hydroxides gives phosphine, which may either ignite spontaneously or explode in air.

Phosphorus trichloride (PC13) reacts with water to form phosphorous acid with HCl evolution; the phosphorous acid decomposes on heating to form phosphine, which may either ignite spontaneously or explode. Care should be taken in opening containers of PCl3, and samples that have been exposed to moisture should not be heated without adequate shielding to protect the operator.

Potassium (K) is much more reactive than sodium; it ignites quickly on exposure to humid air and, therefore, should be handled under the surface of a hydrocarbon solvent such as mineral oil or toluene (see Sodium). Potassium can form explosive peroxides on contact with air. If this happens, the act of cutting a surface crust off the metal can cause a severe explosion.

Residues from vacuum distillations have been known to explode when the still was vented suddenly to the air before the residue was cool. Such explosions can be avoided by venting the still pot with nitrogen, by cooling it before venting, or by restoring the pressure slowly. Sudden venting may produce a shockwave that can detonate sensitive materials.

Sodium (Na) should be stored in a closed container under kerosene, toluene, or mineral oil. Scraps of sodium or potassium should be destroyed by reaction with n-butyl alcohol. Contact with water should be avoided because sodium reacts violently with water to form hydrogen (H2) with evolution of sufficient heat to cause ignition. Carbon dioxide, bicarbonate, and carbon tetrachloride fire extinguishers should not be used on alkali metal fires. Metals like sodium become more reactive as the surface area of the particles increases. Prudence dictates using the largest particle size consistent with the task at hand. For example, use of sodium ''balls" or cubes is preferable to use of sodium "sand" for drying solvents.

Sodium amide (NaNH2) can undergo oxidation on exposure to air to give sodium nitrite in a mixture that is unstable and may explode.

Sulfuric acid (H2SO4) should be avoided, if possible, as a drying agent in desiccators. If it must be used, glass beads should be placed in it to help prevent splashing when the desiccator is moved. To dilute H2SO4, the acid should be added slowly to cold water. Addition of water to the denser H2SO4 can cause localized surface boiling and spattering on the operator.

Trichloroethylene (Cl2CCHCl) reacts under a variety of conditions with potassium or sodium hydroxide to form dichloroacetylene, which ignites spontaneously in air and detonates readily even at dry ice temperatures. The compound itself is highly toxic, and suitable precautions should be taken when it is used.

5.G.7 Chemical Hazards of Incompatible Chemicals

When transporting, storing, using, or disposing of any substance (see Chapter 4, and section 5.C), utmost care must be exercised to ensure that it cannot accidentally come into contact with an incompatible substance (see Chapter 3, section 3.D). Such contact could result in a serious explosion or the formation of substances that are highly toxic or flammable or both. Oxidizing agents and reducing agents should be separated from one another so that no contact is possible in the event of an accident. These reagents can also pose a risk upon contact with the atmosphere. Storage should be appropriate for the chemical under consideration. Glass systems that are to be evacuated should be taped to prevent danger of flying glass on implosion.

5.H WORKING WITH COMPRESSED GASES

5.H.1 Chemical Hazards of Compressed Gases

Compressed gases expose the worker to both chemical and physical hazards. Such hazards and the equipment required for the safe use of compressed gases are discussed in Chapter 6, section 6.D.

Safe storage, monitoring for leaks, and proper labeling are essential for the prudent use of compressed gases. If the gas is flammable, flash points lower than room temperature compounded by rapid diffusion throughout the laboratory present the danger of fire or explosion. Additional hazards can arise from the reactivity and toxicity of the gas, and asphyxiation can be caused by high concentrations of even inert gases such as nitrogen. An additional risk of simple asphyxiants is head injury resulting from falls following rapid loss of oxygen from the brain. Death can also occur after asphyxiation if oxygen levels remain too low to sustain life. Finally, the large amount of potential energy resulting from the compression of the gas makes a highly compressed gas cylinder a potential rocket or fragmentation bomb. On-site chemical generation of a gas should be considered as an alternative to use of a compressed gas if relatively small amounts are needed. Monitoring compressed gas inventories and disposal or return of gases not in current or likely future use are advisable to avoid the development of hazardous situations.

5.H.2 Specific Chemical Hazards of Select Gases

Workers are advised to consult the Laboratory Chemical Safety Summary (LCSS) and the Material Safety Data Sheet



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