When the ferrozine compound binds with Fe2+, a colour change occurs. Ferrozine can thus be used to determine the amount of free Fe2+ in solution: By using standards of ferrozine with known amounts of bound Fe2+ (e.g. no iron, saturated with iron, 50% iron etc.) a standard curve of absorption at a certain wavelength (562nm I think) can be created, and you can use this to determine chelation.

Edit: the ferrozine assay is from this paper - Carter, P. Spectrophotometric determination of serum iron at the submicrogram level with a new reagent (ferrozine). Anal. Biochem. 1971, 40, 450–458.