Mercury(I) chloride (CAS 10112-91-1): storage and disposal
Known on older labels as calomel or mercurous chloride, this white mercury salt is legacy stock in school, college and university stores. It is harmful if swallowed or in contact with skin, irritates skin, eyes and airways, and is very toxic to aquatic life. Its classification is milder than that of mercuric chloride, which makes reading the label carefully the first job. Light is what changes it over time.
- CAS number
- 10112-91-1
- Form
- Solid
- Storage class
- Toxic
- Signal word
- Warning
- Usual EWC code
- 16 05 06*
Harmful or irritant
Hazardous to the environment
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 toxic 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
Read the label twice at the shelf, because mercurous and mercuric differ by a few letters and the two salts carry very different classifications. A faded name should be stored and described as the more severe of the two until someone can confirm it. Either way the jar goes in a locked cabinet with a stock record, away from food and drink areas, acids and oxidisers. Keep it out of the light, since light breaks the salt down. Ammonia solution is a poor neighbour in a shared prep room too: its vapour turns calomel black. An opaque outer container or a dark corner of the cabinet does the job, provided the label stays readable and the entry in the record matches it.
How to tell it has deteriorated
Light is what ages this salt. The white powder darkens as it decomposes to mercury, and the same change also leaves some mercury(II) chloride behind, so a grey or dark jar no longer holds only what its label says. Darkening starts where light has reached the powder, so one side of a jar can look worse than the other. A black surface on stock kept near ammonia has a similar cause and the same meaning: the contents have changed. Beyond colour there is little to see. Caking, a stained cap, a failed seal or a faded label are the remaining checks, and a missing stock record matters as much as any of them. Leave a darkened jar closed and note the colour for the collection.
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 toxic laboratory pack with compatible chemicals.
Collection by a registered waste carrier, bound for a permitted facility, is how calomel leaves the site, and that specialist route is the only one. It never goes into a sink, drain, general waste bin or skip, and it should not be treated first, because it is very toxic to aquatic life. The description matters more than the quantity: give the name on the label, say that it is a mercury salt and mention any darkening, since the mix inside may have changed. Old calomel reference electrodes from a university or college lab should be listed separately, as the glass bodies are fragile and also contain mercury. Grouping follows from that description, with toxic solids clear of acids and oxidisers. Hazardous waste paperwork, or the special waste version in Scotland, travels with the load.
| EWC code, laboratory chemicals | 16 05 06* |
|---|---|
| Discarded inorganic chemical, not a laboratory pack | 16 05 07* |
| 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
- OxidiserToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- AcidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
Named chemicals to keep apart
- Acetic acidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
- Calcium hypochloriteToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- Hydrochloric acidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
- Hydrogen peroxideToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- Nitric acidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
- Potassium nitrateToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- Potassium permanganateToxic and oxidising stock are stored apart. A reaction could release toxic vapours.
- Sulfuric acidToxic stock is stored apart from acids. Several toxic salts release toxic gas in contact with acid.
Common quantities we collect
Usually held as: legacy stock.
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
- H302 Harmful if swallowed.
- H312 Harmful in contact with skin.
- H315 Causes skin irritation.
- H319 Causes serious eye irritation.
- H335 May cause respiratory irritation.
- H410 Very toxic to aquatic life with long lasting effects.
Precautionary statements
- 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.
- P270 Do not eat, drink or smoke when using this product.
- P271 Use only outdoors or in a well-ventilated area.
- P273 Avoid release to the environment.
- P280 Wear protective gloves, protective clothing, eye protection and face protection.
- P301+P317 IF SWALLOWED: Get medical help.
- P302+P352 IF ON SKIN: Wash with plenty of water.
- P304+P340 IF INHALED: Remove person to fresh air and keep comfortable for breathing.
- P305+P351+P338 IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing.
- P319 Get medical help if you feel unwell.
- P321 Specific treatment (see supplemental first aid instruction on this label).
- P330 Rinse mouth.
- P332+P317 If skin irritation 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.
- 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.
- P301+P316 IF SWALLOWED: Get emergency medical help immediately.
- P317 Get medical help.
Substance as classified, dilutions may differ.
Questions
Our old jar says calomel. Is that the same as mercury(I) chloride?
Yes, calomel is the traditional name for mercury(I) chloride, and older stock may also read mercurous chloride. Use the name on the label when you describe it and add the modern name beside it, so nobody confuses it with mercuric chloride, which is a different salt.
How does it differ from mercuric chloride when we store it?
Mercuric chloride carries a much more severe classification, but both belong in the locked toxic cabinet away from acids and oxidisers. Calomel adds a particular need to stay out of light and away from ammonia. If a faded label could be either, keep it as the more severe salt.
The powder has turned grey. Can it still be collected?
Yes, it can still go by specialist collection, but it should be described as darkened. The grey colour shows it has been breaking down to mercury, so the contents no longer match a clean white sample. Keep the lid on, put the jar in the dark and note the change.
Should old calomel electrodes go on the same list as the jar?
List them, but as separate items. They are fragile and contain mercury as well as calomel, so they need describing apart from a jar of powder. Leave them intact, do not dismantle them or drain any filling solution, and give the number held and their condition.
