Sodium Thriosulphate (or hypo) is (was?) used as a photographic 'fixer' for black & white photographs.
It also has a useful reaction with steel producing an intense black colour on the surface of the steel. Diluted hypo, warmed to 75 deg C, and ultra cleaned steel swirled about in it produce this reaction in about a minute. Useful for blacking tools and the like.
But what is the reaction ? Is this a deposit (so maybe transient) or is there a reaction with the atoms on the surface of the steel (so hopefully reasonably well bonded)?
I know that it works but what is happening, anyone know?
Sodium thiosulphate is normally a reducing agent, itself becoming oxidised in the process. However, metallic iron is already fully reduced and won't oxidise anything as it stands.
However, powerful reducing agents will reduce thiosulphate, for example sodium amalgam in water reduces it to give sodium sulphide and sodium sulphite.
Na2S2O3 + 2Na -> Na2S + Na2SO3
It may be that the steel is behaving similarly to sodium amalgam, producing ferrous sulphide and sodium sulphite, and it's the ferrous sulphide that is giving the intense black coating on the surface of the steel.
Na2S2O3 + Fe -> FeS + Na2SO3
Speculation, I'm afraid, and just because an equation can be written doesn't mean it actually happens. Perhaps a way of testing the idea would be to rinse and dry your piece of blackened steel and then put a drop of vinegar or brick acid on it. If it's iron sulphide, you should get a strong smell of hydrogen sulphide, i.e. rotten eggs. As to whether it's bonded, try rubbing it off is the best suggestion I can make.
Note to GP: Ferrous sulphate in water is always pale green. Even when the water of crystallisation is driven off, the resulting colour is brown, not black.
Hydrated ferrous sulphate is pale green. It's much more likely that the reaction is producing ferrous sulphide which is black. Thiosulphate ions readily dissociate into sulphite ions and elemental sulphur.
If I'm right the black coating will readily dissolve in dilute acids to give hydrogen sulphide which is easily detectable in very low concentrations in air.
It is being used as a source of free sulphur in a form that will react with the iron surface to make a mixture of sulphides.
No it is pretty pale green crystals and ferric is a dirty yellow brown. (both the sulphates are water soluble)
ITYM Iron Sulphides which are black. There are various suggested recipes for blacking tools in metalworking books that will perform better than the mix suggested above in terms of colour and longevity.
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