Except for a few metal ions, most notably UO2+, most inorganic ions are not sufficiently fluorescent for a direct analysis. Many metal ions may be determined indirectly by reacting with an organic ligand to form a fluorescent, or less commonly, a phosphorescent metal–ligand complex. One example is the reaction of Al3+ with the sodium salt of 2, 4, 3′-trihydroxyazobenzene-5′-sulfonic acid—also known as alizarin garnet R—which forms the fluorescent metal–ligand complex shown here. The analysis is carried out using an excitation wavelength of 470 nm, monitoring fluorescence at 500 nm.
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