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Separation Anxiety

Here is a statistic that jumped out at me recently: improper storage of chemicals accounts for nearly 25 percent of all chemical accidents. Why is that? This is a sad statistic, given that all these accidents are entirely preventable, yet they continue to happen despite the availability of numerous and excellent resources just a few keystrokes away.

Vince McLeod, CIH

Vince McLeod is an American Board of Industrial Hygiene-certified industrial hygienist and the senior industrial hygienist with Ascend Environmental + Health Hygiene LLC in Winter Garden, Florida. He has more...

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Avoiding Chemical Incompatibility Issues

Here is a statistic that jumped out at me recently: improper storage of chemicals accounts for nearly 25 percent of all chemical accidents.1 Why is that? This is a sad statistic, given that all these accidents are entirely preventable, yet they continue to happen despite the availability of numerous and excellent resources just a few keystrokes away.

Here is a typical incident:

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The collapse of a shelf in a flammable liquid storage cabinet led to an explosion and a fire in a lab at company X, causing the destruction of two labs and damage ranging from $200,000 to $300,000 in repairs. Spills due to the unstable shelf had occurred previously, but no one had tried to repair or replace the defective shelf. In this instance, 12 containers of hexane were being unpacked into a flammable liquid storage cabinet when one of the shelves collapsed. The resulting three-alarm blaze took about 20 fire trucks and 84 firefighters from several area fire stations more than an hour to extinguish.2

And another one, but at least with a positive aspect:

Flammable Liquid Storage Cabinet Prevents Ignition of Flammable Solvents

A laboratory fire started in a refrigerator used for storing experimental samples—small quantities of solvents and other chemicals. It is thought that an electrical fault may have been the cause. Apparently, the fire burned for some time, igniting the plastic refrigerator lining before burning through the door seal and spreading into the room.

The refrigerator was adjacent to the storage cabinet pictured above, which contained a large quantity of flammable solvents. The photo shows the cabinet after the fire. The scorch on the right side of the cabinet was caused by the burning frig (the seat of the fire). The mark at the top of the door was caused by burning material (a plastic light fitting) that dripped onto the cabinet and continued to burn.

Although burning material had dripped into the lap seal above the door, there was no sign of any flame within the cabinet, and the interior paint finish is in original glossy condition. Although the soot was drawn into the cabinet by the ventilation fan, antiflash vents prevented flames from entering the cabinet.2

These two incidents demonstrate common errors in storing chemicals, the first being a defective, overloaded or incorrectly installed shelf, and the second being improper storage of solvents in a standard refrigerator. They also exhibit another common mistake that is the topic of this month’s Safety Guys article—a lack of proper segregation or separation of incompatible materials.

Important determining factors

Most of us would consider good chemical storage in the lab as having a hood with two cabinets below, one for corrosives and the other for solvents, and everything else being placed on shelves throughout the lab or out on the workbenches. If we are lucky, there may be a flammable storage cabinet and possibly a refrigerator for samples and small containers.

But safely storing hazardous chemicals is much more complex than that (as we have just read) and depends on a lot of factors. Perhaps the most obvious is the nature of chemical operations or the research focus of the laboratory. A large, busy organic synthesis research lab is a lot different than a dedicated inorganic water quality lab both in terms of types and quantities of chemicals in use and storage. The next most important factor for the lab manager is the level of employee expertise. We must always keep this in mind and ensure that our people are welltrained. Finally, there are the alwayspresent local and state regulations and building and fire codes.

Determining proper segregation for laboratory chemicals

Not discounting the importance of the wide variety of requirements and limitations mentioned above, we are going to focus here on incompatible chemical storage and proper segregation of these materials. What does “incompatible” mean anyway?

When certain chemicals are mixed or come into contact with each other, chemical reactions occur. The uncontrolled mixing or contact usually happens in circumstances such as spills, leaking containers or partially open containers, and it results in reactions that produce hazards. The hazards include heat or pressure, fire or explosion, violent reactions, toxic or flammable mists, and fumes or gases. Chemicals that react to produce these hazards when mixed together are termed “incompatible.” To prevent uncontrolled hazard production, we need to segregate incompatible materials and store them separately from each other.

How do we know we have adequate separation? Separation is accomplished by distance, partitions, cabinets and containment devices. In reality there are very few strict written guidelines. One example involves compressed gas storage and the applicable fire codes that mandate 20 feet or more of separation between oxygen and flammable gases.3 This is a large distance, necessitated by the physical properties of gases, such as their expansion and dispersion. When we are dealing with solids and liquids, physical separation becomes a judgment call and depends on the quantities stored and the type of storage used. We prefer physical barriers or separate cabinets for incompatible groups, when possible. Containment devices are acceptable and work well if space is limited. These can be as simple as plastic tubs to keep acids and bases separated in a shared cabinet. Just make sure that the device is big enough to hold the entire volume of the largest container being stored.

Determining chemical compatibility

The surest way to determine incompatibility is to check the Material Safety Data Sheet (MSDS) for each chemical. The MSDS will give the chemical family for the material and list incompatible substances in the reactivity data section. However, checking each one could prove tedious, especially if a lot of different chemicals are used in the lab. So, we usually refer to chemical compatibility matrices or lists that separate chemicals based on generic hazard groups. For example, groups that are used most frequently include the following:

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

Many of these compatibility charts and lists have been published (a few are given in the resources below), and we recommend that you find one that suits your needs and keep it posted in the lab in a conspicuous location so that lab personnel can easily refer to it when storing chemicals. With 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 begin to segregate your chemicals.

A few last words

Some words of caution are in order. 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. Beware of this and seek expert advice when you are unsure about safe storage. In closing, here are a few more guidelines for safe chemical segregation:

  • 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 bench tops or in hoods, except for those being used currently.
  • Do not store incompatible materials one above the other on shelving in the lab. Prevent any chance of accidental mixing.
  • Do separate chemicals into their organic and inorganic families and then compatible groups.
  • Do provide a definite storage place for each chemical and return the chemical to that location after each use.
  • Do store volatile toxics and odiferous chemicals in a ventilated cabinet.
  • Do store flammable liquids in approved flammable storage cabinets or safety cans.
  • Do ensure that shelving materials are appropriate and compatible with the chemicals stored on them (e.g., do not store oxidizers on wooden shelves).

Finally, for those of us in seismically active regions, there are additional precautions (and probably regulations) to address. In these areas we should have lipped shelving and secured storage units, at a minimum. Check with your local authorities for additional guidance. As always, safety first.


1. Hazard Investigation: Improving Reactive Hazard Management. U.S. Chemical Safety and Hazard Investigation Board. Report No. 2001-01-H, NTIS No. PB2002- 108795. 2002.
2. Laboratory Safety Incidents. Laboratory Health and Safety Committee, American Industrial Hygiene Association. March 2009.
3. NFPA 55 Compressed Gases and Cryogenic Fluids Code, 2010 edition. National Fire Protection Association, Quincy, MA. 2004.

Additional Resources

Prudent Practices in the Laboratory: Handling and Disposal of Chemicals. National Research Council. National Academy Press. Washington, D.C. Latest edition.
NIOSH Pocket Guide to Chemical Hazards. National Institute of Occupational Safety and Health. Publication 2005-149.
CRC Handbook of Laboratory Safety, 5th edition. CRC Press, LLC, Boca Raton, FL. 2000. Compatibility chart online here:
A Method for Determining the Compatibility of Chemical Mixtures. U.S. Environmental Protection Agency, Cincinnati, OH. EPA-600/2-80-076. 1980. Compatibility chart online here: