A series of standards was prepared by mixing the fluorescein or bromo-cresol green adsorbed on silica gel modified with N,N-diethylaminopro-pyltriethoxysilane with varying amounts of unmodified silica. Photo-acoustic magnitude and phase values were obtained at their respective absorption maxima, 470 and 600 nm. Calibration graphs were prepared by combining the magnitude and phase into the response function. Two-component samples were prepared by mixing the different ratios of the standard samples. The concentration of the dyes in the two-component samples was determined by referring to the respective calibration graphs and solving a pair of simultaneous equations. The relative error in the determination ranged from −5.6% to 5.1% for fluorescein and −16.8% to 38% for bromocresol green when compared to their concentration determined by spectrophotometry.

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