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How can a reagent make a nonabsorbing metal ion measurable by spectrophotometry?

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To make a nonabsorbing metal ion measurable by ultraviolet-visible spectrophotometry, a chemical reagent is added to the sample to react with the analyte, forming a new product that absorbs light strongly in the ultraviolet-visible spectral region.1

When the analyte is a metallic ion, the added reagent is typically a complexing agent or ligand. This reagent binds to the metal ion to form a colored coordination compound. The intensity of the light absorbed by this resulting complex is then measured and used to determine the concentration of the original, nonabsorbing metal ion.123

A classic example of this process is the addition of 1,10-phenanthroline to solutions containing iron(II) ions. The reaction produces a red-colored complex that allows for the precise detection of even trace amounts of iron. This procedure, where a chemical reagent is utilized in a spectrophotometric assay, is referred to as spectrochemical analysis.1