The pKa of 5-carboxy-2′,7′-dichlorosulfonefluorescein is near 4 and thus the dye is potentially an excellent pH indicator for acidic organelles.
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6-CFDA (6-Carboxyfluorescein diacetate, single isomer)
6-CFDA is membrane-permeant and thus can be loaded into cells via incubation. Once inside cells, 6-CFDA is hydrolyzed by intracellular esterases to 6-carboxyfluorescein.