Look around you in the laboratory. Take note of the system used to store the chemicals and the conditions and environment they are stored in. More than likely, you will see at least one of the following examples of poor chemical storage practices there in your laboratory :
Accidents resulting from poor storage techniques are preventable. In most cases, the above poor storage practices have not yet led to disaster. However, the potential for such a disaster is extremely high. This section will provide information on alternative storage systems which are meant to circumvent outdated storage methods and lower the potential for an incompatible reaction. Before discussing categorical storage arrangements, the three alternative storage methods (random, alphabetical and incomplete categorical) will be discussed.
Random storage - By far the worst storage system involves no system at all, that is, random storage. With this system, there are no restrictions to where chemicals are stored and no limit to the number of adverse reactions that may arise due to incompatible contacts. You may find acids next to bases, oxidizers next to flammables, water reactives next to the sink and severe poisons next to the writing desk. This is a laboratory waiting for a disaster to happen.
Alphabetical storage - Probably the most common chemical storage practice in the recent past is that of storing chemicals in alphabetical order. When chemicals are stored alphabetically, the situation is improved over the random storage system, but there is still a great potential of incompatible substances coming in physical contact, particularly during an emergency situation such as a fire, spill or natural disaster. A wide variety of examples are possible to illustrate the problems associated with alphabetical storage (see Brethericks' Handbook of Reactive Chemical Hazards, or NFPA 491M : Manual of Hazardous Chemical Reactions) that may be encountered, and the danger associated with the chance encounter. The following list provides numerous of these examples.
Acetic acid + acetaldehyde
small amounts of acetic acid will cause the acetaldehyde to polymerize, releasing large amounts heat
Acetic anhydride + acetaldehyde
condensation reactions can be violent -- explosive
Acrolein + ammonia, aqueous
extremely violent polymerization reaction of acrolein and any alkali or amine
Aluminum metal + ammonium nitrate
potential explosion
Aluminum metal + antimony trichloride
aluminum metal burns in the presence of antimony trichloride vapor
Aluminum metal + any bromate (or chlorate or iodate)
finely divided aluminum plus these compounds produces potential explosion that is detonated by heat, percussion, friction or light.
Aluminum chloride - self-reacting
upon prolonged storage, explosion occurs when container is opened
Ammonium nitrate + acetic acid
mixture will ignite especially if acid is concentrated
Cupric sulfide + cadmium chlorate
explode on contact
Hydrogen peroxide + ferrous sulfide
vigorous reaction, highly exothermic
Lead perchlorate + methanol
explosive mixture if agitated
Maleic anhydride + magnesium hydroxide
potentially explosive reaction
Mercury nitrate + methanol
mixture has potential of forming mercury fulminate, an explosive
Nitric acid + nitrobenzene
mixtures of nitric acid and nitrobenzene may be detonated
Potassium cyanide + potassium nitrite
potentially explosive mixture if heated
Silver + tartaric acid
explosive mixture
Silver oxide + sulfur
potentially explosive mixture
Sodium + selenium
reaction attended by burning
Sodium + silver bromide, silver chloride, silver fluoride, or silver iodide
forms impact-sensitive systems
Sodium + sulfur
reaction proceeds with explosive violence
Sodium + stannic halides
forms impact-sensitive mixtures
Sodium cyanide + sulfuric acid
release of HCN gas, death
Incomplete or Poorly Chosen Categorical Storage
This system provides some differentiation between hazard classes of chemicals, and as such is an improvement over the alphabetical storage policy. Examples of how chemicals may be divided are listed below.
acids are stored separately, but nitric and perchloric acid are not isolated and perhaps the perchloric acid is stored on wooden (combustible) shelves
solids are stored separately from liquids, but flammable solids are stored next to solid oxidizers
organics are separated from inorganics, but flammables and extreme toxics are not segregated from the less hazardous materials
no provision is made for water reactives, either liquids or solids
Any of these categorical attempts at segregating hazard classes is better than no separation at all, and the resulting potential for dangerous contact between incompatible substances has been greatly decreased. However, undesirable contacts are still possible and more complete classification needs to be done. This is accomplished through a complete categorical storage system.