Effect of acidic conditions on decomposition of methylene blue in aqueous solution by air microbubbles

Methylene blue (MB) has recently been proposed as a new chemical probe to colorimetrically verify the presence of hydroxyl radicals and was applied to the microbubble technique in this report. The degradation of MB in water was observed by air microbubbles under acidic condition or adding H₂O₂. The discoloration of MB induced by air microbubbles supports that hydroxyl radicals are generated by the collapse of microbubbles in water. The decomposition rate of MB increases with increasing the concentration of acids (HCl, HNO₃, H₂SO₄) and H₂O₂ up to ab. 2 mM, and becomes almost constant above ab. 2 mM. Note that the decomposition rate of MB mainly depends on pH, not the kind of acid and that adding salt slightly enhanced the decomposition of MB. The fact that the blue-shift of the absorbance band of MB was observed for not H₂O₂ but acids proposed that the methyl groups of MB are preferentially dissociated under acidic conditions due to the protonation of dimethylamino group.