Our chemistry education begins very early in life. Most of us were taught the consequences of mixing incompatible chemicals even before we could read. Recall those cartoons from your childhood where a mad scientist is mixing a concoction of chemicals while talking to himself and laughing malevolently. This scene usually ends with an explosion that leaves the laboratory in shambles and the mad scientist covered with black blast streaks, funny hair, and a surprised look. What happened? Incompatible chemicals were mixed, of course! Unfortunately, the cartoon depiction is not too far off from real world “accidents;” parts or the whole lab is often left in shambles, but more importantly, the injuries that do occur in real life don’t go away in the next scene as in the cartoon world.

As we say over and over again, chemical safety begins with a good working knowledge of the chemicals you use and their hazardous properties. When incompatible materials contact each other, the result can lead to explosion as described above, the evolution of toxic or flammable gas, or both. As there are many references on the Internet and in hard copy that provide good information on the compatibility of individual chemicals and chemical classes,^2-6 we thought we would instead focus on some of the issues, situations, and consequences of materials coming into contact with one another.

HAZARDOUS WASTE INCIDENTS
By far, the most frequent “explosion” call we have responded to has been the result of the mixing of incompatible wastes. Some of these happen very quickly and can make a mess when things boil over. Hopefully, this is contained in the hood and no one is splashed. Some reactions occur more slowly; these are the ones that go boom in the night (or the day) after minutes or hours of reaction. I’m sure some of you have gone to add some waste to a closed container and heard a hiss when the cap is unscrewed. Something in that container reacted to build up pressure; if allowed to go on long enough and with sufficient reactants present, the container could explode. In our experience, this is often from using a waste container to hold a different class of chemical than the waste being added. Some residual material remained in the original container and reacted with the added waste. Typically, an organic material and strong acid or oxidizer mixed together producing large quantities of gas. We have also seen cases where rapid polymerization occurred from a reactant and catalyst coming into contact to produce lots of heat, rapid expansion of volume, and similar end results. Toxic gases (hydrogen cyanide, chlorine, etc.) may also be produced as a result of these type reactions. Chemical containers should be triple rinsed and dried before being used for waste accumulation if used for any other class of material than the original one it contained. Containers should be prominently labeled and care taken not to inadvertently mix incompatible wastes. Some of the cases involve pouring waste into the wrong container, either through poor labeling, carelessness, or sometimes just pure laziness (We have one waste bottle already. Why do we need another? Besides, it’s such a pain to write what I added on that pesky label. I’ll just slip some in. Who’s to know?). One note of regulatory caution; there may be local or other jurisdictional requirements for waste management that might dictate parameters such as size, type, and specific labeling of waste containers.

CONTAINER STORAGE
The other big issue is with proximate storage of incompatible chemicals that could have “bad” consequences if mixed. This is of particular concern in earthquake-prone areas, but is important in every area. A shelf collapse, small fire, or waste explosion as mentioned above can cause breakage of many containers resulting in catastrophic results. There are two frequently encountered storage practices that present problems and cause safety professionals angst.