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Fluorescein is a synthetic dye made by fusing resorcinol with phthalic anhydride (see acid anhydride) and zinc chloride catalyst. A red crystalline solid, fluorescein dissolves in alkalis to give a deep red solution showing very intense yellow-green fluorescence, visible at very low concentrations. It is used as a water tracer, and was used in World War II to mark spots on the sea.

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