Among the vast series of phenolsulfonephthalein dyes, the nitro derivatives and especially 3,3 0 ,5,5 0tetranitrophenolsulfonephthalein (nitrophenol crimson) remain practically unexplored, whereas the halogen and alkyl derivatives have been studied comprehensively. This striking difference is probably due to the enormous influence of the four NO 2 groups on the properties of the dye. As a result, the protolytic behaviour is unlike even that of tetrabromo phenolsulfonephthalein, and the recognised scheme of acid-base and tautomeric equilibrium of the sulfonephthaleins is unable to explain it. The molecular form H 2 R was isolated as a sultonic tautomer, and an X-ray crystal structure analysis was carried out. Our studies of the UV-vis absorption spectra in water, methanol, dimethyl sulfoxide, acetonitrile, acetone, and dichloromethane, as well as in aqueous micellar solutions of surfactants, allowed us to evaluate the true molar absorptivity of the dianion R 2À , and to elucidate the enormous tendency to form yellow trianionic carbinol ROH 3À , even in the presence of traces of H 2 O. Nuclear magnetic resonance and electrospray data confirm the proposed scheme of ionisation and tautomerism of nitrophenol crimson.

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