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 ( I SCL ) is independent of H 2 O 2 concentration. For pH >10 I SCL increases monotonically with H 2 O 2 concentration up to10 -4 M but decreases as the concentration is increased further. A mechanism is proposed in which HO 2 - and the luminol monoanion competitively reduce sonochemically generated HO •, producing O 2 •- and luminol radical anion, respectively. Luminescence follows the decomposition of a hydroperoxide adduct formed by reaction between O 2 •- and luminol radical anion. EDTA is shown to suppress the background (silent) chemiluminescence of solutions containing luminol and H 2 O 2 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 (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.