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Definition

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Incompatible chemicals give an undesired chemical reaction when mixed. This usually refers to substances that will react to cause an imminent threat to health and safety through an explosion, fire, and/or formation of toxic materials.

The term can also refer to an unwanted change in the physical or mechanical properties of the materials.

Additional Info

Even everyday chemicals have incompatibilities. For example, did you ever notice that containers of bleach have a warning not to mix bleach and ammonia? When mixed, these substances generate the toxic gases chloramine (NH₂Cl) and hydrazine (N₂H₄), which could cause serious injuries or death!

Other examples of incompatibilities include the reaction of alkali metals such as sodium or potassium metal with water. In this case, the products are extremely basic sodium or potassium hydroxide (corrosive), hydrogen gas (explosive) and a lot of heat (an exothermic reaction). The heat generated is so great that the hydrogen generated will usually ignite! See our entry on water reactive chemicals for information on the proper storage of these materials.

Improper storage and disposal of incompatible chemicals has led to a number of accidents. Always properly identify and segregate your chemicals and wastes to avoid the accidental mixing of incompatibles. For example, do not store acids and bases in the same cabinet. Do not store acid waste and organic waste in the same area. See the links under Further Reading for more tips and information.

Books Available

NOTE: We may collect a share of sales or other compensation from the links in the following list:

• "Prudent Practices in the Laboratory: Handling and Management of Chemical Hazards", Hardcover, 360 pages, 2011. Estimated price: $97.87. Info and/or order.
• "Laboratory Waste Management: A Guidebook (ACS Professional Reference Book), 2nd Ed, Hardcover, 256 pages, 2012. Estimated price: $128.99. Info and/or order.
• "Wiley Guide to Chemical Incompatibilities", Paperback, 1,110 pages, 2009. Estimated price: $350.00. Info and/or order.
• "Field Guide to Clandestine Laboratory Identification and Investigation", Hardcover, 144 pages, 2019. Estimated price: $136.60. Info and/or order.
• "Guidelines for Safe Storage and Handling of Reactive Material", Hardcover, 386 pages, 1995. Estimated price: $135.06. Info and/or order.

SDS Relevance

Fortunately, chemical incompatibility is one of the items that OSHA requires on a Safety Data Sheet. You will find incompatibility and safe storage/handling information in Section 7 of the sheet (handling and storage). Read your SDS carefully and make sure the labels on the container include warnings about incompatibilities. If the label lacks such warnings, why not add them?

Most mistakes with chemicals happen not when they are being used properly, but when a process or reaction is being cleaned up. This is when most people tend to let their guards down and when a variety of different kinds of chemical substances can be mixed. If some of these are incompatible, look out! Here's one example of what can happen in these kinds of situations. Note the lessons learned and safety tips that accompany this article.

Further Reading

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• The Chemical Safety Library (free registration required to download reports) contains over 75,000 reagents and is "dedicated to sharing previously inaccessible hazardous reaction information in the interest of increased laboratory and personal safety across the chemical industries". It is a terrific resource for checking that one is not accidentally mixing incompatible materials during laboratory or chemical mixing operations.
• ChemStor is an open source program from UC Riverside that warns users if it's unsafe to mix certain chemicals.
• Practical Segregation of Incompatible Reagents in the Organic Chemistry Laboratory, a proposed system for incompatible chemical reagent segregation (subscription required).
• A handy list of incompatible chemicals (print and save) at the U of KY.
• Chemical Segregation and Storage Table at the NIH Office of Research Services.
• Chemical Storage Guidelines at Lawrence Berkeley National Lab.
• EPA compatibility chart along with extensive chemical waste info at Harvard University.
• Safe Storage of Hazardous Chemicals at UC Berkeley.
• Chemical Compatibility/Resistance Database at Cole-Parmer lists how well certain materials stand up to various chemicals. Very useful.
• 40 CFR Appendix V to Part 264 - Examples of Potentially Incompatible Waste.
• Substituting inorganic kitty litter for regular clay litter at the WIPP nuclear waste repository created an incompatible waste situation which caused an explosion that has cost over two billion dollars to clean up, shut down WIPP for years, and has left some Los Alamos nuclear waste too hazardous to move.
• In 2019, mixing incompatible chemicals resulted in an explosion at AB Specialty Silicones that killed four workers and destroyed the facility.

See also: decomposition, water reactive.

Additional definitions from Google and OneLook.

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