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Chemical collection and stock control for UK schools, universities and hospitals

Picric acid: safe storage and disposal

If you have found old picric acid, stop here before you do anything else. Do not move the container, do not open it and do not touch any crystals you can see. Keep people out of the room and get a specialist assessment. If you believe there is immediate danger, evacuate the area and call 999, and tell them what you have found.

Picric acid (CAS 88-89-1), also called 2,4,6-trinitrophenol, is a yellow crystalline solid that is supplied wet for a reason. Damp, it can be stored and used. Allowed to dry, it is an explosive, and the salts it forms with metals can be more sensitive still. Schools used it for biological stains and practical work, and histology labs still meet it in fixatives and stains. Its GHS signal word is Danger. The hazard statements are H201 Explosive; mass explosion hazard, H301 Toxic if swallowed, H311 Toxic in contact with skin, H317 May cause an allergic skin reaction, H319 Causes serious eye irritation, H331 Toxic if inhaled, H361f Suspected of damaging fertility, H373 May cause damage to organs through prolonged or repeated exposure and H411 Toxic to aquatic life with long lasting effects.

The Department for Education names picric acid, with 2,4-DNPH, among chemicals that need specialist disposal if they have not been stored correctly.

CAS number
88-89-1
Form
Solid
Storage class
Flammable
Signal word
Danger
Usual EWC code
16 05 06*
  • Explosive pictogramExplosive
  • Acute toxicity pictogramAcute toxicity
  • Serious health hazard pictogramSerious health hazard
  • Hazardous to the environment pictogramHazardous to the environment

This chemical is a manual enquiry

This can become unstable or explosive, especially if it is old or has dried out. Nobody should move it until someone competent has looked at it, so we start with a conversation and usually a store assessment.

Explosive if allowed to dry, and forms more sensitive metal picrates.

It cannot be priced through a form. Please do not open or move it. Call us or send an enquiry and we will talk through the safe next step, which is often a store assessment.

What to do this week

  1. Leave it where it is. If it looks old, crystalline or damaged, do not open or move it and keep people away from the shelf.
  2. Write down the label wording, the rough age and where it is stored, from a safe distance.
  3. Send us those details, and a photo if you can take one without touching it.

Safe storage while you wait for a specialist

The only safe storage advice for suspect picric acid is to leave it where it is. Mark the door so nobody enters casually. Remove nothing nearby unless you can do it with no risk of disturbing the bottle. Keep other activities, such as shelving repairs or cleaning, out of the room.

Do not add water. Do not loosen the lid. Do not wipe the bottle, and do not move it to a place you consider safer. Those are the actions most likely to do harm, and they are decisions for the person assessing the bottle.

Tell your head of department and your senior manager straight away. Keep a record of what you saw, when and where. A photograph taken from a distance without moving anything can help the specialist.

How to tell picric acid may have become dangerous

This is a list for reading from where you stand. Do not pick up the bottle to see it better.

  • Dry-looking yellow solid, with no visible water, or a powder that has separated from the walls of the bottle
  • Crystals on the thread of the lid, around the neck or on the outside of the bottle
  • A metal cap, a metal shelf fitting or a corroded area touching the container
  • A stuck lid. A cap that has seized may have crystals in the thread, and turning it can be the trigger
  • Yellow staining on the shelf, the floor or a nearby container
  • An old supplier name, a handwritten label or no date at all

Age alone is a reason for caution. A bottle bought long ago may have lost its water through a poor seal without much visible sign, so an old sealed bottle gets the same response as one you can see has dried.

Loses its water wetting over time. Dry material and metal picrates around the cap are shock sensitive.

How picric acid must be disposed of

Picric acid does not leave by routine collection. It is not accepted for routine collection and requires a specialist chemical store assessment before removal arrangements can be agreed. A specialist decides how and when it can be made safe for transport, and on the arrangements needed for its removal. If the condition of the material means it cannot wait, that is the setting for emergency chemical removal.

The Department for Education also notes that for certain items, such as desensitised explosives, the carrier will need a classification or an authorisation from HSE. Expect this kind of removal to be planned around the individual container.

SettingUsual List of Wastes codeHazardous
Laboratory chemical, including school and university16 05 06*Yes
Discarded organic chemical, not in a laboratory pack16 05 08*Yes
Human healthcare setting18 01 06*Yes

For transport, wetted picric acid has UN numbers including UN 1344 and UN 3364. Dry material is classed as an explosive under UN 0154. The specialist will determine which applies. The picture may be different for a mixture, such as a stain or fixative.

Nothing about picric acid goes down a sink. That includes rinse water from a stained container. Lead and copper pipework can form picrates in old drain traps, so tell any specialist if you think picric acid has gone down a sink in the past.

Waste codes and transport details for Picric acid
EWC code, laboratory chemicals16 05 06*
Hazardous wasteYes

The code depends on where the waste came from and what it is mixed with. Use the waste code finder to see why.

Do not store picric acid with

  • Metals, because it forms shock-sensitive metal picrates with them. That means copper, lead, iron filings, zinc powder, aluminium powder, nickel, tin and magnesium ribbon.
  • Lime, plaster and concrete dust, including calcium carbonate, calcium hydroxide and calcium oxide, which form sensitive picrate salts.
  • Oxidisers, such as hydrogen peroxide, potassium permanganate and potassium nitrate.
  • Acids and corrosive stock, as a storage class.

Think about metal shelving, and about a bottle that sits on a bare plaster shelf in an older building. Report what you see, and leave the container for the specialist.

  • OxidiserOxidisers can ignite flammable materials or make a fire far more intense.
  • AcidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • CorrosiveA leak of corrosive stock can break down flammable containers and feed a fire.

Named chemicals to keep apart

  • Acetic acidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • Aluminium powderPicric acid forms shock sensitive metal picrates with metals.
  • Calcium carbonatePicric acid forms sensitive picrate salts with lime, plaster and concrete.
  • Calcium hydroxidePicric acid forms sensitive picrate salts with lime, plaster and concrete.
  • Calcium hypochloriteOxidisers can ignite flammable materials or make a fire far more intense.
  • Copper metalPicric acid forms shock sensitive metal picrates with metals.
  • Hydrochloric acidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • Hydrogen peroxideOxidisers can ignite flammable materials or make a fire far more intense.
Show 12 more
  • Iron filingsPicric acid forms shock sensitive metal picrates with metals.
  • Lead metalPicric acid forms shock sensitive metal picrates with metals.
  • Magnesium ribbonPicric acid forms shock sensitive metal picrates with metals.
  • Nickel metalPicric acid forms shock sensitive metal picrates with metals.
  • Nitric acidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • Potassium nitrateOxidisers can ignite flammable materials or make a fire far more intense.
  • Potassium permanganateOxidisers can ignite flammable materials or make a fire far more intense.
  • Sulfuric acidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • Thionyl chlorideA leak of corrosive stock can break down flammable containers and feed a fire.
  • Tin metalPicric acid forms shock sensitive metal picrates with metals.
  • Zinc chlorideA leak of corrosive stock can break down flammable containers and feed a fire.
  • Zinc powderPicric acid forms shock sensitive metal picrates with metals.

Common quantities of picric acid found

It is usually one small bottle, or a few. A school may find a jar of crystals in the back of a cupboard. A histology lab or university may hold a few bottles of stain or fixative, including Bouin's fluid, and a small stock of the solid itself. Related compounds may be close by, such as ammonium picrate, and 2,4-DNPH deserves the same attention.

Usually held as: wetted solid, saturated solution, Bouin fixative.

Part-used bottles and full containers are both fine. Tell us the size and how many, and an estimate is enough for a first price.

Picric acid in different sectors

Schools and colleges. Your first calls are to your head of science, your senior team and your advisory service, CLEAPSS in most of the UK or SSERC in Scotland. Keep pupils and staff away from the room, and plan lessons elsewhere if you need to.

Pathology and histology. Check every fixative and stain with picric acid in its formula. Even a bottle in regular use can carry dried crystals on the cap or thread. Ask a specialist before you change how it is handled. Include old stains in closed departments. Chemical waste stays out of clinical streams, under the healthcare classification guidance.

Universities and research. The main risk is the forgotten shelf in a group that has moved on. Your safety office will usually want to be the first to know.

Add it to your emergency plan as well. If anyone does not know a suspect bottle is present, they cannot warn a fire crew.

Hazard data from PubChem GHS classification, retrieved 31 October 2026. Guidance only. Check your own risk assessment and your supplier's safety data sheet.

Hazard statements

  • H201 Explosive; mass explosion hazard.
  • H301 Toxic if swallowed.
  • H311 Toxic in contact with skin.
  • H317 May cause an allergic skin reaction.
  • H319 Causes serious eye irritation.
  • H331 Toxic if inhaled.
  • H361f Suspected of damaging fertility.
  • H373 May cause damage to organs through prolonged or repeated exposure.
  • H411 Toxic to aquatic life with long lasting effects.
Precautionary statements
  • P210 Keep away from heat, hot surfaces, sparks, open flames and other ignition sources. No smoking.
  • P230 Keep wetted with the specified agent.
  • P240 Ground and bond container and receiving equipment.
  • P250 Do not subject to grinding, shock or friction.
  • P261 Avoid breathing dust, fume, gas, mist, vapours or spray.
  • P262 Do not get in eyes, on skin, or on clothing.
  • P264 Wash hands and exposed skin thoroughly after handling.
  • P270 Do not eat, drink or smoke when using this product.
  • P271 Use only outdoors or in a well-ventilated area.
  • P280 Wear protective gloves, protective clothing, eye protection and face protection.
  • P301+P316 IF SWALLOWED: Get emergency medical help immediately.
  • P302+P352 IF ON SKIN: Wash with plenty of water.
  • P304+P340 IF INHALED: Remove person to fresh air and keep comfortable for breathing.
  • P316 Get emergency medical help immediately.
  • P321 Specific treatment (see supplemental first aid instruction on this label).
  • P330 Rinse mouth.
  • P361+P364 Take off immediately all contaminated clothing. And wash it before reuse.
  • P370+P380 In case of fire: Evacuate area.
  • P372 Explosion risk.
  • P373 DO NOT fight fire when fire reaches explosives.
  • P401 Store in accordance with local, regional, national and international regulations.
  • P403+P233 Store in a well-ventilated place. Keep container tightly closed.
  • P405 Store locked up.
  • P501 Dispose of contents and container in accordance with local, regional, national and international regulations.
  • P203 Obtain, read and follow all safety instructions before use.
  • P260 Do not breathe dust, fume, gas, mist, vapours or spray.
  • P264+P265 Wash hands and exposed skin thoroughly after handling. Do not touch eyes.
  • P272 Contaminated work clothing should not be allowed out of the workplace.
  • P273 Avoid release to the environment.
  • P305+P351+P338 IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.
  • P318 If exposed or concerned, get medical advice.
  • P319 Get medical help if you feel unwell.
  • P333+P317 If skin irritation or rash occurs: Get medical help.
  • P337+P317 If eye irritation persists: Get medical help.
  • P362+P364 Take off contaminated clothing. And wash it before reuse.
  • P391 Collect spillage.

Substance as classified, dilutions may differ.

Questions

Our Bouin fixative bottle has a yellow crust round the cap. Is that the same risk as solid picric acid?

Treat it as a warning sign. The crust may be dried picric acid sitting in the threads, so do not unscrew the cap or carry the bottle away. Keep people clear of it and call for specialist help, describing exactly what you can see.

Will an explosives team have to come out for an old school bottle?

Sometimes, if warning signs are present and an assessor or your authority decides an explosives specialist is needed. Many bottles do not reach that point. You should not make the judgement yourself: report what you can see from a distance and leave the bottle untouched.

A solution bottle has been on a metal shelf for years. What should we do?

Leave it there for now. Any leak or dried residue where it meets the metal could have formed picrates, so sliding it off the shelf is the very thing to avoid. Keep people away from that shelf and raise an enquiry that mentions the metal.

We still use wet picric acid routinely. How often should someone look at it?

Make it part of every stock check, looking at the level, the cap and the threads, and record the result against the bottle. If the level has dropped or crystals have appeared, stop using it and treat it as a warning sign rather than topping it up.

Storing sound, in-use picric acid

If your lab uses picric acid and buys it new, your own risk assessment and the supplier's safety data sheet govern how it is kept. As a general guide, it is isolated in storage, clear of metals and plaster, in its original non-metallic closure and checked on a fixed schedule. It sits under flammables because the wetted solid is classed as a flammable solid.

The better question is if you need to hold it at all. If it has no current use, plan for it to leave while it is still in good condition.

Guidance only. Check your own risk assessment and your supplier's safety data sheet.

Hazard data source: PubChem GHS Classification, retrieved 31 Oct 2026. Formula C6H3N3O7. EC number 201-865-9.

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