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

Potassium metal (CAS 7440-09-7): storage and disposal

A small jar of potassium lumps under mineral oil is usually a long held purchase for alkali metal work in a school, college or university. It reacts with water more readily than sodium, and it causes severe skin burns and eye damage. Age is what decides its handling: over time it forms a superoxide crust that can explode when cut or scraped, so old stock is never a routine item and is not quoted online.

CAS number
7440-09-7
Form
Solid
Storage class
Flammable
Signal word
Danger
Usual EWC code
16 05 06*
  • Flammable pictogramFlammable
  • Corrosive pictogramCorrosive

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.

Reacts violently with water and forms a shock sensitive superoxide crust as it ages.

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 in a school or lab

Water heads the list of things to keep away, since it reacts violently and more readily than sodium does, so ammonia solution and every other water based bottle belong elsewhere. Chloroform, dichloromethane and carbon tetrachloride are the neighbours people forget, as alkali metals can react explosively with chlorinated solvents. Bromine and carbon disulfide need a different store as well. Potassium appears on some Class A peroxide-former lists because potentially explosive oxidation products can develop during storage. Receipt and opening dates should be recorded, but a periodic visual review does not show that aged metal can be kept. Do not carry out peroxide testing on potassium metal. Aged or suspect jars require specialist assessment without opening or moving them. If a jar has no date, or the review has lapsed by a long way, do not open or move it.

How to tell it has deteriorated

Begin with the label: no date, or a date nobody can place, is itself a warning on this metal. Next read the oil level from outside the jar, because metal left exposed is where the trouble starts. Only then look through the glass at the lumps. Fresh metal tarnishes quickly, so a dull surface says little by itself. A yellow or orange crust is another matter. It means superoxide has formed, and cutting or scraping crusted metal can cause an explosion. If you see that colour, or the jar is old and undated, do not open it, cut the metal or handle the container, and do not tilt it for a better view. Call for specialist help and keep the area dry.

Forms a superoxide crust that can explode on cutting or scraping.

This can form peroxides as it ages, especially after opening. Age and the opened date matter more than appearance, so a container with no opened date should be treated as old.

Review after 3 months: Review interval after opening from the cited peroxide former lists, not an expiry date from a manufacturer. Class C and all other chemicals are left null.

How it must be disposed of

This does not go through our standard collection route. It is arranged by manual enquiry, and never goes down the drain or into general waste.

No lab pack grouping can be worked out from a description of this metal, because a crusted lump may not be fit to handle at all. It is not quoted online, and the online quote path does not accept it. Send a manual enquiry giving the date on the jar, the oil level and any colour visible on the metal. A store assessment is recommended, with a competent chemist seeing it in person, and a yellow or orange crust may call for an explosives or emergency specialist instead. Do not open it, move it or try to make it safe before advice comes. It must never go into a sink, a drain, a bin or a skip. School and university holdings normally take code 16 05 06*.

Waste codes and transport details for Potassium metal
EWC code, laboratory chemicals16 05 06*
Discarded inorganic chemical, not a laboratory pack16 05 07*
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 with

  • 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

  • 1,1,1-TrichloroethaneAlkali metals can react explosively with chlorinated solvents.
  • 1,2-DichloroethaneAlkali metals can react explosively with chlorinated solvents.
  • Acetic acidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • Ammonia solutionReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
  • BromineBromine reacts violently with ammonia, reactive metals and many solvents.
  • Calcium hypochloriteOxidisers can ignite flammable materials or make a fire far more intense.
  • Carbon disulfideCarbon disulfide reacts violently with these materials.
  • Carbon tetrachlorideAlkali metals can react explosively with chlorinated solvents.
Show 15 more
  • ChloroformAlkali metals can react explosively with chlorinated solvents.
  • DichloromethaneAlkali metals can react explosively with chlorinated solvents.
  • FormalinReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
  • Hydrochloric acidReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
  • Hydrogen peroxideReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
  • 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.
  • Sodium hydroxideReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
  • Sodium hypochloriteReacts with water, releasing heat and often a flammable or corrosive gas, so keep it away from all water based stock.
  • Sulfuric acidOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
  • TetrachloroethyleneAlkali metals can react explosively with chlorinated solvents.
  • Thionyl chlorideA leak of corrosive stock can break down flammable containers and feed a fire.
  • TrichloroethyleneAlkali metals can react explosively with chlorinated solvents.
  • Zinc chlorideA leak of corrosive stock can break down flammable containers and feed a fire.

Common quantities we collect

Usually held as: lumps under oil.

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.

Where it is commonly held

Hazard statements

  • H260 In contact with water releases flammable gases which may ignite spontaneously.
  • H314 Causes severe skin burns and eye damage.
Precautionary statements
  • P223 Do not allow contact with water.
  • P231+P232 Handle and store contents under inert gas. Protect from moisture.
  • P260 Do not breathe dust, fume, gas, mist, vapours or spray.
  • P264 Wash hands and exposed skin thoroughly after handling.
  • P280 Wear protective gloves, protective clothing, eye protection and face protection.
  • P301+P330+P331 IF SWALLOWED: Rinse mouth. Do NOT induce vomiting.
  • P302+P335+P334 IF ON SKIN: Brush off loose particles from skin. Immerse in cool water or wrap in wet bandages.
  • P302+P361+P354 IF ON SKIN: Take off immediately all contaminated clothing. Immediately rinse with water for several minutes.
  • P304+P340 IF INHALED: Remove person to fresh air and keep comfortable for breathing.
  • P305+P354+P338 IF IN EYES: Immediately rinse with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.
  • P316 Get emergency medical help immediately.
  • P321 Specific treatment (see supplemental first aid instruction on this label).
  • P363 Wash contaminated clothing before reuse.
  • P370+P378 In case of fire: Use suitable extinguishing media to extinguish.
  • P402+P404 Store in a dry place. Store in a closed container.
  • P405 Store locked up.
  • P501 Dispose of contents and container in accordance with local, regional, national and international regulations.

Substance as classified, dilutions may differ.

Questions

Can a technician cut the crust off old potassium to save the metal?

No. The crust is the danger, and cutting or scraping it can cause an explosion. There is nothing worth saving in an old jar. Leave it closed and where it stands, and send an enquiry describing the colour you can see through the glass.

Why does a metal appear on peroxide former lists?

With age, potassium forms a superoxide on its surface, and that behaves like the dangerous products of classic peroxide formers. It is listed in class A for that reason, with a short review interval. Potassium metal must not undergo standard peroxide testing. A crusted or suspect jar needs specialist assessment without opening, moving or disturbing it.

How often should we look at an opened jar of potassium?

Potassium metal is listed among high-risk peroxide-forming materials in some institutional safety schemes, but a three-month review does not establish that an opened jar remains safe to retain. Follow the applicable institutional procedure and obtain specialist assessment of aged or suspect stock. Each review is a look from outside: oil level, date on the label and any yellow or orange on the metal. Nothing about it requires the jar to be opened.

Our potassium jar has no date at all. What happens next?

Treat it as old, even if the metal looks clean from where you stand. It stays unopened and in place while you send a manual enquiry that says the date is missing. A competent chemist then needs to see it in person.

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 K. EC number 231-119-8.

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