Spectrophotometric study of the effects of surfactants and ethanol on the acidity constants of fluorescein

In the present paper, a sensitive, fast and suitable method for the calculation of pKa values of fluorescein is proposed. The effects of sodium dodecyl sulfate (SDS) and Triton X-100 (TX-100) as a surface-active agent on the acidic and basic forms, and the spectral properties of fluorescein were studied by the spectrophotometric method. The study was performed in sub-micelle surfactant concentration, and absorption spectra at 300–550 nm intervals were recorded. Stepwise acidity constants of fluorescein at pH range 1.50–10.00 and at constant ionic strength 0.1 M and 25 °C were determined using DATAN program using pH-spectrophotometric titration data. The method is efficient, however the component spectra showed intensive overlapping. The calculated acidity constants of fluorescein in water at ionic strength 0.1 are pKa1 = 2.20, pKa2 = 4.30 and pKa3 = 6.43. The acidity constants of the dye in ethanol–water solution were studied by the same procedure. Effect of surfactants and ethanol on acidity constants and pure spectrum of each component are also discussed.