Studies on fluorescein-II The solubility and acid dissociation constants of fluorescein in water solution

The solubility of yellow fluorescein and of red fluorescein as a function of pH has been measured in water at ionic strength 0.10. The pH of minimum solubility is the same for both, 3.28. The intrinsic solubility, defined as the solubility of the undissociated species, H(2)Fl, and assumed to be constant and independent of pH, was calculated from the observed solubilities on the low-pH side of the minimum: S(i, yellow) = 3.80 x 10(-4)M: S(i, red) = 1.45 x 10(-4)M. The first dissociation constants were evaluated from the intrinsic solubilities and the observed solubilities on the low-pH side: both fluoresceins yielded the same value, pK(H3Fl) = 2.13. In using the observed solubilities on the high-pH side of the minimum to evaluate the intrinsic solubility and the second dissociation constant it was necessary to modify the existing theoretical approach by taking into account the presence of the fully dissociated anion. Appropriate mathematical treatments were devised to handle the more complex equations. Both fluoresceins yielded the same value for the second dissociation constant, pK(H2Fl) = 4.44. Both fluoresceins give the same yellow colour in saturated solution and the results just reported for the pH of minimum solubility and for the dissociation constants also indicate that for each of the three prototropic forms of fluorescein present in solution, H(3)Fl(+), H(2)Fl, and HFl(-), only one structure exists.