Abstract
The relationship between the structure and the spectrum of fluorescein in aqueous solution was investigated by mainly comparing the absorption and fluorescence properties of fluorescein with those of monomethylfluorescein, dimethylfluorescein, and fluorescein methyl ester. Fluorescein was proved to be predominantly present in its univalent cation (V), neutral lactonoid (I), univalent anion (III) and bivalent anion (IV) forms at pH<1, pH3, pH 5-6 and pH>8, respectively. All forms except I were fluorescent and IV fluoresced most intensely. The neutral quinonoid form (II) was not detected in aqueous solution against to the results of previous authors.
