Reduction of the naphthazarin molecule as studied by pulse radiolysis. Part 2.—Second one-electron step

Abstract

The disproportionation of the radicals formed by one-electron reduction of naphthazarin has been observed as a function of pH in the range 1–14. The radicals are least prone to disproportionate at pH 8–9, the stability constant at pH ca. 8.5 being 0.4. The data have been combined with the one-electron reduction potentials of naphthazarin to obtain the one-electron reduction potentials of the radicals (i.e. the second one-electron reduction potentials of the parent) as a function of pH. E¹₇ of the radicals is equal to E²₇ of the parent and is –15 mV. The two-electron reduction potential of naphthazarin is –62 mV at pH 7 and –351 mV at pH 0. The optical absorption spectra of 1,4,5,8-tetrahydroxynaphthalene prepared in different ionised forms by radiolysis and by chemical reduction have been determined. The redox data and the absorption spectra show that this compound has pK_a values of 7.6 and 8.7 and has no further pK_a values below ca. 14.