Resazurin

Resazurin (7-Hydroxy-3H-phenoxazin-3-one 10-oxide) is a blue dye, itself weakly fluorescent until it is irreversibly reduced to the pink colored and highly red fluorescent resorufin. It is used as an oxidation-reduction indicator in cell viability assays for bacteria and mammalian cells, and for measuring aerobic respiration and exchange with the hyporheic zone in streams. Usually it is available commercially as the sodium salt.