroxides in the sample.
  • A brown colour indicates a high concentration.
  • A blank determination should also be performed.
  • Always prepare a fresh new solution of iodide/glacial acetic acid before use, as it will be seen that after a while the blank sample will be brown due to air oxidation.

Ferric thiocyanate test for peroxides

  • One drop of the solution to be tested is mixed with one drop of reagent solution.
  • A pink or red colour indicates the presence of peroxides.
  • Reagent solution:9 g of iron(II) sulfate heptahydrate (FeSO4·7H2O) in 50 ml of 18% hydrochloric acid (HCl). Add 0.5–1.0 g of granulated zinc (Zn), followed by 5 g of sodium thiocyanate (NaSCN).

When the transparent red colouring disappears, add 12 g or more of sodium thiocyanate (NaSCN) and decant the solution from the unused zinc (Zn) into a clean bottle.

Titanium(IV) oxysulfate test for peroxides

  • Remove a 1 ml sample and add 1 ml of reagent.
  • Shake and allow the mixture to stand for at least 2 minutes; observe the colour.
  • Compare against the calibration/standard curve.
  • Reagent solution:Is a non-prepared titanium(IV) oxysulfate (TiOSO4) solution in 27–31% sulfuric acid (H2SO4). Fluka no. 89532.

See diagram for standard curve for peroxides.In order to be on the safe side when determining the colour, it is recommended that the concentration should not exceed 1500 ppm.The concentration is given as hydrogen peroxide (H2O2).

Peroxide test strips

  • Test strips are commercially available from a number of chemical suppliers.
  • Use the test strips as described in the instructions for use.

Examples of suppliers and their respective product numbers:Sigma-Aldrich: Aldrich, Quantofix no. Z249254 (0–25 mg/l).Aldrich, Quantofix no. Z101680 (0–100 mg/l).

 

Removal

If peroxides are proven to be present in the solvent, these must be removed before use. The concentration of peroxide must not exceed 400 ppm (mg/l). One of the following methods described here can be used. Following removal of the peroxides, the chemical is retested using one of the test methods.

Method 1 – Activated alumina

  • Any peroxides formed can be removed from the solution by allowing it to pass through a short column filled with activated alumina.
  • The method is effective for both water soluble and non-water soluble solvents. Exceptions are alcohols of low molecular weight.
  • The method does not destroy the peroxides, so the alumina must be treated after use with a diluted acidic solution of potassium iodide (KI) or iron(II) sulfate (FeSO4) in order to remove the peroxides.

Method 2 – Iron(II) salt:

  • 60 g of iron(II) sulfate (FeSO4) + 6 ml of concentrated sulfuric acid (H2SO4) + 110 ml of water.
  • Or: 100 g of iron(II) sulfate (FeSO4) + 42 ml of concentrated hydrochloric acid (HCl) + 85 ml of water.