[Fmeub]

Hi!

I know that the crystal surfaces of potassiumtris(oxalato)ferrate(III) will turn yellow if they are not stored in the dark, but how do I explain it by using reaction equations?

I suppose the light gives energy in the form hv, but I'm not sure whether any bonds break.

[resc]

Hi

The photosensitive nature of this compound is used in actinometry (look up ferrioxalate actinometry). Light reduces the Fe³⁺ to Fe²⁺ and the oxalate form CO₂:

2Fe³⁺ + C₂O₄^2- (hv) --> 2Fe²⁺ + 2CO₂

You can then add something like 1,10-phenanthroline if you want to quantify the Fe²⁺ and hence the amount of photons, by measuring the absorption of the iron-phenanthroline complex.

So in answer to your question, bonds do break as the complex goes via an excited state, and one of the pathways deactivating this excited state is decomposition.