SNARF-4F 5-(AND-6)-CAR 20 X 50 UG，S23921，Invitrogen

The cell-permeant ratiometric pH indicator SNARF®-4F 5-(and-6)-carboxylic acid, acetoxymethyl ester, acetate (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.