Storage of acids and bases: what to keep apart

Storage of acids and bases in a laboratory starts with an inventory of bottles, not with choosing a cabinet. The countertop chemical resistance table, which laboratories use when choosing a countertop, lists 17 acids and bases (test date unknown; this concerns the countertop resistance test in the source table). An inventory of the bottles at a workstation makes it easier to choose chemical storage cabinets and to write the workstation instruction.

Which acids and bases on the list have a CLP entry?#

The list of acids and bases in the countertop chemical resistance table runs from citric to hydrofluoric acid, and the bases are two sodium hydroxide solutions and two ammonia solutions.

The corrosive pictogram is the most common in the entries: 10 reagents on the list have it. The numbers in the pictogram table below do not add up to 17, because one reagent can have several pictograms.

The skull pictogram appears in the entry of 3 reagents, and each of them also has the corrosive pictogram. The oxidiser pictogram appears in one kind of entry only. It belongs to nitric acid, that is, the two nitric acid solutions on the list. The flame pictogram likewise appears in one kind of entry only. It belongs to acetic acid, that is, the two acetic acid solutions. Which reagent has which entry is shown by the full list in the section on the method.

6 reagents on the list have no selected entry. Chromic mixture is a mixture, so its hazards depend on its composition. For both ammonia solutions (ammonia 25% and ammonium hydroxide 28%) and for phosphoric acid an entry exists, but the selection rule did not match it. For chromic acid and oleic acid there is no entry under that name. In each of these cases the source is the supplier's safety data sheet.

Why are acids and bases not stored side by side?#

An acid and a base react with each other, and the CIOP-PIB booklet recommends separating chemicals that may react with each other (opens in a new tab) on the basis of data from the safety data sheet. This is the institute's recommendation for chemical warehouses, but the same principle is useful for a cabinet in a laboratory.

The same booklet gives a simple rule: packages marked with the same warning labels may be stored together. For acids and bases the rule is not enough, because an acid and a base often carry the same corrosive pictogram and still react with each other. That is why acids and bases stand on separate shelves or in separate spill trays, and the safety data sheet settles the rest.

For more on how to divide reagents into groups, see the article on storing hazardous substances in a laboratory.

Which acid needs a place of its own?#

The booklet does not give the threshold above which an acid counts as concentrated, so whether a given solution from the list qualifies has to be checked in its safety data sheet. For concentrated nitric acid the table lists: ozone, oxidisers, hydroiodic acid, concentrated sulfuric acid, powdered metals, carbides, picrates, organic compounds and hydrogen sulfide. Acetic, citric and oleic acid from the list are organic compounds, so they should not stand next to concentrated nitric acid.

For concentrated sulfuric acid the same table lists, among others, nitric acid and hydrochloric acid. In practice, concentrated nitric acid and concentrated sulfuric acid stand apart from each other, and organic acids are kept away from concentrated nitric acid.

Acetic acid needs a closer look. Its entry also has the flame pictogram. Whether the flame applies to a specific solution (5% acetic acid or glacial acetic acid) is stated in its safety data sheet, and that decides whether the bottle goes into a flammable liquids cabinet. Hydrofluoric acid has a separate article, hydrofluoric acid in the laboratory. Its entry has the skull pictogram.

Storage of acids and bases: a checklist#

A division plan can be made from the list of reagents in five steps. Each step relies on the safety data sheet of the specific solution, because an Annex VI entry describes the substance, not the bottle.

1. List the acids and bases at the workstation

   Name, concentration from the label and supplier. Attach the current safety data sheet to each item.

2. Copy the warning pictograms from the sheet

   The corrosive, oxidiser, skull and flame pictograms. Packages with the same pictograms may stand together unless the sheet says otherwise, except for acids and bases (step three) and concentrated nitric acid (step four).

3. Separate the acids from the bases

   Separate shelves or separate spill trays, because an acid and a base react with each other.

4. Set concentrated nitric acid apart

   Away from oxidisers, concentrated sulfuric acid and organic compounds, including organic acids.

5. Check items with several pictograms

   A reagent that is corrosive and flammable, or corrosive and toxic, goes where its most serious hazard according to the safety data sheet requires.

When choosing a cabinet, the article on how to choose laboratory cabinets for reagents is useful.

Acids and bases from the resistance table: results, method and data#

The list covers every acid and every base in the countertop chemical resistance table, that is, 17 reagents. The entries were read on 15 October 2026 from PubChem, for example from the PubChem entry for nitric acid (opens in a new tab), where nitric acid has two entries depending on the concentration of the solution. PubChem is the place the entries were read, not the text of the regulation.

A hazard group is a pictogram from the entry: oxidising, corrosive, toxic, flammable or health hazard. In the full list below, the group describes the entry of the substance, not the solution at the concentration given in the countertop resistance table. For 8 reagents the entry carries Note B, and the threshold limits of these entries (SCL) are not in the data.