Phosphoric acid (CAS 7664-38-2): storage and disposal
Phosphoric acid is held in school and college prep rooms, university labs and pathology departments as a syrupy 85% liquid, and in dental surgeries as an etch gel of about 37%. It causes severe skin burns and eye damage, and it attacks some metals. Where it is stored, what is nearby and what the container looks like decide how it is described and collected.
- CAS number
- 7664-38-2
- Form
- Solution
- Storage class
- Acid
- Signal word
- Danger
- Usual EWC code
- 16 05 06*
Corrosive
What to do this week
- Check the label is readable and write down the opened date if it is not already there.
- Check it is stored as acid stock and apart from the items listed under "Do not store with".
- If you no longer need it, add it to your disposal list so it goes on the next collection.
Safe storage in a school or lab
Cyanide salts, ferricyanides, ferrocyanides, chlorates, sulfides and hypochlorites all need to be kept away from this acid. Contact with acids frees toxic hydrogen cyanide, toxic hydrogen sulfide or chlorine, and chlorates can form explosive chlorine dioxide. Keep the syrupy 85% bottle low down on an acid-resistant tray in a ventilated corrosive store. It attacks some metals and releases hydrogen, so avoid metal lids, bare metal shelving and metal trays. A dental surgery should keep the etch gel in the same corrosive arrangement and not loose in a drawer with instruments or in a cupboard beside alkalis.
How to tell it has deteriorated
Few visible signs of ageing show up in stock of this kind, so the container and its surroundings tell the story. Look for perished or brittle caps, a crust around the neck, cracked containers and any discolouration. Corroded fittings nearby are a meaningful warning, as the acid attacks some metals. In a dental surgery, check that etch gel containers are sealed, labelled and intact, and that no residue has run onto the shelf below. A faded label matters as much as any physical change, because the strength must be known. Stock that cannot be identified or that has leaked should be left in place and flagged.
How it must be disposed of
By specialist collection only. It never goes down the drain or into general waste. It is normally packed in a corrosive acid laboratory pack with compatible chemicals.
It must never be poured down a sink, tipped into a drain or binned with general waste, and nobody should try to dilute or neutralise it first. Specialist collection is the only route. A dental surgery holding used or expired etch gel describes it under 18 01 06*, while a school or college clearing a bottle of the 85% liquid uses 16 05 06* as laboratory stock. Record the strength, whether each item is the syrupy liquid or the gel, and how many containers there are. A bottle with a crusted neck or a sticky outside is awkward to lift, so mention it. The specialist collector determines compatible packing arrangements, keeping the acid apart from cyanides and other incompatible materials. Retain a copy of the consignment note.
| EWC code, laboratory chemicals | 16 05 06* |
|---|---|
| Discarded inorganic chemical, not a laboratory pack | 16 05 07* |
| Human healthcare setting | 18 01 06* |
| Hazardous waste | Yes |
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
- FlammableOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
- AlkaliAcids and alkalis react with strong heat and spattering if they meet.
- OxidiserMany oxidisers release toxic gas or become unstable in contact with acid.
- ToxicToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
Named chemicals to keep apart
- AcetoneOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
- Ammonia solutionAcids and alkalis react with strong heat and spattering if they meet.
- Ammonium sulfide solutionSulfides release toxic hydrogen sulfide gas with acids.
- Calcium hypochloriteHypochlorites release toxic chlorine gas in contact with acids.
- Chloramine THypochlorites release toxic chlorine gas in contact with acids.
- EthanolOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
- FormalinToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
- HexaneOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
Show 16 more
- Hydrogen peroxideMany oxidisers release toxic gas or become unstable in contact with acid.
- Iron(II) sulfideSulfides release toxic hydrogen sulfide gas with acids.
- Mercury(II) chlorideToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
- MethanolOxidising acids can ignite flammable materials, and acid fumes attack containers and seals.
- Potassium chlorateChlorates and strong acids can form explosive chlorine dioxide.
- Potassium cyanideCyanides release highly toxic hydrogen cyanide gas with acids.
- Potassium ferricyanideHeating or strong acids can release toxic hydrogen cyanide from these complexes.
- Potassium ferrocyanideHeating or strong acids can release toxic hydrogen cyanide from these complexes.
- Potassium hydroxideAcids and alkalis react with strong heat and spattering if they meet.
- Sodium azideAzides release highly toxic and explosive hydrazoic acid with acids.
- Sodium chlorateChlorates and strong acids can form explosive chlorine dioxide.
- Sodium cyanideCyanides release highly toxic hydrogen cyanide gas with acids.
- Sodium dichloroisocyanurateHypochlorites release toxic chlorine gas in contact with acids.
- Sodium hypochloriteHypochlorites release toxic chlorine gas in contact with acids.
- Sodium nitriteNitrites release toxic nitrogen oxides with acids.
- Sodium sulfideSulfides release toxic hydrogen sulfide gas with acids.
Common quantities we collect
Usually held as: 85%, dental etch gel about 37%.
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
- H314 Causes severe skin burns and eye damage.
Precautionary statements
- 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+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.
- 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 expired dental etch gel go in the clinical waste bin?
No. It is a corrosive acid that causes severe burns, so it must never go in the clinical bin, a sink or general waste. In a human healthcare setting it is handled as hazardous waste through specialist collection.
Why shouldn't phosphoric acid be stored on a metal shelf?
It attacks some metals and releases hydrogen in the process. A resistant tray low down in a ventilated corrosive store is the better choice, with metal fittings kept away from the bottle.
How can we tell whether an old bottle is still usable?
It gives few visible signs, so check the cap, the neck for crust, the glass for cracks, any discolouration and whether the label is readable. If any of those is wrong, or the strength is unknown, list it for collection.
What must be kept away from it in a shared store?
Cyanides, ferricyanides, sulfides, hypochlorite bleach and chlorates. Acids release toxic gas from several of them, and chlorates can form explosive chlorine dioxide. Keep alkalis apart too, as acids and alkalis react with heat and spattering.
