Abstract: A composition for determining pH of a solution comprises a fluorescent carbazine dye covalently bound to a solid support. A method of determining pH of a solution comprises placing the composition in the solution, contacting the composition with a selected wavelength of light to excite fluorescence by the carbazine dye, measuring intensities of the fluorescence at two selected wavelengths, calculating a ratio of fluorescence intensities at the two selected wavelengths, and correlating the ratio with a predetermined relationship of such ratios to pH. A fiber optic system for measuring pH of a solution with the carbazine-dye-containing composition is also disclosed.