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Among the fluorescein dyes, the behaviour of their nitro derivatives in solution is most unusual, and yet at the same time rarely studied. In this study, their 13 C nuclear magnetic resonance and electrospray spectra are reported, which confirm, in concert with their ultraviolet-visible spectra, the molecular structures of the neutral and ionic species of 2,4,5,7-tetranitrofluorescein observed in different solvents and at various pH values. A peculiar property of this dye consists in its lactonisation of the double-charged anion, contrary to the wellknown coloured and fluorescent dianions of fluorescein, eosin, etc. Also, this dianion-lactone exhibits high ability for opening the pyran ring. This cleavage takes place at much lower pH values as compared with those for fluorescein and its halogen derivatives. The ruptured species can be formed as either lactone or carbinol; the latter appears as a result of the nucleophilic attack of a HO À on the central carbon atom. The corresponding molecular structures are identified via the electrospray method and X-ray analysis. The behaviour of some other nitrofluoresceins in alkaline media is also considered. Coloration TechnologySociety of Dyers and Colourists † Just now, an article "Syntheses of novel acid-base indicators by nitration of phenolphthalein and fluorescein" by Y. Negishi,

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The application of the previously developed original method for data processing of conductometric titration of solutions of weak electrolytes of low concentration (less than 2•10-4 mol/l) for determining the acidity constants of basic titrants, dibasic acids and ampholytes in three aqueousorganic systems: water-ethanol, water-acetone, and waterdimethyl sulfoxide is demonstrated. The determined acidity constants and total limiting conductance of acetic acid, triethylamine, Tris and carbonic acid in selected aqueousorganic mixtures simplify the usage of the method to measure рК а of other objects in those media.