Extraordinary character of the solvent influence on protolytic equilibria: inversion of the fluorescein ionization constants in H2O–DMSO mixtures

Abstract

The protolytic equilibria of fluorescein and sulfonefluorescein in the H₂O–DMSO system are studied spectrophotometrically. The pK_a values, as well as the absorption spectra, attributed to individual ionic (molecular) forms of the dyes, are determined. The pa*_H scales used in pK_a calculations [at DMSO contents (wt.%) of 21.6, 42.2, 60.3, 81.4 and 91.3] were estimated by the indicator method (sulfonephthalein series) through the overlapping procedure.

The character of the solvent influence on the fluorescein stepwise ionization (H₃R⁺⇄ H₂R ⇄ HR^–⇄ R^2–; K_ao, K_a1 and K_a2, respectively) results in extraordinary interrelations between the K_a constants, up to inversion: pK_a1 > pK_a2. The K_a1/K_a2 ratio changes gradually from 224 in H₂O to 0.045 in 91% DMSO; meanwhile the K_a0/K_a1 ratio increases from 204 to 7 × 10¹⁰.

The anomalous character of the fluorescein pK_a dependence on the content of DMSO (up to pK_a1 > pK_a2) is explained through the tautomeric equilibria shift and the nature of the functional groups. The tautomerization constants of the fluorescein neutral form are given as obtained on the basis of the extrathermodynamic assumption proposed previously (N. O. Mchedlov-Petrossyan, Zh. Anal. Khim., 1979, 34, 1055). This allowed calculation of the ionization microconstants (k), which were found to be in agreement with pK_a values of model compounds.