Differences between Phthaleins and Sulfonphthaleins

Abstract

Differences in color and molecular structure between phthalein-pigments and sulfonphthalein-pigments were investigated using X-ray crystallography and absorption spectrophotometry of their aqueous solutions. The molecular structure of sulfonefluorescein (H₂⁺SF^- ) was determined as a zwitter ion, 2-(3-hydroxonio-6-hydroxy-3H-xanthen-9-yl) benzenesulfonate. The absorption spectra of H₂⁺SF^- demonstrated the dissociation profile of a dibasic acid, while those of fluorescein (H₂FL) indicated a tribasic acid and further, at pH>10, SF^2- and FL^2-, while at pH<0, H₂⁺SF^- and H₃⁺FL to be dominant. The spectra of H₂⁺SF^- were analyzed to obtain the values of pK₁ and pK₂ together with the spectrum of HSF^- . Similarly, from the spectra of H₃⁺FL the values of pK₁, pK₂ and pK₃ together with the spectra of HFL^- and H₂FL were obtained. Further by adding 1/5 of the H₃⁺FL spectrum to 2/15 of HFL^- spectrum, an indicated spectrum as that of H₂FL was obtained. From these results, the features of dissociation of H₂SF^- and H₃⁺FL were estimated. The molecular structure of phenolsulfonphtalein (H₂PS^-) was determined as a zwitter-ion, α-(4-hydroxonio-2, 5-cyclohexadiene-1-ylidene)-α-(4-hydroxy-phenyl)-2-toluenesulfonate. The absorption spectra of H₂PS^- demonstrated that H₂PS^-, HPS^- and PS^2- became dominant at pH«0, pH=4 and pH>11, respectively. On the contrary, phenolphthalein (H₂PP) displayed only one type of absorption spectrum in the visual region, the shape of which was similar to that of PS^2- while the molar extinction coefficient was smaller. The spectra were analyzed to obtain the valus of pK₁=9.05 and pK₂=9.5O. The spectra also demonstrated a slow addition reaction of OH^- to PP^2-= and pK₃= 12 was obtained by measuring the absorbance after equilibration. From these results, the features of dissociation and coloration of H₂⁺PS^- and H₂PP were estimated.