SNARF(R)-4F 5-(AND-6)-CAR 1 MG，S23920，Invitrogen

The cell-impermeant ratiometric pH indicator SNARF®-4F 5-(and-6)-carboxylic acid (pK_a of ~6.4) exhibits a significant pH-dependent emission shift from yellow-orange to deep red fluorescence under acidic and basic conditions, respectively. This pH dependence allows the ratio of the fluorescence intensities from the dye at two emission wavelengths - typically 580 nm and 640 nm - to be used for quantitative determinations of pH.