Disodium hydrogen phosphate (CAS 7558-79-4): storage and disposal

Buffer shelves in school, college, university, pathology and histology labs hold disodium hydrogen phosphate as the basic half of the common phosphate pair. It is not classified as hazardous on the sources we checked, but it is still a chemical waste that needs a correct description. The difficulty is the stock: a jar whose hydrate level nobody recorded.

CAS number: 7558-79-4
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

Nothing in particular needs to be kept apart from it, and the main storage job is a clear label. Disodium hydrogen phosphate is sold in more than one hydrated form, and old tubs often lose or gain water, so the label should show the exact form and the date. It sits in a dry, well organised store, away from food grade supplies and beside, not mixed with, the dihydrogen salt. Histology benches that keep ready-made buffers should store them in labelled bottles with the date of making, on a tray, and not on a bench where splashes reach them.

How to tell it has deteriorated

The tub can change its character because the salt trades water with the room. A powder may cake, form a crust at the lid or look wet, and a tub of crystals may turn chalky white on the surface. These are changes of water content rather than hazard. Made-up buffer may go cloudy or grow film. Faded labels and missing dates are the more important problems. A jar topped up from another container loses its identity and should be recorded as mixed stock.

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.

Record the quantity, the form named on the label and the condition of the cap. It is not classified as hazardous on the sources we checked, but it stays a chemical waste, so it is described by what it is and collected by specialist carrier, never rinsed into a drain or put in a skip. The waste code for a non-hazardous discarded chemical applies, with the healthcare entry for pathology and histology holders. A waste transfer note covers it. Unlabelled jars are described by their possible contents.

Waste codes and transport details for Disodium hydrogen phosphate
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.

Common quantities we collect

Usually held as: buffer 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

Questions

Does the hydrate form matter when we list it for collection?

It helps the carrier describe the stock accurately. Copy the form from the label if you can read it, and if the writing has faded, say so on the request instead of guessing.

Does ready-made buffer need specialist collection?

Yes. Even without a hazard classification on the sources we checked, it is chemical waste and must never go into a drain. List the volume and the date made, and arrange collection.

Why has the powder in our tub gone chalky?

The salt exchanges water with the room, so powder can cake, crust or turn chalky. It shows the cap is not sealing, not a new hazard.