Silica gel (CAS 112926-00-8): storage and disposal

Silica gel is the desiccant in a school desiccator, the sachet in a dental instrument case and the packed powder in a chromatography store. It is not classified as hazardous on the sources we checked, but what it has dried or carried can change that. Indicator grades may contain cobalt chloride, which changes the hazard, so the first job is knowing which grade is in the container.

CAS number: 112926-00-8
Form: Solid
Storage class: General
Usual EWC code: 16 05 09

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 general 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

A school keeps silica gel in a stock jar beside the desiccators, which is fine on a dry, labelled shelf in a general store. Keep the lid tight, since its whole purpose is to take water from the air and a loose lid turns the jar into spent stock. Pathology, histology and dental stores keep it in cabinets or sachets, where it should be labelled as the desiccant it is and not left loose among instruments. Hydrofluoric acid attacks glass and silicates and releases a harmful gas, so silica of any grade stays well away from it. Hold coloured indicator beads apart from plain beads.

How to tell it has deteriorated

Start with the lid and label. A jar that has stood open, or a sachet that has split, has probably done all the drying it ever will. Indicator beads change colour as they fill with water, so spent desiccant usually shows a colour change, such as blue beads turning pink, and a jar that has all changed is used up. Plain beads show nothing, so clumping or a damp feel is the main hint. Chromatography grade powder in a half used bottle may have caked or been scooped with a damp spoon. A label that has lost its grade, or beads of unknown type, are recorded as unknown.

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 specialist carrier collects silica gel, grouped with compatible solids in a lab pack and described with its grade. Plain silica uses 16 05 09 with a waste transfer note, and a dental surgery or hospital pathology lab uses 18 01 07. The wrinkle is the indicator grade: where cobalt chloride may be present, say so at booking, because the hazard changes and the description has to follow it. Silica from a chromatography column carries whatever solvents and compounds it held, and is described by those. It must not go to general waste, a drain or a skip, and nobody should try to dry out or treat it first.

Waste codes and transport details for Silica gel
EWC code, laboratory chemicals	16 05 09
Human healthcare setting	18 01 07
Hazardous waste	No

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

Named chemicals to keep apart

Hydrofluoric acidHydrofluoric acid attacks glass and silicates, releasing toxic gas.

Common quantities we collect

Usually held as: desiccant, chromatography grade.

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

How can we tell whether our silica gel contains cobalt chloride?

Coloured indicator beads, often blue when dry and pink when spent, are the usual sign, but the supplier label or safety data sheet is the proper answer. If you cannot establish the grade, say so at booking and the description will be treated cautiously.

Can spent desiccant from a desiccator go in the ordinary bin?

No. It is a chemical waste, and what it has dried may matter to the description. It goes by specialist collection with an accurate note of its grade and the setting it came from.

How do we describe silica left over from a chromatography column?

By what it carries. Used column material holds solvents and the compounds that were separated, so list those at booking rather than calling it plain silica. Keep it in a closed, labelled container apart from clean stock.

Do silica gel sachets expire?

They stop working once they have taken up their fill of water, whatever the date on the packet. Unopened sachets in sealed bags keep longer, and spent ones are a reason to book collection rather than keep them.