Polarography of Anthraquinones and Dianthrones of Rhubarb

The 1, 8-dihydroxyanthraquinones such as emodin (I), physcion (II), crysophanol (III), aloe-emodin (IV) and rhein (V) show reversible 2-electron reduction wave forming the hydroquinones in aqueous isopropanol solutions. Their semiquinones are stable in dimethylformamide. Sennosides (VI) show irreversible 2-electron reduction wave accompanied with 2 adsorption waves, which are ascribed to reductive cleavage of the C-C bridge to the anthrone (VII). The carbonyl in VII is successively reduced to the corresponding alcohol (XI), which seems to dehydrate to the anthracene (XII). Then, the I-V and VI in rhubarb can be determined by polarography.