Mechanistic and Spatial Study of Ultrasonically Induced Luminol Chemiluminescence

Abstract

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 (ISCL) is independent of H2O2 concentration. For pH >10 ISCL 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 ISCL. 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, ISCL decays exponentially with perpendicular distance from the surface of a plane-ended ultrasound transducer horn. Spatially resolved ISCL data is used to determine the acoustic attenuation coefficient (α) 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.