How to grow green potassium ferrioxalate crystals from iron rust

Potassium trioxalato ferrate (III) is photosensitive, i.e. it decomposes when exposed to light, reforming iron (II) oxalate.

https://www.dimanregional.org/cms/lib/MA01929605/Centricity/...

I believe that the reason it turns green again in the dark is because the oxygen of the air oxidizes the iron back to the iron (III) form. This proceeds easily if there is excess oxalic acid. In the absence of excess oxalic acid, this could still happen via a disproportionation sort of reaction that turns some of the iron (II) oxalate into insoluble iron crud (mixed oxides/hydroxides) and frees its complexed oxalate to reform the iron (III) complex.

You could test to see if it's oxygen from the air by exposing some of the solution to sunlight until it turns yellow, then storing half in a capped container with no head space for air, and the other half in an open topped container. Does the solution in the open topped container turn green faster? Does the solution in the closed container turn green again at all?

If there is excess oxalic acid present, then the extra oxalate ions can form ferrioxalate ions in the absence of light, causing the solution to revert to green.

UV induced fluorescence would have been cooler tho

I store my crystals in a sealed container, and they are fine after a month. People who have left them outside do not report dehydration, but the surface still turns white due to exposure to light.

Some crystals can be successfully sealed in resin to prevent dehydration. Preventing light from photodecomposing potassium ferrioxalate crystals would be very tricky. I do not know whether it is possible.

If you do this, get it fairly dry first because surface moisture will form a thin layer between the oil and the crystal making a shiny surface from different refractive indices.

Ooh, epitaxial growth! Could try that with a series of alums - Al(iii), Cr(iii) and Fe(iii) is all readily available.

I heated it at 200C for 1 hour on a hot plate. You might need to heat a bigger batch for longer. Generally, I do double the time for it to get completely dry, to make sure all the iron hydroxide is converted to iron oxide.

But either way, as I understand if, you do it slowly you're _mainly_ going to make bleach instead, with small amounts of chlorine gas.

Probably not the best idea to breathe it anyway, but shouldn't take much ventilation to make it perfectly safe.

Not sure if that was added later.

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