The use of highly flammable or explosive substances or mixtures of substances is relatively common in organic chemistry laboratories. If proper precautions are taken nothing should happen, however, ignoring these aspects can be dangerous.
As general guidelines, the following considerations may be taken into account:
- If a substance is known to be explosive seek, if possible, an alternative.
- When the use of an explosive or flammable substance is strictly necessary, it should be used in as small quantities as possible and precautions should be taken:
- Work in a fume cupboard and in the absence of flames or heat sources.
- Use adequate refrigeration.
- Use inert atmosphere.
- Use protective shields.
- Add reagents gradually.
The following compounds or groups of compounds may be explosive, by themselves or in the presence of sources of heat, impact or friction:
- Acetylene and its metal salts, such as silver or copper.
- Hydrazoic acid, azides and their organic and inorganic salts (only sodium azide is safe).
- Diazonium salts and diazocompounds.
- Inorganic nitrates, especially ammonium nitrate.
- Alkali metal salts of nitrophenols.
- Peroxides, such as, for example, concentrated hydrogen peroxide solution.
Hazardous mixtures of compounds
Strong oxidizers such as:
- Perchloric acid and perchlorates.
- Chromium trioxide, chromates and dichromates.
- Nitric acid and nitrates.
- Concentrated hydrogen peroxide.
- Liquid air and liquid oxygen.
They are particularly dangerous when mixed with easily oxidizable organic substances such as:
- Alcohols and polyols.
- Materials containing cellulose. For example, paper.
- Garments containing wool or cotton.
These oxidizers are also hazardous when mixed with elements such as sulfur, phosphorus or finely divided metals such as magnesium.
Specific explosion hazards
The following products may present some specific explosion hazards:
This is one of the most common causes of explosions in organic chemistry laboratories. Common substances such as diethyl ether, diisopropylether, dioxane or THF form peroxides when exposed to air or light, although they are not the only ones (see Table 1).
|They form explosive levels of peroxides without the need to concentrate the sample.||Butadieno,1
Cloruro de Vinilideno,
|They form explosive levels of peroxides by concentration of the sample.||Acetal,
Dietilenglicol dimetil éter,
Etilenglicol dimetil éter,
Metil isobutil cetona,
|They can self-polymerize by accumulation of peroxides.||Acetato de vinilo,
Cloruro de vinilo,
Cloruro de vinilo,
|1When stored as a monomer liquid.
2Although they form peroxides, no explosions have been described from these monomers
3When stored as a liquid they form explosive levels of peroxides without the need for concentration. Store in gaseous state in steel cylinders.
Due to their wide use in laboratories, we must especially mention ethers. If they are purified by distillation, there is a progressive increase of peroxides in the residue with the consequent risk of explosion.
To minimize this risk, the following should be taken into account:
- Use peroxide inhibitors (most ethers are marketed with such inhibitors).
- In case of prolonged storage, add inhibitors again.
- Use safety containers.
- Never store these solvents for long periods of time.
- In the case of ethyl ether, there is doubt about the presence of peroxides, so proceed with extraction using sodium sulfite.
Sodium amide and metal potassium
Oxidation products are formed on the surface of both substances. If they are removed with a knife or spatula, explosions may occur. Do not crush in mortars or similar. Use directly.
Excess sodium amide can be destroyed with propan-2-ol. On the other hand, metal potassium is destroyed by mixing it with solid ammonium chloride.
Alkali metals vs. chlorinated solvents
Alkali metals (lithium, sodium and potassium) and others such as aluminum or magnesium, especially when finely divided, react violently with halogenated compounds such as chlorinated solvents (e.g. CCl4) so shavings or residues of these compounds should never be washed with chlorinated solvents.
Perchloric acid can react violently with materials such as cork, rubber, wool or natural fibers (cotton, linen, etc.). If it comes into contact with these materials it can be rapidly absorbed by them, generating high explosion risks.
Chromic acid / nitric acid
Mixtures of chromic acid and nitric acid are often used as material cleaning agents. Therefore, heating these mixtures to wash residues from soiled flasks or glassware can be dangerous.
Azides, with the exception of sodium azide, are explosive. Their inadvertent occurrence in some reactions, such as Sandmeyer’s reaction, can lead to explosions.
Liquid nitrogen (boiling point -196 °C) contains some liquid oxygen (boiling point -186 °C). Excessive evaporation can increase the proportion of oxygen. Contact of this oxygen with flammable or combustible materials is highly dangerous.
Glass joints under high vacuum
If the glass material, which is used to work under vacuum, is not in perfect conditions, or its thickness and resistance are not adequate, implosions may occur once the vacuum is established. Special attention must be paid to the rotary evaporators.
Glass ampoules opening
Some reagents are supplied in hermetically sealed glass ampoules. If volatile reagents are suspected, they should be cooled before opening.
Compressed gas cylinders
Under certain circumstances, metal bullets containing compressed gases (N2, NH3, O2, CO2, etc.) may constitute a risk of explosion, despite their robust construction. A sudden rupture releases an immense volume of gas which can also, depending on its nature, fan flames, etc.
It is therefore advisable to anchor the gas bullets with a chain.
Experimental work under pressure
In the case of flash chromatography, plastic jackets are often used for protection or to minimize risks.