4 ms·

>Green potassium ferrioxalate solution exposed to light will turn yellow, and then brown, releasing carbon dioxide in the process. After keeping them in the dar

by Levitz 5y ago

>Green potassium ferrioxalate solution exposed to light will turn yellow, and then brown, releasing carbon dioxide in the process. After keeping them in the dark for some time, they become green again. Adding small amounts of oxalic acid greatly helps to speed up the process. I understand why they would turn yellow, but how can they turn back to green when keeping them in the dark? They absorb carbon dioxide passively somehow?

selimthegrim 5y ago
They are probably co-crystallized with it in the same way many crystals are with hydroxide - https://en.wikipedia.org/wiki/Water_of_crystallization#Other_solvents_of_crystallization https://en.wikipedia.org/wiki/Water_of_crystallization#Other... UV induced fluorescence would have been cooler tho
philipkglass 5y ago
The solution turns yellow in sunlight due to a light driven redox reaction: 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/Domain/1021/Iron-1.pdf 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?
- kragen 5y ago
  I'm pretty sure it's oxygen. The same thing happens with solarized (sun-bleached) cyanotypes, according to the Cyanomicon, except that there the colors are white and blue rather than yellow and green. I think Chase is right that with oxalate this requires excess oxalic acid, so you can't repeat it indefinitely, while the ferricyanide/ferrocyanide redox cycle is reversible.
crystalchase21 5y ago
Light reduces the iron center of the ferrioxalate ion, and oxidizes the oxalate ion into carbon dioxide. Since the green color of the solution is due to ferrioxalate ions, a loss of them will make the solution browner, which is the color of iron. 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.

by Levitz 5y ago

selimthegrim 5y ago

They are probably co-crystallized with it in the same way many crystals are with hydroxide - https://en.wikipedia.org/wiki/Water_of_crystallization#Other_solvents_of_crystallization https://en.wikipedia.org/wiki/Water_of_crystallization#Other... UV induced fluorescence would have been cooler tho

philipkglass 5y ago

The solution turns yellow in sunlight due to a light driven redox reaction: 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/Domain/1021/Iron-1.pdf 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?

kragen 5y ago
I'm pretty sure it's oxygen. The same thing happens with solarized (sun-bleached) cyanotypes, according to the Cyanomicon, except that there the colors are white and blue rather than yellow and green. I think Chase is right that with oxalate this requires excess oxalic acid, so you can't repeat it indefinitely, while the ferricyanide/ferrocyanide redox cycle is reversible.

crystalchase21 5y ago

Light reduces the iron center of the ferrioxalate ion, and oxidizes the oxalate ion into carbon dioxide. Since the green color of the solution is due to ferrioxalate ions, a loss of them will make the solution browner, which is the color of iron. 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.