Sodium thiosulfate (CAS 7772-98-7): storage and disposal

Where the stock usually sits is the problem. Thiosulfate is commonly used alongside iodine solution in titrations, but working compatibility during an experiment does not mean the reagents should share storage or spill containment. Keep each in its appropriate storage location. Keep thiosulfate apart from acids, which decompose it and release sulfur dioxide, and from oxidising chemicals. Keep the crystals in a dry, labelled, well organised store, dated on arrival and apart from food grade supplies. Solutions in bottles for titration should carry the concentration and the date made up. Crystals kept in an open tub change with the room, so the cap needs to be closed after use. Histology benches should keep working solutions on a tray, labelled for the stain they belong to.

Crystals of the pentahydrate can absorb moisture and turn damp or sticky in humid conditions, and can lose water and turn powdery in warm, dry conditions. Cloudiness or a pale deposit may indicate contamination or a change in the solution, while an old 0.1 M bottle should not be relied upon as a standard without verification. Look also for faded labels, a cap crusted with dried solution, and a missing date. If nobody can say what concentration a bottle holds, treat it as unknown.

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.

Crystals and solution are collected as separate items, so list each one. It is not classified as hazardous on the sources we checked, but it needs a clear description rather than a rinse down the sink. A waste transfer note covers the movement, and histology or pathology holders use the healthcare entry. Record the container size, the stated concentration of any solution and whether the label is intact. Do not treat it with another chemical before collection. The specialist carrier packs it with other low hazard stock from the same visit.

Usually held as: pentahydrate, 0.1 M.

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.