Stability of Fe(II)

How stable is Fe(II) in a solution? I am doing simple photometry with fresh samples and I feel like it doesn't oxidize that quickly, but have to use a stabilizing agent (dihydroxyfumaric acid) nonetheless. Couldn't find the pKa of the acid, but I suppose the decrease in the pH will stabilize the iron. Is that right? If yes, why?
Also, are the reducing properties of the dihydroxyfumaric acid important for keeping the iron in the +2 state?


