Kinetics of nitrous acid formation in nitric acid solutions under the effect of power ultrasound

Sonochemical nitrous acid formation was investigated in 0.1-4.0 mol dm-3 aqueous nitric acid solutions under the effect of power ultrasound with 20 kHz frequency. HNO2 steady-state concentration was obtained under long-time sonication; the excess HNO2 formed is decomposed and evoluted from the solution as NO and NO2 gases. The HNO2 steady-state concentration and the HNO2 initial formation rate depend linearly on the HNO3 concentration and acoustic intensity (1.8-3.5 W cm-2) and decrease with rising temperature in the range 21-50°C. The HNO2 formation rate depends on the type of saturating gas as follows: Ar > N2 > He > air. NO and O2 are the major gaseous products of HNO3 sonication. The NO2 accumulation in the gas phase is observed only when the decomposition of HNO2 formed becomes noticeable. The gaseous products formation rates depend on the HNO3 concentration, acoustic intensity, and the type of saturating gas. The mechanism of HNO2 sonochemical formation is assumed to be the thermal decomposition of HNO3 in the gaseous vicinity of collapsing bubbles or in the overheated liquid reaction zone surrounding the cavitational bubbles.