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Methods of absorption and fluorescent spectroscopy and fluorescent chronoscopy are used to investigate aqueous fluorescein solutions with pH values increasing from 1.79 to 11.67. For each method, the calibration curves are drawn for the fraction of dianion fluorescein species, quantum fluorescence yield, and amplitude dianion contribution to the fluorescence intensity on the pH values of aqueous solutions. The calibration curves allow the pH values of aqueous fluorescein solutions with gelatin (pH = 6.4 ± 0.1, 6.6 ± 0.2, and 6.6 ± 0.1), starch (pH = 6.3 ± 0.1, 6.3 ± 0.2, and 6.4 ± 0.1), and acid-soluble chitosan additions (pH = 5.4 ± 0.1, 5.6 ± 02, and 5.8 ± 0.1) to be estimated by these three methods. The pH values of biopolymer solutions obtained by the potentiometer method for starch (6.51) and gelatin (6.69) and by the three spectral and fluorescent methods are close; they are significantly underestimated (3.85) for chitosan solutions.

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