Potassium chromate (CAS 7789-00-6): storage and disposal
Yellow crystals of potassium chromate, and the 5% indicator solution used in silver titrations, sit in school, college and university stores. It is a chromium(VI) compound that may cause cancer by inhalation and genetic defects, may cause an allergic skin reaction, and is very toxic to aquatic life. Because the cancer risk comes through breathing it in, the state of the solid and any dried crust on an indicator bottle decide how it is handled.
- CAS number
- 7789-00-6
- Form
- Solid
- Storage class
- Toxic
- Signal word
- Danger
- Usual EWC code
- 16 05 06*
Harmful or irritant
Serious health hazard
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
Inhaled dust is what decides where this goes, which points to the locked toxic cabinet with a stock record and well away from the open indicator rack. A dropper bottle of 5% solution left in a titration tray on a shared bench is the usual slip, because it stops being treated as chromium(VI) stock once it looks like routine kit. A weaker solution may be less hazardous than the solid, but it still belongs under the same stock record. Acids need their own store, and acid contact turns yellow chromate towards orange dichromate. Oxidisers are also kept apart. Check that any rubber teat is sound, and label each bottle clearly, since a faded yellow liquid is easily mistaken for something ordinary.
How to tell it has deteriorated
Good stock is bright yellow, either as dry free running crystals or as a clear yellow solution. Orange means acid has reached it, turning some chromate to dichromate. A green tint shows that chromium has been reduced, most likely by something organic getting in. The more common finding is a dried yellow crust on a dropper or stopper, where the solution has evaporated at the neck. That crust matters because it can become dust. Perished rubber teats, faded labels and yellow marks on the shelf also deserve attention. Otherwise it gives few visual signs with age. Do not rub the crust away or squeeze a stiff teat to see whether it still works. Leave the bottle closed and describe what you see.
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.
Neither the solid nor the indicator may go into a sink, a drain, a general waste bin or a skip, and nobody should try to treat either first. Even a small dropper bottle counts. Specialist collection is the only route. Say whether you hold crystals or 5% solution, how many bottles, and whether any crust is present. Silver titration residues deserve their own line on the list, because those flasks hold silver as well as chromate, and they should not be added to the indicator stock. This material is toxic stock in a collection, so it sits away from acids and oxidisers in its group. School and university holdings normally take code 16 05 06*. The note for the movement can record the solid and solution separately.
| 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: solid, 5% indicator.
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.
- H317 May cause an allergic skin reaction.
- H319 Causes serious eye irritation.
- H335 May cause respiratory irritation.
- H340 May cause genetic defects.
- H350i May cause cancer by inhalation.
- H410 Very toxic to aquatic life with long lasting effects.
Precautionary statements
- P203 Obtain, read and follow all safety instructions before use.
- 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.
- P272 Contaminated work clothing should not be allowed out of the workplace.
- P273 Avoid release to the environment.
- P280 Wear protective gloves, protective clothing, eye protection and face protection.
- 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.
- P318 If exposed or concerned, get medical advice.
- 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.
- 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.
- 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
Are leftover titration residues with this indicator a separate waste?
Yes. Residues from a silver titration hold silver and chromate together, often as a reddish solid, so they are not the same as the unused indicator. Keep them in their own labelled container and list them separately, with a note of what the titration used.
Our indicator bottle has gone orange. What does that mean?
Orange usually means acid has got into the bottle, perhaps from a contaminated dropper, turning some chromate into dichromate. It is still chromium(VI). Stop using the bottle, keep it closed, and note the colour on your list so that it is described correctly when collected.
Does one small dropper bottle at 5% really need to be listed?
Yes. A small bottle at 5% may be less hazardous than the solid, but it still contains a chromium(VI) compound and it is very toxic to aquatic life. Add it to the list with its strength, and it can join the rest of your stock in the collection.
