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Extraordinary Character of the Solvent Influence on Protolytic Equilibria: Inversion of the Fluorescein Ionization Constants in H2O-DMSO Mixtures

DOI: 10.1039/ft9928803025

Source and publish data:

Journal of the Chemical Society - Faraday Transactions p. 3025 - 3032 (1992)

Update date:2022-08-11

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Authors:

Mchedlov-Petrossyan, Nikolay O.Mchedlov-Petrossyan, Nikolay O.

Mayorga, Rafael SalinasMayorga, Rafael Salinas

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Article abstract of DOI:10.1039/ft9928803025

The protolytic equilibria of fluorescein and sulfonefluorescein in the H2O-DMSO system are studied spectrophotometrically.The pKa values, as well as the absorption spectra, attributed to individual ionic (molecular) forms of the dyes, are determined.The paH* scales used in pKa calculations 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-); Ka0, Ka1 and Ka2, respectively) results in extraordinary interrelations between the Ka constants, up to inversion: pKa1 > pKa2.The Ka1/Ka2 ratio changes gradually from 224 in H2O to 0.045 in 91percent DMSO; meanwhile the Ka0/Ka1 ratio increases from 204 to 7*1010.The anomalous character of the fluorescein pKa dependence on the content of DMSO (up to pKa1 > pKa2) 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 pKa values of model compounds.

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