posted by smiley .
Why should an Fe3+ solution be used in calibrating the spectrophotometer for the spectrophotometric determination of iron in Fe2+ complexes?
What is the procedure? Are you complexing the ferric with thiocyanate? Or are you running the experiment just with ferric ion? Again, you haven't given much information about what you are doing but I suspect that any ferrous iron in the sample is oxidized to ferric (and most ferrous solution have some ferric ion in them anyway), and it's the ferric ion that forms the final product with the complexing agent just before measuring the absorbance. Please clarify and repost if I have misinterpreted the problem.