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How to Prevent Uncontrolled Mixing of Chemicals in the Lab

How to Prevent Uncontrolled Mixing of Chemicals in the Lab

To prevent uncontrolled mixing, we need to segregate incompatible materials

Vince McLeod, CIH

Between 2001 and 2018, the US Chemical Safety and Hazard Investigation Board reports there were 262 chemical incidents resulting in 490 injuries and 10 fatalities. What is disturbing is that all of these were entirely preventable.

Take, for example, one recent incident:

Oxidizer Solvent Explosion

A corrosive storage cabinet under a chemical hood in a university undergraduate laboratory was the site of an early morning explosion. Luckily, no one was standing in front of the hood when the explosion occurred. We believe the explosion resulted from nitric acid (an oxidizer) and an organic solvent being mixed in a closed container.

All lab professionals should understand that nitric acid reacts violently with organics, producing heat and gas in exothermic reactions. Placed in a sealed container, we should know that pressures would build, perhaps beyond the breaking point of the container. But, how many of us would have stopped before adding the acid waste to a “clean” container or pause to verify the state of the receiving container?

Consider this all too common scenario:

Glass Waste Bottle Ruptures, Possible Reaction of Incompatible Chemical Wastes

A graduate student sitting at a lab computer was surprised by a chemical waste bottle which burst and sprayed nitric acid and shards of glass all over the lab.

Approximately 2 L of nitric acid waste had been accumulated in a chemical waste bottle which originally contained methanol. Over the course of 12 to 16 hours, it is likely that some residual methanol reacted with the nitric acid waste and created enough carbon dioxide to overpressurize the container. Two other waste containers in the hood were severely damaged and several others were cracked or leaking.

Fortunately, the laboratory worker was not injured.

These two incidents demonstrate common errors in mishandling incompatible chemicals. They serve as evidence that a lack of proper segregation or separation of incompatible materials can lead to dangerous outcomes.

Begin with proper chemical storage

Storing hazardous chemicals is complex and depends on a number of factors. The nature of chemical operations or research focus of the laboratory determines the chemicals in use and the resulting waste streams. Research labs are quite different from production labs and analytical labs differ from synthesis labs. Another important factor is the level of employee expertise. As lab managers, we must always keep this in mind and ensure our people are competent and well trained.

Define incompatible chemicals and proper segregation

The focus is on incompatible chemical storage and proper segregation of these materials. We must understand that certain chemicals, if mixed or allowed to come into contact with each other, produce reactions. The uncontrolled mixing or contact usually happens in circumstances such as spills, leaking containers, or incompletely closed containers and results in reactions that produce hazards. Chemicals which react to produce heat, pressure, fire, explosion, or violent reactions when mixed together are incompatible. To prevent uncontrolled mixing, we need to segregate incompatible materials.


Related On-Demand Webinar: Managing Lab Chemicals Safely


How do we determine adequate separation? Distance, partitions, cabinets, and containment devices are all acceptable measures to use. When we store solids and liquids, physical separation is a judgment call and depends on the quantities stored and the type of storage used. Most safety professionals prefer physical barriers or separate cabinets for incompatible groups. Containment devices are acceptable and work well if space is limited. Just make sure the containment is large enough to hold the entire volume of the biggest container being stored.

Storage based on chemical compatibility

The surest way to check incompatibility is to refer to the safety data sheet (SDS) for each chemical. The SDS will give the chemical family for the material and list incompatible substances in the reactivity data section. However, this could prove tedious, especially if there are a lot of different chemicals used in the lab. Lab managers usually refer to chemical compatibility matrices or lists that separate chemicals based on generic hazard groups. For example, major groups that are used most frequently include:

  • Flammable/combustible liquids (and organic acids)
  • Flammable solids
  • Mineral acids
  • Caustics (bases)
  • Oxidizers
  • Perchloric acid
  • Compressed gases

Many compatibility charts and lists are published online and links to a few are provided in the resources below. It is recommended you find one that suits your needs and refer to it when storing chemicals. Using your favorite compatibility reference and the lab’s chemical inventory, you can quickly determine how many different groups (and thus, storage spaces) are needed and segregate your chemicals.

There are always exceptions

No matter how complete your list seems or how complex the compatibility matrix appears, there is always the exception chemical, the one that falls into two (or more) groups. Seek expert advice when you are unsure about safe storage.

If you are in seismically active regions, additional precautions (and probably regulations) might apply. In these areas, storage shelving should have lips and secure storage units at a minimum. Provided below are bulleted brief guidelines for safe chemical segregation:

  • Separate chemicals into their organic and inorganic families and then compatible groups.
  • Provide a definite storage place for each chemical and return the chemical to that location after each use.
  • Store volatile toxics and odiferous chemicals in a ventilated cabinet or hood.
  • Store flammable liquids in approved flammable storage cabinets or safety cans.
  • Ensure shelving materials are appropriate and compatible with the chemicals stored on them. (e.g., do not store oxidizers on wooden shelves).
  • Do not store chemicals alphabetically as a general group. Separate into compatible groups first.
  • Do not store chemicals on high shelves or in high cabinets. A good rule is to store them at eye level or below.
  • Do not store chemicals on benchtops or in hoods except for those being used currently.
  • Do not store incompatible materials one over the other on shelving in the lab. Prevent any chance of accidental mixing.
  • Remember, safety first.

Additional resources:

  • Prudent Practices in the Laboratory: Handling and Disposal of Chemicals. National Research Council. National Academy Press. Washington, DC. Latest edition.
  • NIOSH Pocket Guide to Chemical Hazards. National Institute for Occupational Safety and Health. Publication 2005-149. http://www.cdc.gov/niosh/npg/