How does 1,10-phenanthroline help measure iron(II) ions?

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In chemical analysis, 1,10-phenanthroline is used as a complexing agent to facilitate the measurement of iron(II) ions, which do not always absorb light strongly enough for direct detection.1

When added to a solution containing iron(II), 1,10-phenanthroline reacts to form a stable, red-colored coordination complex. This reaction product absorbs light strongly in the visible region of the spectrum, allowing for spectrochemical analysis via ultraviolet-visible spectrophotometry. By measuring the intensity of this absorption, analysts can accurately determine the concentration of iron(II) in a sample, even when only very small amounts are present. Additionally, because the complex changes color upon oxidation, it can also function as an oxidation-reduction indicator.21