Segregation Based on Incompatibility
There is no clear consensus on what and how many classes of chemicals should be segregated. To a large extent, how the chemical groups are divided and assigned will depend largely upon the amount of space available. More elaborate classification schemes are used by some institutions with specialized needs, the U. S. Coast Guard for instance, which breaks chemical storage into 43 separate classes.
The risk associated with incompatible chemicals coming into contact must be avoided wherever chemicals are handled or stored. In general, when chemicals react to form compounds, energy is consumed or released. When incompatible chemicals react, the generation of energy may be extremely violent resulting in catastrophic explosions. Gaseous products may be formed which are dangerously flammable, giving off vapors which can travel along benchtops to an ignition source, thus creating a dangerous fire situation. Reaction products may also release toxic vapors capable of overcoming nearby laboratory personnel. Finally, even non-hazardous vapors may be harmful if given off in a great enough volume to displace the oxygen in an enclosed area thus creating an oxygen deficient environment.
The mixing of incompatible chemicals can occur either through the accidental mixing of two reactants or when two chemicals are purposefully mixed together, such as during an experiment. In either case, disaster can be avoided if care is exercised before chemicals are handled or stored. As discussed in the previous sections, isolation of chemicals into hazard classes will eliminate most accidental adverse reactions that may occur due to breakage in the storage areas. Careful analysis of chemical properties will curtail adverse reactions involving intentional mixing of chemicals.
Chemical compatibility charts are available which outline general classes of incompatible chemicals. An example, taken from the Coast Guard's CHRIS Hazardous Chemical Data is given below which shows chemicals broken into a more elaborate storage scheme based on 24 segregated groups. Also included are examples of each reactivity group. Other excellent sources of information on chemical incompatibility include The National Fire Protection Association's publication 491M - Hazardous Chemical Reactions, and the National Research Council's Prudent Practices for Handling Hazardous Chemicals in Laboratories.