Chemical Safety Begins with a Good Working Knowledge of the Chemicals You Use and their Hazardous Properties
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, in real life the injuries that do occur 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, the evolution of toxic or flammable gas, or both. As there are many, 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 happened in the hood, is contained, 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 built-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 that was previously used 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 of any other class of material than the original one it contained. Containers should be prominently labeled and care should be 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?).
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 with catastrophic results. There are two frequently encountered storage practices that present problems and cause safety professionals angst.
Largely Uncontrolled Storage:This ranges from random storage, storage to meet convenience, or the anarchist method in laboratories where everyone manages their own chemicals however they choose. There are few restrictions regarding where chemicals are stored and, as a result, the possibility of adverse reaction is almost limitless. You might find acids with bases, oxidizers with organics, organic acids with inorganic acids and oxidizers, water reactives near water sources, and two-part kit systems with the reactants next to one another.
Alphabetical Storage: Though convenient, this common chemical storage practice still can result in highly incompatible materials in close proximity. This is especially a concern in an emergency situation such as a fire or natural disaster.
The best approach is to sort each chemical according to its general reaction class and separate them accordingly. The proper sorting does take specific knowledge and consideration. Perchloric acid, for example, should not be stored with sulfuric acid. Both are called acids, so what’s the problem? Perchloric acid is a strong oxidizer and becomes unstable and can explode when concentrated. Sulfuric acid is a very effective dehydrating agent. It removes the water from other solutions and can, when mixed with perchloric acid, sufficiently concentrate it to become unstable and, if contaminated with an organic, explode. Know your materials!
How Close is Too Close?
So how close is too close when storing incompatible chemicals? This is a judgment call depending upon the materials and quantities present. We prefer to look for physical separation by a barrier of some sort that would prevent mixing. This might be cabinets devoted to certain reactive chemical classes (e.g., inorganic acids beneath the hood, organic liquids in the flammable cabinet). The storage area itself should be compatible with the materials stored (e.g., you shouldn’t store strong oxidizers directly on wooden shelving because of their ability to react with the wood). Secondary containment tubs help provide flexibility and can limit the spread of chemicals should a container break or leak. One should not store incompatible materials one over the other on shelving in the lab even if containment tubs are used. Depending on the materials and quantities present, effective separation of incompatible chemicals could require many separate spaces.
Once again we have barely scratched the surface. There are many excellent references available to assist with chemical compatibility issues and decisions. In all likelihood storage requirements are also referenced in institutional policy or governmental regulations. The next time you take a spin around the lab look at your chemical storage with a fresh eye; hopefully you will like what you see, and if you don’t, you’ll be glad you did!
3. Prudent Practices in the Laboratory, National Research Council, National Academy Press, Washington, D.C., 1995.
4. "Bretherick’s Handbook of Reactive Chemical Hazards,” Academic Press; 7 edition 2006.