Calcium hydroxide (CAS 1305-62-0): storage and disposal
School and college prep rooms hold calcium hydroxide as slaked lime powder and as limewater, and dental surgeries keep it as a paste. It is classified Danger for serious eye damage, with skin and respiratory irritation. A bottle of limewater looks like plain water, which is why it often outlives its label, and the powder must be kept clear of picric acid and Bouin's fluid.
- CAS number
- 1305-62-0
- Form
- Solid
- Storage class
- Alkali
- Signal word
- Danger
- Usual EWC code
- 16 05 06*
Corrosive
Harmful or irritant
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 alkali 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
Check the lid first: a powder tub that does not seal takes carbon dioxide from the air and slowly turns to calcium carbonate, so the stock stops being what the label says. Keep the tub low on a resistant tray in the alkali store, away from acids. Picric acid and Bouin's fluid must be kept apart because picric acid forms sensitive picrate salts with lime, plaster and concrete, which makes spillage on a plaster shelf or a concrete floor a particular problem. Dental paste should stay in its original packaging with the expiry label. Dust from the powder irritates the airways, so open tubs gently.
How to tell it has deteriorated
Limewater tells its own story. A clear bottle that has been opened turns cloudy, with a white film on the surface and chalky sediment, as calcium carbonate forms from carbon dioxide in the air. That is the normal result of use and age. Powder tubs show it as hard crusting at the rim and a gritty, caked texture. Dental paste that has dried, separated or hardened in its container is past use. The label on a limewater bottle is often handwritten and fades, so an unlabelled clear liquid is the real problem. General checks apply too, including cracked containers and unknown receipt dates.
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 alkali laboratory pack with compatible chemicals.
Collection is by specialist route only. Limewater, powder and paste must not go into a sink, a drain, a general waste bin or a skip, and nobody should neutralise limewater first, however clear it looks. Powder and solution are packed in different groups, and acids and picric acid are never packed alongside. Record the form of each item, whether powder, limewater or dental paste, with the container size. Because the classification includes serious eye damage, hazardous waste paperwork applies, and the record should reflect that.
| 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
- AcidAcids and alkalis react with strong heat and spattering if they meet.
- OxidiserSome oxidisers react strongly with alkalis, and ammonia based stock is especially at risk.
Named chemicals to keep apart
- Acetic acidAcids and alkalis react with strong heat and spattering if they meet.
- Ammonium picratePicric acid forms sensitive picrate salts with lime, plaster and concrete.
- Calcium hypochloriteSome oxidisers react strongly with alkalis, and ammonia based stock is especially at risk.
- Hydrochloric acidAcids and alkalis react with strong heat and spattering if they meet.
- Hydrogen peroxideSome oxidisers react strongly with alkalis, and ammonia based stock is especially at risk.
- Nitric acidAcids and alkalis react with strong heat and spattering if they meet.
- Picric acidPicric acid forms sensitive picrate salts with lime, plaster and concrete.
- Potassium nitrateSome oxidisers react strongly with alkalis, and ammonia based stock is especially at risk.
Show 2 more
- Potassium permanganateSome oxidisers react strongly with alkalis, and ammonia based stock is especially at risk.
- Sulfuric acidAcids and alkalis react with strong heat and spattering if they meet.
Common quantities we collect
Usually held as: solid, limewater, dental paste.
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
- H315 Causes skin irritation.
- H318 Causes serious eye damage.
- H335 May cause respiratory irritation.
Precautionary statements
- P260 Do not breathe dust, fume, gas, mist, vapours or spray.
- P261 Avoid breathing dust, fume, gas, mist, vapours or spray.
- P264 Wash hands and exposed skin thoroughly after handling.
- P264+P265 Wash hands and exposed skin thoroughly after handling. Do not touch eyes.
- P271 Use only outdoors or in a well-ventilated area.
- P280 Wear protective gloves, protective clothing, eye protection and face protection.
- P301+P330+P331 IF SWALLOWED: Rinse mouth. Do NOT induce vomiting.
- P302+P352 IF ON SKIN: Wash with plenty of water.
- 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.
- P317 Get medical help.
- P319 Get medical help if you feel unwell.
- P321 Specific treatment (see supplemental first aid instruction on this label).
- P332+P317 If skin irritation occurs: Get medical help.
- P362+P364 Take off contaminated clothing. And wash it before reuse.
- P363 Wash contaminated clothing before reuse.
- 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.
Substance as classified, dilutions may differ.
Questions
Why has our limewater gone cloudy?
Carbon dioxide from the air reacts with it to form a chalky white deposit, so a cloudy bottle with a skin on the surface has simply been open or old. It is no longer fresh stock. Label it, keep it closed and include it in collection.
Where do we keep calcium hydroxide if we also hold picric acid?
On separate shelves, ideally in separate cabinets. Picric acid forms sensitive picrate salts with lime, plaster and concrete. Keep picric acid and Bouin's fluid away from calcium hydroxide and from plaster or concrete surfaces.
Can dental calcium hydroxide paste go in clinical waste?
No. It is a chemical waste with a hazardous classification, so keep it separate from clinical and general bins. Record the product, container size and expiry date and hold it for specialist collection.
