The one-stop guide to working with oxidizers: from hazard classification and extinguishing oxidizer fires to mitigating risks

What are oxidizers?

Oxidizers (or oxidizing agents) are substances that contribute to or cause the combustion of other materials by (generally) providing oxygen. Oxidizing gases are said to contribute more than air, while oxidizing liquids and solids themselves are not necessarily combustible.

The ‘flame over circle’ pictogram represents three GHS physical hazard classes 

Mode of action

Not only can oxidizers intensify fires, but they can also cause them or explosions. This is because they widen the flammable range of flammable gases and liquids and lower the flashpoints and ignition temperatures of combustible material, so they ignite more readily. Moreover, the oxidation-reduction potential of the materials involved influences the intensity of the reaction: when strong oxidizing agents encounter strong reducing agents such as metals, metal hydrides or flammable organic solvents, fire or explosion is most possible.¹ The explosive potential can also be increased by exposure to heat, shock, or friction.

Atmospheric oxygen is not required for oxidizer-fed fires

Combustion or burning is a high-temperature exothermic redox reaction between a combustible material (reductant) and an oxidant, usually atmospheric oxygen that affords (often gaseous) oxidized products. As heat is applied to a solid or liquid combustible material, it undergoes thermal degradation (called pyrolysis) into smaller free radical fragments, which form a bond with oxygen in the atmosphere, suppling the energy necessary for pyrolysis to continue.  An oxidizer intensifies a fire by increasing the concentration of oxygen available, thereby increasing the rate of reaction and the amount of heat produced.² This kind of fire does not require atmospheric oxygen, therefore typical firefighting methods are less effective.

What fire extinguisher should I use?

Using the wrong type of extinguisher with certain oxidizer fires can result in a violent reaction or explosion. Always refer to ‘Section 5: Firefighting measures” of the SDS to determine the correct extinguisher. Generally, do not use a halon or halocarbon clean agent extinguisher because they both may react with the oxidizer. Moreover, dry powder extinguishers containing ammonium compounds should not be used with oxidizers that contain chlorine as they may react to produce explosive NCl₃³₃. Also, carbon dioxide or other extinguishing agents that smother atmospheric oxygen are not useful in oxidizer-fed fires.¹¹

Drenching amounts of water may be the agent of choice to fully extinguish an oxidizer-fed fire. However, water extinguishers should not be used if non-miscible flammable liquids are involved as it will lead to the fire spreading.²²

The combustion products of oxidizer-fed fires are generally more toxic

Complete combustion of methane in air generates carbon dioxide and water. However, burned in a chlorine environment, the combustion products are carbon tetrachloride vapor and hydrogen chloride gas, which is corrosive to lung tissue and other mucous membranes.

Compressed gases that are oxidizers

Spontaneous ignition may occur if a gauge or piping system is contaminated with hydrocarbons, resulting in an explosion accompanied by fragmentation of the equipment and possible fire. Moreover, cylinder leaks may lead to oxidizer-enriched environments that widen the flammable range of flammable gases and liquids at both the upper and lower flammable limits.²² Flash points and ignition temperatures may also be lowered; in the extreme case lowered to the point that a substance can ignite at room temperature.

Clothing or other material that is exposed to oxidizer-enriched environments may become saturated, retaining the oxidizer for a minimum of 30 minutes, posing a threat for easy ignition and burning with great intensity.²²

Note also that oxidizing gases are generally very toxic (except oxygen), so tissues such as lung, skin, and eyes are at risk.

How to mitigate the risks of working with oxidizers

1. Receiving:

• Keep a record of the chemicals in your lab. This will allow you to readily locate materials, prevent an excess of inventory, and properly dispose expired oxidizers.

 2. Storage:

• Store oxidizing agents in a metal cabinet away form organic materials, flammables, combustibles, active metals, reducing agents, or chemicals that produce toxic gas.
• Ensure the storage area is cool and dry with no direct sunlight or nearby heat sources.
• Store cylinders of oxidizing agents in an upright position, protect the valve, and secure to prevent them from being knocked over.
• Equip the storage area with proper ventilation and emergency equipment including spill kits, firefighting equipment, and first aid kits.

 3. Handling:

• Perform all reactions involving oxidizers in the fume hood with the sash down, especially if there is a splash hazard or the reaction is exothermic.
• Remove all combustibles, flammables, organic materials, finely divided metals, reducing agents, and other incompatible substances (listed in the SDS) from the work area. Also remove all ignition sources and unnecessary materials.
• Do not use reactive oxidizer mixtures outside of accepted temperature ranges, which may initiate violent reactions.
• For compressed oxidizing gases, verify that gauges, valves, and piping are free of contaminants (such as hydrocarbons) and only being used for that specific gas. If these fittings have previously been used with other chemicals, wash them thoroughly or use new fittings.
• Wear appropriate PPE. This may include nitrile or neoprene gloves, close toed shoes, fire resistant lab coats, goggles, or eye protection with side shields or face shields if necessary.

 4. Emergencies and spills:

• Before working with oxidizers, review SDS documents and familiarise yourself with safety measures.
• Use inert absorbent pads for liquid spills (not combustible solids such as paper towels, clothing, or lab coats). If an acid is spilled, neutralize with bicarbonate.
• For solid spills, the material should be swept up and collected in a lidded waste container.
• If a major spill occurs, immediately remove electronics from the area, leave the area and call for assistance.
• If an oxidizer is spilled on someone, immediately wash the area with water for at least 15 minutes and call emergency services.
• In case of inhalation or ingestion, get fresh air immediately and rinse the mouth out with water but do not induce vomiting. Contact emergency services for assistance.
• Ensure an appropriate fire extinguisher is accessible and properly working.

 5. Alternatives:

• If possible, review your SOP to determine if there are any safer chemical alternatives.
• Begin with the smallest amount possible for novel work to minimize the risk of adverse reactions. Consider the use of dilute oxidizer solutions to reduce the reactivity and degree of health risk.

 6. Waste:

• Treat empty oxidizer containers as oxidizing agents. Do not use empty containers for waste as trace amounts may initiate a fire and/or explosion with incompatible material.
• Treat solid waste including tissues saturated with oxidizing agents as hazardous waste. Do not mix solid oxidizer waste with organic materials.
• Do not dispose of oxidizers down the drain or in a standard garbage can.
• Review the SDS to determine whether a particular waste container is compatible with the oxidizer. Make sure waste containers are properly labelled to prevent mixing of incompatible material.
• Store oxidizer waste away from organic, flammable or combustible waste, and arrange for it to be safely removed as soon as possible.

Reference:

1. Oxidizing Agents – AFNS Safety (ualberta.ca)

2. Safe Operating Procedure (unl.edu)
3. selection-of-fire-extinguishers-for-fires-involving-oxidizers-swimming-pool-chemicals-57e9f82b.pdf (amerex-fire.com)