Solutions of iron tend to oxidize in the presence of air. Ferrous iron oxidizes quickly to Ferric. Ferric iron seems to precipitate at pH > 3 or 3.5.
I also noticed a solution of Ferric Nitrate (prepared with deionized water), originally acidified to pH 1.5 with nitric acid, when diluted 50 times, starts to yellow/brown after a day. Doing a total iron determination using hydroxylamine and/or thioglycolic methods on this diluted sample seems to result in lower results the next day than when it was freshly diluted (this drop is about 20% per day from the initial results).
The diluted Fe(NO3)3 pH is about 3.3 - 3.5, so I'm sure it has something to do with hydroxides/oxides forming but I'm not sure if that tells the whole story. The reducing agents used didn't seem to work for this "days old, less acidified" sample. Note: The sample is stored in a 500-mL volumetric flask at room temperature under fluorescent lighting.
What can I try to make these reducing agents work with my situation? Boiling didn't seem to help too.
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