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

Sodium sulfate (CAS 7757-82-6): storage and disposal

College and university prep rooms hold sodium sulfate mainly as a drying agent, and school stores keep a jar for salt experiments. It is not classified as hazardous on the sources we checked, but it is still a chemical waste that must be described accurately. The jar that matters most is often the one full of used drying agent.

CAS number
7757-82-6
Form
Solid
Storage class
General
Usual EWC code
16 05 09

What to do this week

  1. Check the label is readable and write down the opened date if it is not already there.
  2. Check it is stored as general stock and apart from the items listed under "Do not store with".
  3. 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 jar that has been used for drying, not the one in the cupboard. Anhydrous sodium sulfate takes up water readily, so a poorly closed tub clumps and loses its use. Keep it in a dry, labelled, well organised store with a date on arrival, apart from food grade supplies. There are no particular chemical partners to separate it from. In a prep room the usual mix-up is between sodium sulfate, sodium sulfite and sodium sulfide on neighbouring shelves, whose names differ by a single letter and whose hazards differ greatly. Put the name in full on the label and leave a gap between them.

How to tell it has deteriorated

Hard lumps in the tub show that anhydrous sodium sulfate has absorbed water and set. Crystals may look wet or stick to the sides of the tub. Used drying agent can carry a smell of whatever it was drying, and it should never go back into the stock jar. Other checks are a split tub, a damaged cap, a faded label and a missing receipt date. Few other visible changes mark this salt, so label condition and cross-contamination matter more than appearance.

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 non-regulated or low hazard laboratory pack with compatible chemicals.

A waste transfer note covers sodium sulfate in its original tub. It is not classified as hazardous on the sources we checked, but it stays a chemical waste, so it does not go into a bin, a drain or a skip. Used drying agent is a different matter: it is described by what it has been drying, such as a solvent residue, and must be kept in a separate labelled container. Record the quantity and the container condition. The specialist carrier packs low hazard solids together.

Waste codes and transport details for Sodium sulfate
EWC code, laboratory chemicals16 05 09
Hazardous wasteNo

The code depends on where the waste came from and what it is mixed with. Use the waste code finder to see why.

Common quantities we collect

Usually held as: solid.

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

Questions

Can used sodium sulfate from drying go back in the tub?

No. Used drying agent carries traces of what it dried and is described by that, so it needs its own labelled container and a separate line on the collection request.

Is sodium sulfate the same as sodium sulfite?

No. They are different chemicals with different classifications, and the names differ by a single letter. Check the full name on the label every time and never rely on memory or shelf position.

Why has our tub set solid?

Anhydrous sodium sulfate absorbs water from the air and sets into lumps. It means the cap has not been closed tightly, and the contents are no longer a reliable drying agent.

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 Na2O4S. EC number 231-820-9.

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