[limpet chicken]

I am in need of some sodium dithionite, and it is pretty expensive (£40 odd per kilo), and I just do not need anything remotely near that amount (20g tops)

I had an idea for synthesising dithionite, pass SO2 through a saturated solution of NaOH, would that work? or better yet, pump SO2 through fused sodium hydroxide, to avoid potential nastiness when adding dithionite to water in bulk.

[woelen]

No, that will not work. If you bubble SO2 through NaoH (either dissolved or fused), then you will get Na2SO3, sodium sulfite.

You can make dithionite by reducing a slightly aicdified solution of a sulfite with zinc metal. This zinc will reduce the sulphur dioxide in an acidic sulfite-mix to dithionite:

Zn + 2SO2 --> Zn(2+) + S2O4(2-)

Keep in mind though that the synthesis of dithionite is a real pain. It must be performed in an environment absolutely free of air, otherwise your dithionite is oxidized by oxygen from the air immediately. Isolating the dithionite from the solution is yet another issue.

Dithionite is very unstable. Only the thoroughly dried powder can be stored reasonably well. As soon as it becomes humid it decomposes and is very easily oxidized.

Why is that sodium dithionite so expensive? Over here, I can buy it at appr. EUR 12 per kilo (appr. GBP 8 per kilo) at over 85% purity (the rest is mainly sodium sulfite). Try to look at shops, who sell chemicals for textile processing. Dithionite is used extensively for treating textile and ordering it on-line does not make you suspect (at least not in NL).