Mechanistic and spatial study of ultrasonically induced luminol chemiluminescence

Aqueous solutions containing 10^-3 M luminol and varying concentrations of hydrogen peroxide are irradiated with 20 kHz ultrasound at 50 °C. The intensity of sonogenerated chemiluminescence (SCL) is shown to increase linearly with ultrasound power and to be strongly pH dependent, reaching a maximum at pH 12. For pH > 10 SCL intensity (I_SCL) is independent of H₂U₂ concentration. For pH > 10 I_SCL increases monotonically with H₂O₂ concentration up to10^-4 M but decreases as the concentration is increased further. A mechanism is proposed in which HO₂^- and the luminol monoanion competitively reduce sonochemically generated HO^-, producing O₂^- and luminol radical anion, respectively. Luminescence follows the decomposition of a hydroperoxide adduct formed by reaction between O₂^- and luminol radical anion. EDTA is shown to suppress the background (silent) chemiluminescence of solutions containing luminol and H₂O₂ without significantly affecting I_SCL. Digital images of SCL emission occurring near the transducer - solution interface are analyzed to determine the spatial distribution of sonochemical activity. It is shown that, in the absence of standing waves, I_SCL, decays exponentially with perpendicular distance from the surface of a plane-ended ultrasound transducer horn. Spatially resolved I_SCL data is used to determine the acoustic attenuation coefficient (a) in acoustically cavitating water noninvasively. It is shown that α values at the cavitation-producing frequency increase with transducer output power and may be many orders of magnitude greater than is the case for homogeneous water.