Polarographic studies of the interaction of boric acid with 1-hydroxy- and 1,2-dihydroxy-9,10-anthraquinones and -anthrahydroquinones

Abstract

The interaction of 1-hydroxy-9,10-anthrahydroquinones (1-HOAHQ) with boric acid in aqueous borate buffer solutions at pH 9.5 causes a decrease in the anodic limiting current of the 1-HOAHQ and the simultaneous appearance of a new diffusion-controlled anodic wave at more positive potentials. This is attributed to the formation of a cyclic borate ester of the 1-HOAHQ involving the peri-hydroxyl groups. Ester formation is much slower but still complete when the boric acid – borate to 1-HOAHQ molar ratio exceeds 10–15. These esters are most stable at pH 7.5–9.5 and completely unstable above pH 12. 1,2-Dihydroxy-9,10-anthraquinones (1,2-diHOAQ) react with boric acid to give cyclic esters involving the 1- and 2-hydroxyl groups, but of lower stability than those from 1-HOAHQ. On reduction of a 1,2-diHOAQ in the presence of boric acid at pH 9.5, cyclic borate esters involving the peri-hydroxyl groups are rapidly produced in preference to those involving the ortho-hydroxyl groups.