Systems, including compositions and methods, for measuring pH, particularly in cells, organelles, and other samples. The compositions include pH-sensitive fluorescent and fluorogenic 2',7'-dialkylfluorescein derivatives and associated nonfluorescent precursor compounds of the formula shown below. The compositions may permit ratiometric measurement in the excitation spectrum and the emission spectrum. The methods include adding a precursor compound to a sample cell, incubating the sample cell to release the free indicator, illuminating the sample cell, and detecting the fluorescence response of the free indicator.