Characterization of the ionization and spectral properties of sulfonephthalein indicators. Correlation with substituent effects and structural features.

The pK values of the second ionization of a set of substituted sulfonephthaleins are studied by spectrophotometry and (13)C NMR spectroscopy. A study of the correlation between equilibrium and spectral data on the one hand and the substituent effects on the other is presented, using the dual substituent analysis of Swain and Lupton. This shows a complete dependence of pK values on the F field variable, and of the wavelengths of the bands of basic forms on the R resonance variable.