– The cell-permeant pH indicator, carboxy SNARF-1, acetoxymethyl ester, acetate (CAS #126208-13-7) has a pKa of ~7.5 after de-esterification, thus is useful for measuring pH changes between pH 7 and pH 8. Carboxy-SNARF-1 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.