Photochemistry of Peroxoborates: Borate Inhibition of the Photodecomposition of Hydrogen Peroxide

The UV absorbance and photochemical decomposition kinetics of hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. At higher pH borate/boric acid inhibits the photodecomposition of hydrogen peroxide (molar absorptivity and quantum yield of H 2 O 2 and HO 2 − , (19.0±0.3)  m −1 cm −1 and 1, and (237±7)  m −1 cm −1 and 0.8±0.1, respectively). The results are consistent with the equilibrium formation of the anions monoperoxoborate, K BOOH =[H + ][HOOB(OH) 3 − ]/([B(OH) 3 ][H 2 O 2 ]), 2.0×10 −8 , R. Pizer, C. Tihal, Inorg. Chem . 1987 , 26 , 3639–3642, and monoperoxodiborate, K BOOB =[BOOB 2− ]/([B(OH) 4 − ][HOOB(OH) 3 − ]), 1.0±0.3 or 4.3±0.9, depending upon the conditions, with molar absorptivity, (19±1)  m −1 cm −1 and (86±15)  m −1 cm −1 , respectively, and respective quantum yields, 1.1±0.1 and 0.04±0.04. The low quantum yield of monoperoxodiborate is discussed in terms of the slower diffusion apart of incipient . OB(OH) 3 − radicals than may be possible for . OH radicals, or a possible oxygen‐bridged cyclic structure of the monoperoxodiborate.