Potassium ferrocyanide (CAS 13943-58-3): storage and disposal

A school prep room normally keeps the jar with the dry inorganic salts, dated and in a general store. Histology is where the risk appears. The Perls iron stain uses it alongside hydrochloric acid, so the two bottles often end up together on a bench or in a staining cupboard, and that is exactly the pairing to separate. Nitric, sulfuric, phosphoric and trichloroacetic acid need the same distance. Avoid a warm spot too, as heating is the other way hydrogen cyanide can be released. The label deserves a second look, since ferrocyanide and ferricyanide are easily confused on a stock list: this one is yellow. Rusty metal shelving is worth avoiding, because contact produces a blue stain that makes the shelf look far worse than it is.

Clear lemon yellow crystals are normal for the trihydrate, and they stay stable for a long time in a closed jar. In warm dry storage the surface can lose water and turn pale or whitish, which is a change of hydration and no great concern. A blue colour is different. It means the salt has met iron(III), most often from rust on a cap or shelf bracket, forming Prussian blue. Caking is likely in a damp room. Beyond that it gives few visual signs, and it does not become more dangerous simply through age. Look instead for a stained or missing label, and anything suggesting acid has reached it: wet necks, corroded caps or an acid bottle standing close by.

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.

Before you ask for a collection, copy the exact label wording, checking whether it says ferrocyanide or ferricyanide, and note whether it is the trihydrate. Record any working solution separately, because used Perls solution contains acid as well as the salt, which changes how it is grouped. Say if a jar has been standing close to acids. Solid stock sits with other inorganic salts in a lab pack, away from any acid. Specialist collection is the only route, and it does not go into a sink, a drain, a general waste bin or a skip, nor should anyone try to treat it first. Hospital histology labs normally use code 18 01 06*, while schools and colleges use 16 05 06*.

Usually held as: trihydrate.

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.