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 degrees C. T he 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 H2O2 concentration. For pH > 10 I-SCL increases monotonically with H2O2 concentr ation up to 10(-4) M but decreases as the concentration is increased furthe r. A mechanism is proposed in which HO2- and the luminol monoanion competit ively reduce sonochemically generated HO., producing O-2(.-) and luminol ra dical anion, respectively. Luminescence follows the decomposition of a hydr operoxide adduct formed by reaction between O-2(.-) and luminol radical ani on. 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 inte rface are analyzed to determine the spatial distribution of sonochemical ac tivity. It is shown that, in the absence of standing waves, I-SCL decays ex ponentially with perpendicular distance from the surface of a plane-ended u ltrasound transducer hem. Spatially resolved I-SCL data is used to determin e the acoustic attenuation coefficient (alpha) in acoustically cavitating w ater noninvasively. It is shown that or values at the cavitation-producing frequency increase with transducer output power and may be many orders of m agnitude greater than is the case for homogeneous water.