Chemical storage classes

A working scheme, with limits

These seven classes are our working scheme, not a legal classification. HSE's chemical warehousing guidance uses the transport classes, and your employer or advisory body may have a scheme of its own. If you are a CLEAPSS or SSERC member, keep following their storage advice. Where our class and theirs disagree, follow theirs.

Many chemicals fit more than one description. Glacial ethanoic acid is both an acid and flammable. Our database applies fixed rules in a set order, so each chemical gets one class, and shows its other hazards and incompatibles separately. HSE makes the same point about its own table: class-based segregation does not take account of every chemical incompatibility, and two substances shown as compatible may still react. Check the safety data sheet for anything you are unsure of.

The seven classes

Flammable. Solvents and other liquids that catch fire easily, plus flammable solids and water-reactive metals. Think ethanol, propan-2-ol, acetone, xylene and diethyl ether. Keep them away from oxidisers and heat, in a suitable fire-resisting cabinet or store, and hold only as much as you need in the room you are using. HSE's HSG51 and the Dangerous Substances and Explosive Atmospheres Regulations are the references here. Sodium metal sits in this class but needs special attention, because of how it reacts with water.

Oxidiser. These supply oxygen to a fire or react strongly with organic matter. Potassium permanganate, potassium nitrate, sodium chlorate and chromium(VI) oxide are examples. Store them away from flammables, and from acids. A shelf of oxidisers needs a clear gap from anything combustible, including paper and cardboard packaging.

Acid. Hydrochloric, sulfuric, phosphoric and ethanoic acids, for example. Keep them low, below eye level, and away from alkalis. Acid fumes attack metal fittings, so check cabinets for corrosion. Treat nitric acid and perchloric acid separately, because they are also oxidisers. Hydrofluoric acid needs its own arrangements and should be discussed with your adviser.

Alkali. Sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonia solution. Keep them low and away from acids. Concentrated ammonia also needs to be kept cool and away from oxidisers. Solid hydroxides absorb moisture and carbon dioxide from the air and can seize a lid in place.

Toxic. Substances that can do serious harm in small amounts, or that have long-term effects. Mercury and its salts, lead and barium compounds, and carcinogens are examples. Phenol, chloroform and ethidium bromide are among those to treat with care. Keep them locked, with access limited to those who need it, and separate each one according to its safety data sheet.

Corrosive. Corrosive chemicals that are not simple acids or alkalis, such as zinc chloride, aluminium chloride, thionyl chloride and benzoyl chloride. Many react with water, so keep them dry and tightly closed. Older containers can build pressure. If one looks swollen, leaking or otherwise abnormal, leave it where it is and get specialist advice.

General. Chemicals with no storage class hazard under our rules, such as many inert salts and buffers. General does not mean harmless. It means they do not need separating under this scheme, and a harmful or irritant label may still apply. Do not rely on them as a barrier between incompatible groups.

What must be kept apart

HSE's chemical warehousing guidance gives four pairings that are worth learning by heart:

HSE adds that separating acids from other substances goes some way to making sure incompatible substances are not stored together.

Beyond those, the separations in our scheme are as follows:

A small store may not have room for seven separate cabinets. Separate cupboards are best, then separate shelves with secondary containment trays where your risk assessment supports that, and the most reactive pairs always go furthest apart.

Chemicals that change as they age

Some stock is more dangerous old than new, and a storage class does not show that.

Peroxide formers. Diethyl ether and similar chemicals can form explosive peroxides once opened, particularly with air and light. Write the opened date on the bottle. Review them by their opened date. Do not open or move an old bottle with crystals around the cap, or a bottle whose date you do not know.

Picric acid. Picric acid is kept wet for good reason, as it can become a dangerous explosive when dry. DfE guidance names picric acid and 2,4-DNPH as chemicals that need specialist disposal if they have not been stored correctly. Do not try to rewet or open a bottle that has dried out. Leave it and get specialist advice.

Reactive metals. Sodium and potassium are stored under oil. An old sample that has lost its oil or grown a crust needs care, and should not be handled as ordinary stock.

Labelling a store

The aim is that someone who has never been in the store, including a firefighter, can read it at a glance. Label cabinets and shelves with the class in words, not just a colour. Keep the original supplier label on each bottle, and replace one that is failing while you can still read it. Date every bottle when it arrives and when it is opened.

Keep the stock list current. DfE guidance says it is essential to have an up-to-date inventory and notes that the fire service will need to see it in the event of a fire. Keep the oldest stock accessible so it is reviewed first. The guidance also says the store should be locked, with access restricted to science staff.

Hospital and university stores

The same reasoning applies at a larger scale, with some additions. Bench quantities should be kept as small as your work allows, particularly solvents such as xylene on a histology bench. Formalin and its waste need separate thought, as it is toxic and also sensitising. Research groups may hold unusual stock that a general class fails to describe, so the safety data sheet carries more weight. In all cases, your own risk assessment takes priority over a general scheme.

How the register uses these classes

In the Chemical Register, each row carries its class as a labelled chip. You can group a store by class, and an alert is raised when incompatibles share a location or a row's class does not fit its location. Our database shows each chemical's class and incompatibles.