The solution turns yellow in sunlight due to a light driven redox reaction: Potassium trioxalato ferrate (III) is photosensitive, i.e. it decomposes when expos

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?