Solving Chemical Incompatibility Issues

Do you realize that improper storage of chemicals accounts for nearly 25 percent of all chemical accidents? ¹ 

Sadly, all of these are entirely preventable, yet they continue to occur despite the easy availability of excellent resources just a few mouse clicks away.

Example of a 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.² 

Most lab professionals understand that nitric acid reacts violently with organics, producing heat and gas in exothermic reactions. Placed in a sealed container, we know that pressures would build, perhaps beyond the breaking point of the container. Not a good scenario.

But how many of us would have stopped before adding the acid waste to a “clean” container to check or verify the state of the receiving container?

And here is another example to make our point:

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 that burst and sprayed nitric acid and shards of glass all over the lab. 

Approximately 2L of nitric acid waste had been accumulated in a chemical waste bottle that originally contained methanol. Over the course of 12-16 hours, it is likely that some residual methanol reacted with the nitric acid waste and created enough carbon dioxide to over-pressurize 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.² 

These two incidents demonstrate common errors in handling incompatible chemicals. They serve as examples for our topic of this month’s Safety Guys article— the lack of proper segregation or separation of incompatible materials.

Proper chemical storage is paramount

In the good old days, before things got so complicated, we considered acceptable chemical storage in the lab as having a fume hood with two cabinets below—one for corrosives and the other for solvents—and everything else was placed on shelves throughout the lab or out on the work benches. If we were lucky, a flammable storage cabinet might have been included, and possibly a refrigerator for samples and small containers.

Related Article: 8 Chemical Storage Plan Fundamentals

But storing hazardous chemicals is a little more complex than that and depends on a number of factors. Of primary importance is the nature of chemical operations or research focus of the laboratory. Research labs are quite different from production labs, and analytical labs differ from synthesis labs. Another important factor is the level of employee expertise. We lab managers must always keep this in mind and ensure that our people are competent and well-trained.

Defining incompatible chemicals and proper segregation

Let’s focus on incompatible chemical storage and proper segregation of these materials. What does “incompatible” mean anyway? Simply, 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 that react to produce heat, pressure, fire, explosion, or violent reactions when mixed together are termed “incompatible.” To prevent uncontrolled mixing, we need to segregate incompatible materials.

Related Article: Provide Secure, Adequately Spaced, Ventilated Storage of Chemicals

What defines adequate separation? Distance, partitions, cabinets, and containment devices are all acceptable measures to use. In actuality, there are very few strict written guidelines. When we store solids and liquids, physical separation becomes a judgment call and depends on the quantities stored and the type of storage used. The Safety Guys 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 the device is big enough to hold the entire volume of the largest container being stored.

Determining 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. So, we 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 we have provided links to a few in the resources below. We recommend 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 to segregate your chemicals.

Beware the exception

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, you should have lipped shelving and secured storage units at a minimum. Below we provide 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 bench tops 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 shelving in the lab. Prevent any chance of accidental mixing.

As always, safety first. Comments or questions are always welcome. Contact thesafetyguys@labx.com.

References

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. Lab Safety Explosion Incidents, American Industrial Hygiene Association, Falls Church, VA. https://www.aiha.org/get-involved/VolunteerGroups/LabHSCommittee/Incident%20Pages/Lab-Safety-Explosions-Incidents.aspx 

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. https://www.cdc.gov/niosh/npg/CRC

Handbook of Laboratory Safety, 5th edition. CRC Press, LLC, Boca Raton, FL. 2000. Compatibility chart online at https://rehs.rutgers.edu/pdf_files/Chemical_Comp_Chart.pdf

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 at https://rehs.rutgers.edu/pdf_files/Chemical_compatibility.html