[paul cotter (OP)]

My eldest daughter had a flimsy white garment(cotton/synthetic mix) that she intended to use in a charity event but which had a nasty rust stain which seemed indelible and I was asked to remove said stain. My first idea was a sequestrant acid such as edta, nta or atmp but not having any of these I thought maybe citric acid would do the job but this failed miserably. A rethink was needed: from the colour I reckoned it was ferric iron and maybe a reducing agent would return it to the ferrous state whereby it would dissolve in dilute acid. Hydrazine hydrate and sodium dithionite came to mind but again I have neither. I decided to try sodium metabisulphite, sprinkling some over the stain and adding a few drops of water. I left this sit for several hours but again it seemed to have no effect and I intended to rinse the garment out and give up. Fatigue overcame me and I forgot to rinse the garment out and to my surprise next morning the stain had disappeared. A note of caution: this was a white garment and attempting to use this method on a coloured item may cause damage to the dye.

[alancalverd]

Good thinking!

[Bored chemist]

Sodium metabisulphite- often used in home brewing- also removes wine stains.
Which is convenient...

The classic choice is oxalic acid but that's rather toxic.

[paul cotter (OP)]

Thanks BC, I had not heard of that. I have oxalic(+ h2o x2,i think) and I briefly considered trying it but when citric failed I dropped the idea of using an acid. PS: I tried to write h2o x2 in words but the forum software kept changing the spelling to "dehydrate" with an e replacing the desired I! 

[Bored chemist]

Bisulphite is a moderately strong acid and, on oxidation by air,  it becomes bisulphate which is a pretty strong acid- possibly strong enough to trash some fabric.

[paul cotter (OP)]

Indeed BC. I was hoping for reduction to Fe(2) followed by dissolution as Fe(2)sulphite/sulphate. Why it took so long is a bit of a puzzle. What is the mechanism with oxalic- I know oxalic can perform reduction under the right circumstances(heat?) but it is not a regular reductant like dithionite and metabisulphite. Being a moderately strong(for a carboxylic) acid it obviously would be good for dissolving a reduced iron species. How would formic acid perform on this?