Interference of ferric ions with ferrous iron quantification using the ferrozine assay

The ferrozine assay is a widely used colorimetric method for determining soluble iron concentrations. We provide evidence for a heretofore unrecognized interference of ferric ions (Fe^3 +) on ferrous iron (Fe^2 +) measurements performed in the dark. Fe^3 + concentrations affected the absorbance measurements, which linearly increased with incubation time.