It doesn’t pick up chelated iron, only free/ferrous form.
From Claude:
The HI721 uses the phenanthroline method, which requires iron to be in the Fe²⁺ (ferrous) form to react. The reagent includes a reducing agent to convert Fe³⁺ to Fe²⁺ before the reaction. However:
Fe-EDTA and Fe-DTPA — the chelating agent competes with phenanthroline for the iron. The reagent’s reducing/acid environment does partially strip iron from weaker chelates like EDTA, so you’ll get some reading, but it may underreport true total iron.
• Fe-DTPA (what APT 3 uses) is a stronger chelate than EDTA, meaning phenanthroline has a harder time displacing it. Readings for DTPA-chelated iron tend to be lower than the actual dosed concentration.
• Fe-HEEDTA (common in many all-in-ones) sits somewhere in between.
Still useful for trends, just not for exact amounts.