Redox and acidic properties of the borate radical B(OH) ˙4. A flash photolysis study

Abstract

Flash photolysis of an aqueous solution of sodium metaborate and potassium peroxydisulphate at pH 11.5 gives rise to a new transient species with an absorption maxima at 590 nm (ε= 80 m² mol^–1) and 650 nm (ε= 90 m² mol^–1). The laser-Raman and ¹¹B n.m.r spectra of metaborate solutions at pH 11.5 are consistent with the presence of the tetrahydroxyborate ion, B(OH)^–₄. The pK_a and reduction potential of the radical B(OH)^˙₄, produced on flashing a mixture of B(OH)^–₄ and S₂O^2–₈ are found to be 10.75 ± 0.02 and + 1.4 V, respectively. The rate constants for the oxidation of several amines and phenols by this one-electron oxidant have been determined. Salt-effect studies are consistent with the reactive species being B(OH)₃O^˙–.