Sodium bisulphite

Sodium bisulphite (NaHSO3) is an interesting compound with many uses. Upon contact with many strong acids, it reacts to form sulphur dioxide gas. Sodium bisulphite also forms adducts with many aldehydes. It's also a food additive (E222). My use for it, however, is to prepare sodium hydroxyethylsulphonate.

Making sodium bisulphite is fairly simple. I had a go at it.

I started by preparing a dilute solution of sodium hydroxide in water. I then set up a sulphur dioxide generator using burning sulphur. Burning sulphur produces very dirty impure sulphur dioxide but it's good enough for this purpose. Anyway, I bubbled sulphur dioxide through the sodium hydroxide solution for 1 hour. After this, the solution had turned quite yellow, and I began boiling it down until I saw a few crystals precipitating. At this point I stopped boiling down the solution and filtered out the crystals. These are sodium metabisulphite. I then transferred the filtrate to a beaker in an icebath and left it there for 1 hour. After this, a huge mass of crystalline sodium bisulphite had precipitated.

The crystals were collected via filtration and are shown here:

NaOH + SO2 ==> NaHSO3    NaHSO3 + NaOH ==> Na2SO3 + H2O