Effects of pH and surfactant on the ultrasound-induced chemiluminescence of luminol

The pH dependence of the sonochemi-luminescence (SCL) of luminol at 141 kHz was investigated and compared to the effect of pH on its fluorescence emission intensity. From the fluorescence intensity. the pK(a1)(*) and the pK(a2)(*), values were determined to be 6.4 and 10.5 respectively, which was consistent with changes observed in the SCL intensity of luminol when the pH was varied. An inverse relationship was found to exist between the SCL and the fluorescence intensities against pH, i.e., the luminol SCL spectrum at pH = 10.8 (mono-anionic form) had the same shape as the fluorescence emission spectra in acidic pH solutions. where luminol exists in either the mono-cationic or neutral form. The addition of an anionic surfactant. sodium dodecyl sulfate (SIDS). significantly decreased the luminol SCL intensity at high pH. It is proposed that the generation of a negative electric potential at the surface of cavitation bubbles effectively prevented the negatively charged form of luminol from approaching the bubbles, thereby inhibiting the reaction of luminol with OH radicals.