Syntheses of fluorescent dyes. XIV. Standards for fluorescence measurements in the near neutral pH‐range

Highly fluorescent cationic indicators have been obtained by reaction of heterocyclic nitrogen bases with 1,3‐propane sultone. Unlike the bases themselves, the fluorescence intensity of the cationic salts is practically p H‐independent in the neutral p H‐range, which makes them useful reference substances in measuring fluorescence intensities in physiological studies. Furthermore, they are sufficiently water soluble, the fluorescence maxima covering the wavelength range from 440 to 530 nm. 2‐(3‐Sulfonatopropyl)harmanium, ( 5 ) appears to be the most useful compound for intra‐ and extracellular liquids, since its fluorescence is not quenched by chloride ion.