Appendix E

SHOCK SENSITIVE AND HIGHLY REACTIVE CHEMICALS 

HIGHLY REACTIVE CHEMICALS

When the term “routine chemical reaction” is used, it usually implies a safe reaction—safe because the reaction rate is relatively slow or can be easily controlled. Highly reactive chemicals can lead to reactions that differ from routine mainly in the rate at which they progress. Reaction rates almost always increase rapidly as the temperature increases. If the heat evolved in a reaction is not dissipated, the reaction rate can increase until an explosion occurs. This factor must be considered, particularly when scaling up experiments, so that sufficient cooling and surface for heat exchange can be provided.

Some chemicals decompose when heated. Slow decomposition may not be noticeable on a small scale, but on a large scale or if the evolved heat and gases are confined, an explosive situation can develop. The heat-initiated decomposition of some substances, such as certain peroxides, is almost instantaneous.

Light, mechanical shock, and certain catalysts are also initiators of explosive reactions. Hydrogen and chlorine react explosively in the presence of light. Examples of shock sensitive materials include acetylides, azides, organic nitrates, nitro compounds, and many peroxides. Acids, bases, and other substances catalyze the explosive polymerization of acrolein, and many metal ions can catalyze the violent decomposition of hydrogen peroxide.

ORGANIC PEROXIDES

Organic peroxides are a special class of compounds that have unusual stability problems that make them among the most hazardous substances normally handled in laboratories.

As a class, they are low-power explosives, hazardous because of their extreme sensitivity to shock, sparks, and other forms of accidental ignition. Many peroxides that are routinely handled in laboratories are far more sensitive to shock than most primary explosives (e.g., TNT). Peroxides have a specific half-life, or low rate of decomposition, under any given set of conditions. A low rate of decomposition may autoaccelerate and cause a violent explosion, especially in bulk quantities of peroxide. These compounds are sensitive to heat, friction, impact, and light, as well as to strong oxidizing and reducing agents. All organic peroxides are highly flammable, and fires involving bulk quantities of peroxides should be approached with extreme caution. A peroxide present as a contaminant in a reagent or solvent can change the course of a planned reaction.

Precautions for handling peroxides:

1.     The quantity of peroxide should be limited to the minimum amount required. Unused peroxides should not be returned to the container.

2.     All spills should be cleaned up immediately. Solutions of peroxides can be absorbed on vermiculite.

3.     The sensitivity of most peroxides to shock and heat can be reduced by dilution with inert solvents, such as alipathic hydrocarbons. However, toluene is known to induce the decomposition of diacyl peroxides.

4.     Solutions of peroxides in volatile solvents should not be used under conditions in which the solvent might be vaporized because this will increase the peroxide concentration in the solution.

5.     Metal spatulas should not be used to handle peroxides because contamination by metals can lead to explosive decomposition. Ceramic or wooden spatulas may be used.

6.     Smoking, open flames, and other sources of heat should not be permitted near peroxides.

7.     Friction, grinding, and all forms of impact should be avoided near peroxides (especially solid ones). Glass containers that have screw-cap lids or glass stoppers should not be used. Polyethylene bottles that have screw-cap lids may be used.

8.     To minimize the rate of decomposition, peroxides should be stored at the lowest possible temperature consistent with their solubility or freezing point. Liquid or solutions of peroxides should not be stored at or lower than the temperature at which the peroxide freezes or precipitates because peroxides in these forms are extremely sensitive to shock and heat.

PEROXIDE-FORMING COMPOUNDS

Chemicals that form dangerously explosive peroxides are commonly stored and used in laboratories on our campus. Ethers and other peroxidizable materials kept for prolonged periods of time after they have been opened will form peroxides that can react explosively when the container cap is removed or when they are concentrated during laboratory activities.

The following guidelines should be used to handle these products safely:

1.     Limit quantities to minimum amount required.

2.     Date all containers when they are received, when they are opened, and include a target date for disposal on the label.

3.     Test containers for peroxides with either wet chemicals or test strips when opened, after each month of storage, or prior to heating.

4.     If peroxides are found, the material should be decontaminated (e.g., filtering through a column of chromatographic, basic-grade aluminum oxide until the test is negative) or disposed of (contact RS).

5.     Dispose of any peroxide-forming chemical according to expiration date or:

Within three months of opening of: 

Within six months of opening of:

Or within one year of opening of:

Unopened containers of potential peroxide-formers should be disposed of no later than 12 months after the date of receipt.

6.     If stored in a refrigerator, the refrigerator must be explosion-proof.

7.     All peroxide-formers should be stored in dark, cool, dry places; but never in a freezer.

8.     Never distill unless known to be free of peroxides.

9.     Never distill to dryness.

Ethers represent a class of materials which can become more dangerous with prolonged storage because they tend to form explosive peroxides with age. Exposure to light and air enhance the formation of the peroxides. A partially empty container increases the amount of air available, and hence the rate at which peroxides form in the container. It is preferable, therefore, to use small containers which can be completely emptied, rather than take amounts needed for immediate use from a larger container over a period of time unless the rate of use is sufficiently high so that peroxides will have a minimal time in which to form.

POLYNITRO COMPOUNDS 

Many polynitroaromatic compounds are shock-sensitive as are some aliphatic compounds containing more than one nitro group. Many of these compounds are sold and stored with 10 to 20 percent water, which desensitizes their reaction to shock.

Picric Acid

Dry picric acid is highly explosive and should be brought into the laboratory only when specifically required. Users should have a thorough understanding of its hazards. Although not explosive when wetted, picric acid solutions may evaporate to leave the hazardous solid. Properly hydrated picric acid will have a moist, yellow appearance. The weight of a new bottle of picric should be recorded on the label before opening, and again each time before and after removing any material for use. Water should be added as a matter of course. If the tare weight has decreased markedly between usages, it may be attributed to the loss of water and resulting instability. Picric acid should be stored away from combustible materials and should not be kept for extended periods (any material older than two years should be disposed). Old containers of picric acid should not be moved or handled. Contact RS for removal.

If you find shock-sensitive compounds that are old or if you are unsure of their stability, or if precipitate (crystals around neck or cap of bottle) or oily layers appear, do not try to open them.

Shock-sensitive compounds such as picric acid, ethers, dioxane, and tetrahydrofuran should always be handled with regard for their explosive power.

SODIUM AZIDE

Sodium azide, a bactericide widely used in medical research, represent a risk due to the possible formation of explosive azides with copper, lead, and other heavy metals. Several explosions have been documented where sodium azide solutions had been used in laboratory equipment or discarded in waste-water piping systems. These explosions usually occurred when service personnel applied heat or friction to azide contaminated metallic surfaces.

Heavy metal azides are highly explosive and are formed whenever sodium azide is allowed to react with metals such as lead or copper. The formation of metallic azides in sewer systems is thought to result when water combines with azide leading to the formation of hydrazoic acid. Hydrazoic acid, itself an explosive is then able to react with lead or copper to form highly explosive metallic azides. 

To prevent azide formation, the following actions should be considered:

Substitution - Several commercial antibacterial products are available which do not use sodium azide (Clear-Bath, Roccal, etc.)

Storage - Sodium azide solutions should not be stored in cabinets or refrigerators with exposed copper or lead parts.

Decontamination - Decontamination should be performed prior to repair or discard of all sodium azide contaminated metallic components or equipment. The decontamination process is as follows:

1.     Make a dilute (2-10%) solution of NaOH

2.     Pour the NaOH solution into the equipment to flush all contaminated surfaces

3.     Treated materials should remain undisturbed for at least 16 hours

4.     Repeat two more times at intervals of one week

Disposal- Contact RS for disposal of sodium azide.

LIST OF PEROXIDIZABLE COMPOUNDS

This is only a partial list of peroxidizable compounds; not intended to be all inclusive.