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Rhodamine B hydrazide is a good probe for sulfite, with colorless and non-fluorescent properties. While the emission is related to the concentration of sulfite (5-800 ng/mL; detection limit=1.4 ng/mL (3σ)). Sulfite reduces dissolved oxygen to yield superoxide radicals, which binds to Rhodamine B hydrazide to form Rhodamine B. Rhodamine B hydrazide gives Rhodamine B-like fluorescence in the presence of sulfite, which is enhanced by Tween 80 surfactant micelles. Rhodamine B hydrazide has an absorption maximum at 554 nm and a fluorescence emission maximum at 574 nm[1].

[1] Yang X F, et al. Novel spectrofluorimetric method for the determination of sulfite with rhodamine B hydrazide in a micellar medium[J]. Analytica Chimica Acta, 2002, 456(1): 121-128.