It depends on the column, your protein variant (what is the pI) and yet the best would be to experimentally determine the elution, e.g. by SDS-PAGE (if it's sufficiently pure or in high quantities), Western blot or for example by measuring iron content in your fractions.

Doesn't iron have some specific absorbance? Maybe you could measure directly UV absorption during elution?

Thank you for your answer, I ended up using pepsin as a protein marker which is stable in acidic pH.