Chemical Storage and Incompatible Chemicals

Walk into a well-run chemical store and you won’t find the bottles arranged alphabetically. That seems odd at first; wouldn’t A-to-Z be the easiest way to find things? The problem is that alphabetical order puts chemicals side by side that should never meet. Acetic acid could sit next to acetaldehyde; potassium permanganate next to propanol; sodium next to a bottle of water-based solution. If one leaked, broke or caught fire, the neighbours could react violently.

Safe storage is about keeping incompatible chemicals apart, so that a single accident stays a single accident.

What makes chemicals incompatible?

Two substances are incompatible if mixing them could produce:

• a lot of heat (possibly boiling, splattering or ignition)
• a fire or explosion
• toxic or flammable gases
• a violent, uncontrolled reaction

Most incompatibilities follow from basic chemistry you already know: acids react with bases, oxidising agents react with reducing agents, and reactive metals react with water.

Key incompatible pairs

Keep apart	Why
Acids and bases	Neutralisation is exothermic; concentrated solutions can boil and splatter.
Oxidising agents (nitrates, permanganates, chlorates, peroxides, concentrated nitric acid) and flammable or organic materials	Oxidisers supply oxygen, so fires start more easily and burn more fiercely; some mixtures are explosive.
Acids and cyanides or sulfides	Release extremely toxic hydrogen cyanide or hydrogen sulfide gas.
Acids and bleach (hypochlorite)	Release toxic chlorine gas.
Ammonia and bleach	Release toxic chloramines.
Water-reactive substances (sodium, potassium, lithium, calcium carbide, some metal hydrides) and water or aqueous solutions	Release flammable gases (hydrogen, ethyne) and heat, possibly igniting. See most reactive metal.
Concentrated sulfuric acid and water or organic materials	Very exothermic dilution; it chars many organic substances.
Oxidising acids (nitric, perchloric) and organic acids such as ethanoic acid	Oxidising acids can react with organic compounds, so they’re often stored separately from other acids.

This isn’t a complete list. The safety data sheet for each chemical (sections 7 and 10) lists its specific incompatibilities.

Segregating by hazard class

Most labs group chemicals by their main hazard, then arrange alphabetically within each group if they want. A typical scheme separates:

1. Flammable liquids (solvents such as ethanol, propanone, hexane): in a dedicated flammables cabinet.
2. Acids (inorganic): in a ventilated, corrosion-resistant acid cabinet, often with nitric acid kept separately or in its own tray.
3. Organic acids (ethanoic acid and similar): separate from oxidising acids; often with or near flammables because many are flammable too.
4. Bases/alkalis (sodium hydroxide, ammonia solution): separate from acids.
5. Oxidising agents: away from flammables, organics and reducing agents.
6. Toxic substances: in a locked cabinet, with access recorded.
7. Water-reactive substances: stored dry, away from water sources (including sprinklers and sinks), with reactive metals such as sodium under oil.
8. General inorganic salts that are relatively low hazard (sodium chloride, calcium carbonate): on general shelving.
9. Compressed gases: cylinders chained upright, away from heat, separated by type (flammable from oxidising).

The exact scheme differs between institutions, and national education safety services often publish recommended schemes for schools.

Storage equipment

• Flammables cabinets are fire-resistant, often self-closing, and have a lip or sump to contain spills. They slow the spread of fire and protect contents from external heat.
• Acid and corrosives cabinets resist corrosion (often lined with plastic rather than bare metal, which acid fumes would attack).
• Secondary containment trays under bottles catch leaks and keep incompatible chemicals from mixing if a bottle breaks.
• Low shelves for large and heavy containers, especially corrosives. Never store them above eye level, where a dropped bottle would splash your face.
• Shelf lips or edges stop bottles sliding off.
• Refrigerators for chemicals must be laboratory-grade or spark-free if they hold flammables. Ordinary domestic fridges have internal switches and lights that can ignite solvent vapour. Food must never be stored in a chemical fridge.

Labelling

Every container must be clearly labelled with:

• the full chemical name (not just a formula or abbreviation)
• the concentration for solutions
• the hazard pictograms (see hazard symbols)
• the date received and date opened, especially for chemicals that deteriorate

When making up solutions, label the new container immediately. Unlabelled bottles are one of the most common and most dangerous problems in labs, because disposing of an unknown chemical safely is difficult and expensive.

Chemicals that change on storage

Some chemicals become more dangerous with time:

• Peroxide-forming chemicals, including some ethers (such as diethyl ether and tetrahydrofuran), slowly react with air to form organic peroxides, which can explode when concentrated by evaporation or disturbed. They must be dated, tested for peroxides regularly, and disposed of within a set period.
• Picric acid is stored damp; if it dries out, it becomes a sensitive explosive.
• Hydrogen peroxide decomposes slowly, releasing oxygen, so containers have vented caps.
• Sodium hydroxide absorbs water and carbon dioxide from the air. See strong bases.
• Some indicators and reagents simply lose effectiveness.

Good stock management

• Buy only what you need. Large stocks increase risk and eventually become waste.
• First in, first out: use older stock before newer.
• Keep an inventory of what’s stored and where, including quantities. Emergency services need this in case of fire.
• Check regularly for leaks, damaged labels, corroded caps, crystals forming around lids and chemicals past their disposal date.
• Dispose of unwanted chemicals properly rather than keeping them “just in case”. See disposing of chemical waste responsibly.

Worked example: sorting a shelf

A technician finds these bottles together on one shelf: concentrated nitric acid, ethanol, sodium hydroxide pellets, potassium manganate(VII), dilute hydrochloric acid and calcium carbide. How should they be separated?

• Ethanol → flammables cabinet.
• Potassium manganate(VII) → oxidisers, well away from the ethanol (permanganate and ethanol can react vigorously, even igniting).
• Concentrated nitric acid → acid cabinet, in its own tray, away from organic materials and other acids where possible, because it is both corrosive and a strong oxidiser.
• Dilute hydrochloric acid → acid cabinet.
• Sodium hydroxide → alkali storage, separate from all acids.
• Calcium carbide → dry storage away from water; it reacts with water to release flammable ethyne gas.

Six bottles, five different locations. That’s normal for a safe store.

A school prep room checklist

• Flammables in a flammables cabinet, away from heat and ignition sources.
• Acids separate from alkalis; nitric acid separate from organic materials.
• Oxidisers away from flammables.
• Reactive metals under oil, stored dry.
• Heavy corrosive containers on low shelves in trays.
• All bottles labelled and dated.
• Toxic substances locked away.
• A fume cupboard is not used as a storage cabinet.
• The store is ventilated, secure and inaccessible to students.

Key takeaways

• Chemicals are stored by hazard group, not alphabetically, to keep incompatible substances apart.
• Key pairs to separate: acids and bases, oxidisers and flammables, acids and cyanides/sulfides/bleach, and water-reactive substances and water.
• Use flammables cabinets, corrosion-resistant acid cabinets, secondary containment and low shelves for heavy corrosives.
• Label everything fully and date it; watch for peroxide-formers and other chemicals that change with time.
• Keep stock small, recorded and regularly checked.