V(e). Chemical Safety - Reactives
A. What Are Reactive Chemicals?
Reactive chemicals are chemicals that can, under certain conditions, release very large and potentially dangerous amounts of energy. Reactive chemicals can lead to reactions that differ from the routine mainly in the rate at which they progress. A chemical reaction can be considered routine if the reaction rate is relatively slow or can be easily controlled. It is this question of rate of reaction and ability to control that rate that marks certain chemicals as warranting special precautions and the label “reactive chemical”.
The primary difficulty in identifying reactives stems from the variety of conditions under which certain chemicals can undergo an uncontrollable hazardous reaction. Some chemicals are simply unstable and can vigorously polymerize, decompose or condense, or become self-reactive. Other chemicals can react violently when exposed to common environmental chemicals or conditions. The following discussion highlights the most common groups of reactives and includes examples of chemicals in each group.
Some chemicals react spontaneously with very common chemicals in the environment such as water or the components of the atmosphere. Many pure metals, for example, will oxidize on exposure to the atmosphere. Many chemicals are stable except when combined with certain other chemicals. These hazardous combinations are listed in the table “Classes Of Incompatible Chemicals”in Section V(c).
Some chemicals require very little energy of activation to initiate a spontaneous reaction. If the reaction is exothermic, the energy initially produced may accelerate a continued reaction and a release of energy too violent to be controlled. Temperature, shock, static, or light may trigger an uncontrollable reaction. In some combinations one chemical will act as a catalyst reducing the amount of energy normally needed to initiate or sustain a reaction.
Spontaneous decomposition or changes in physical state, even at a slow rate, may create a reactive hazard by creating a less stable chemical. For some chemicals this decomposition is rapid and violent. For others it is so slow as to be imperceptible but results in a byproduct with a much higher reactivity hazard. Peroxides that can form when certain organic chemicals are exposed to air radically increases the hazards of working with those chemicals. The formation of shock sensitive picric acid crystals from an aqueous solution is a serious hazard created by a simple physical state change in the same chemical.
There are some additional hazardous conditions that are not usually attributed to “reactive chemicals” but should be mentioned. Extreme differences in physical state can cause an uncontrollable release of energy. For example, bringing a hot liquid such as an oil into contact with a liquid with a lower boiling point such as water will cause instantaneous vaporization of the lower boiling point liquid and a violent release of energy.
B. Examples Of Reactive Chemicals
The following list of examples is compiled from several general references.  Manufacturer’s Material Safety Data Sheets or the references cited should be consulted to determine the specific reactive characteristics of a particular chemical.
Oxidizers are chemicals that can readily provide reactive oxygen under certain conditions. When contaminated with organic materials, (e.g., wood. paper, organic chemicals), or other easily oxidizable chemicals, (e.g., metal powders), oxidizers can form unstable and explosive compounds sensitive to shock.
|bromine and compounds||nitrites|
|chlorine and compounds||nitrogen trioxide|
|chromates and dichromates||permanganates|
|iodine and compounds||picrates|
|manganese dioxide||sodium bismuthate|
AETC, 1988. The Potential of Reactive Chemicals, Videotape.
Flinn Scientific, 1987. Chemical Catalog/Reference Manual. Batavia, IL.
Furr, A.K., 1995. CRC Handbook of Laboratory Safety, 3rd Edition. CRC Press, Boca Raton.
National Research Council, 1995. Prudent Practices for Handling Hazardous Chemicals in Laboratories. National Academy Press, Washington, D.C.
NFPA, 1994. Standard 49: Hazardous Chemicals Data. National Fire Protection Association, Quincy, MA.
NFPA, 1994. Standard 325M: Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids. National Fire Protection Association, Quincy, MA.
U.S. Department of Transportation, 1999. 49 CFR Hazardous Materials Table
Water Exposure Sensitive
Water reactive chemicals can develop pressure; generate flammable, explosive, corrosive or toxic gases; or ignite or explode when exposed to water or moisture.
alkali and alkaline-earth metals (sodium, lithium, calcium, potassium, magnesium)
anhydrous metal halides (aluminum tribromide, germanium tetrachloride)
anhydrous metal oxides (calcium oxide)
nonmetal halides (boron tribromide, phosphorous pentachloride)
nonmetal halide oxides (inorganic acid halides, phosphoryl chloride, sulfuryl chloride, chlorosulfonic acid)
nonmetal oxides (acid anhydrides, trioxide)
Air Exposure SensitiveAir exposure sensitive chemicals can develop pressure, generate flammable or explosive gases, ignite or explode when exposed to air.
alkylmetal derivatives (ethoxydiethylaluminum and dimethylbismuth chloride)
analogous derivatives of nonmetals including diborane, dimethylphosphine, triethylarsine, dichloro(methyl)silane
carbonylmetals (pentacarbonyliron and octacarbonyldicobalt)
finely divided metals (calcium, titanium)
metal hydrides (potassium hydride and germane)
partially or fully alkylated metal hydrides (diethylaluminum hydride, triethylbismuth)
Temperature sensitive chemicals may decompose when held above their maximum safe storage temperature resulting in pressure buildup, flammable or explosive gas generation, ignition or explosion.
certain oxidizers (perchlorates, chlorates, nitrates, bromates, chlorites, iodates)
certain “azo” compounds
Spontaneous Decomposition - chemicals which change structure over time and with no apparent stimulation will develop pressure, generate flammable or explosive gases, ignite or explode.
benzoyl peroxide (dry)
contaminated concentrated hydrogen peroxide
Shock, Friction And Static Discharge Sensitive
Shock, Friction, and Static Discharge Sensitive - chemicals that will violently decompose when initiated by shock, friction, or static discharge.
|contaminated oxidizers||organic nitrates|
|diazo compounds||organic and inorganic peroxides (see below)|
|fulminates||picric acid (trinitrophenol)|
Many common laboratory compounds can form peroxides when exposed to air over a period of time. A single opening of a container to remove some of the contents can introduce enough air for peroxide formation to occur. Peroxides are sensitive to heat, friction, impact, and light and are among the most hazardous chemicals that are encountered in laboratories. Their hazard potential is all the greater because they may not be suspected or detected in commonly used solvents or reagents. Many explosions have occurred during distillation of peroxide-containing substances particularly when the distillation has been taken to or near to dryness.
Crystal formation or cloudy appearance inside a container is a possible sign of peroxide formation. Crystal formation is most likely (and most hazardous) around the cap. Friction caused just by turning the cap can cause an explosion that ignites flammable solvent in the container.
Peroxide formation can also occur in many polymerizable unsaturated compounds. These peroxides can initiate an uncontrolled, and sometimes explosive, polymerization reaction.
Structural groups of chemicals that can form peroxides, listed in approximate order of decreasing hazard, include:
ethers and acetals with alpha hydrogen atoms
olefins with allylic hydrogen atoms
chloroolefins and fluoroolefins
vinyl halides, esters, and ethers
vinylacetylenes with alpha hydrogen atoms
alkylacetylenes with alpha hydrogen atoms
alkylarenes that contain tertiary hydrogen atoms
alkanes and cycloalkanes that contain tertiary hydrogen atoms
acrylates and methacrylates
ketones that contain alpha hydrogen atoms
ureas, amides, and lactams that have a hydrogen atom on a carbon atomattached to nitrogen
alkali metals, especially potassium, rubidium, and cesium
organometallic compounds with a metal atom bonded to carbon
C. Labeling And Ranking Standards
Because of the diversity of reactive materials and their potential hazard, several organizations have developed labeling systems for reactive materials. These systems are designed to give general hazard information quickly and to provide some sense of magnitude of the hazard.
Of these systems, the most widely used is the Standard System for the Identification of the Fire Hazards of Materials published by the National Fire Protection Association in NFPA 704. The label used is the popular “NFPA diamond” used on many manufacturer’s labels and storage tanks. Reactivity information is displayed in the right hand, yellow portion of the diamond. The reactivity hazard is ranked, as are the fire and health hazards, using an ordinal ranking system with values of 0 to 4. In addition, the lower portion of the diamond is used to note Special Warnings including water (or moisture) reactives and oxidizing materials. Some suppliers of laboratory chemicals display the NFPA diamond on container labels. Fisher Scientific does this and also includes a yellow storage code for reactives and oxidizing reagents.
Although it is the most common system NFPA 704 has several important limitations. First the influence of quantity still requires judgment by the person using the chemical. For example, how much of a very reactive material can be safely handled with a given procedure. The second limitation is evident from the official title of the Standard. Its original purpose was to “safeguard the lives of those individuals who may be concerned with fires occurring in an industrial plant or storage location where the fire hazards of materials may not be readily apparent”. It was not designed directly for laboratory decision-making.
Even with these limitations, the NFPA labeling system is a very useful first reference for reactivity hazards and an important emergency response information system. The following are definitions from the NFPA 704 system for reactivity. Appendix V(e)-A lists example chemicals for each rating as listed in NFPA 325M.
|NFPA Reactivity Rating|
|The assignment of degrees in the reactivity category is based upon the susceptibility of materials to release energy by themselves or in combination with water. Fire exposure was one of the factors considered along with conditions of shock and pressure.|
|4 Materials that in themselves are readily capable of detonation or of explosive decomposition or explosive reaction at normal temperatures and pressures. This degree usually includes materials that are sensitive to localized thermal or mechanical shock at normal temperatures and pressures.|
|3 Materials that in themselves are capable of detonation or of explosive decomposition or explosive reaction but that require a strong initiating source that must be heated under confinement before initiation. This degree usually includes: materials that are sensitive to thermal or mechanical shock at elevated temperatures and pressures; materials that react explosively with water without requiring heat or confinement.|
|2 Materials that readily undergo violent chemical change at elevated temperatures and pressures. This degree usually includes: materials that exhibit an exotherm at temperatures less than or equal to 150oC when tested by differential scanning calorimetry; and that may react violently with water or form potentially explosive mixtures with water.|
|1 Materials that in themselves are normally stable, but which can become unstable at elevated temperatures and pressures. This degree usually includes: materials that change or decompose on exposure to air, light, or moisture; materials that exhibit an exotherm at temperatures greater than 150oC, but less than or equal to 300oC, when tested by differential scanning calorimetry.|
|0 Materials that in themselves are normally stable, even under fire exposure conditions, and which are not reactive with water.|
D. General Safety Procedures For Working With Reactive Chemicals
- Find out as much as possible about the reagents and procedures before the experiment.
- Investigate the purity of the materials. Determine whether impurities or spontaneous decomposition products (such as peroxides) will make the experiment more hazardous.
- Conduct small scale preliminary experiments to assess the thermodynamic and physical properties of the reaction.
- Use as little of the chemical or as dilute a solution as possible.
- Consider all methods of controlling reaction variables. The rate of addition can be controlled as well as the rate at which the energy of activation is supplied. Cool exothermic reactions adequately to control the reaction rate. Remember to provide cooling arrangements for both liquid and vapor stages if appropriate. Pressure relief valves should be included in pressurized systems and checked before adding chemicals to the system.
- Determine the proper degree of agitation and mixing rate. Add oxidants slowly with appropriate cooling or mixing.
- Use a face shield in addition to goggles when appropriate.
- Work in a fume hood using the sash as a protective shield.
- Have emergency equipment at hand. Be certain that you know where the nearest fire extinguisher is and that it is appropriate for the type of potential fire hazard (see Chapter IV, for Classes of Fire Extinguishers). It is important to consider not only which type of extinguisher would be most effective but also if a particular type of extinguishing medium would cause an increased hazard. For example, diborane, pentaborane, and diethyl zinc react violently with halogenated extinguishing agents.
- Notify people in the laboratory of any new or unique hazards that could potentially be created by use of a reactive chemical.
E. Student Use Protocols
If a research student will be using reactive chemicals with the exception of routine use of small quantities of oxidizers or peroxide formers, the faculty member must develop a written protocol outlining the experimental procedure to be followed, necessary protective equipment and safety precautions, and emergency procedures. This procedure must be reviewed with and given to the student.
F. Special Precautions for Pyrophoric Materials
Direct faculty staff supervision is required for student use of pyrophoric materials. Fire resistant lab coats must be worn.
G. Special Procedures For Peroxide Forming Chemicals
It is important that information on the age of peroxide forming chemicals be maintained and that these chemicals be tested or disposed of on a regular basis.
The following peroxidizable compounds should be labeled upon receipt with preprinted labels that read:
Peroxidizable Compound, Date Opened __________, Discard Or Test Within ___ Months After Opening (or similar wording).
These labels should also be placed on any other compounds known to be peroxide formers. The date and discard period should be filled-in the first time the container is opened.
The level of peroxides can be tested using peroxide test strips that are available from the stockroom. The following are recommendations for testing or disposal of potential peroxide forming chemicals.
Group A: Severe peroxide hazard on storage with exposure to air.
Discard Within 3 Months.
diisopropyl ether (isopropyl ether) [108-20-3]
potassium metal [7440-09-7]
sodium amide (sodamide) [7782-92-5]
vinylidene chloride (1,1-dichloroethylene)* [75-35-4]
Group B: Peroxide hazard on concentration; do not distill or evaporate without first testing for the presence of peroxides.
Discard Or Test For Peroxides After 6 Month
acetaldehyde diethyl acetal (acetal) [75-07-0]
cumene (isopropylbenzene) [98-82-8]
decalin (decahydronaphthalene) [91-17-8]
diethyl ether (ether, ethyl ether) [60-29-7]
diethylene glycol dimethyl ether (diglyme) [11-96-6]
ethylene glycol dimethyl ether (glyme) [110-71-4]
ethylene glycol ether acetates
ethylene glycol monoether (cellosolves)
methyl isobutyl ketone [108-10-1]
tetrahydrofuran (THF) [109-99-9]
tetralin (tetrahydronaphthalene) [119-64-2]
Group C. Hazard of rapid polymerization initiated by internally formed peroxides.
Normal Liquids; Discard Or Test For Peroxides After 6 Months.**
chloroprene (2-chloro-1,3-butadiene)+ [126-99-8]
vinyl acetate [108-05-4]
Normal Gases; Discard After 12 Months.++
tetrafluoroethylene (TFE) [116-14-3]
vinyl chloride [75-10-4]
- Polymerizable monomers should be stored with a polymerization inhibitor from which the monomer can be separated by distillation just before use.
- Although common acrylic monomers such as acrylonitrile, acrylic acid, ethyl acrylate, and methyl methacrylate can form peroxides, they have not been reported to develop hazardous levels in normal use and storage.
- The hazard from peroxides in these compounds is substantially greater when they are stored in the liquid phase, and if stored without an inhibitor they should be considered as in group A.
- Although air will not enter a gas cylinder in which gases are stored under pressure, these gases are sometimes transferred from the original cylinder to another in the laboratory, and it is difficult to be sure that there is no residual air in the receiving cylinder. An inhibitor should be put into any secondary cylinder before one of these gases is transferred into it. The hazard posed by these gases is much greater if there is a liquid phase in such a secondary container, and even inhibited gases that have been put into a secondary container under conditions that create a liquid phase should be discarded within 12 months.