Quantitation and evaluation of NO2−, NO3−, and H2O2 in the sonolysis of aqueous NaOH solution under air and air-Ar mixture: Effects of solution temperature, ultrasonic power, and ratio of gas mixture

Abstract

When an aqueous solution containing dissolved air is sonicated, H2O2, HNO2, and HNO3 are formed. This is a result of the formation of active bubbles with extremely high-temperature and high-pressure. The yields of H2O2, NO2−, and NO3− are representative indexes for understanding the chemical effects of ultrasonic cavitation in water. However, these yields often vary under the acidic conditions caused by sonication. In this study, we measured the yields of H2O2, NO2−, and NO3− in the presence of NaOH, which suppresses the reaction between NO2− and H2O2 and prevents the formation of NO3− in a bulk solution. Therefore, the yields obtained should correspond to the actual yields just after bubble collapse, directly reflecting the chemical effects of the active bubbles themselves. It was confirmed that the yields of NO2− and NO3− decreased, while the ratio of [NO3−] to [NO2−] ([NO3−]/[NO2−] ratio) increased with increasing solution temperature, suggesting that the temperature and pressure in collapsing bubbles decreased with an increase in the solution temperature. Ultrasonic power clearly affected the yields of NO2− and NO3−, but it did not affect the [NO3−]/[NO2−] ratio, suggesting that 1) the quality of the active bubbles did not change largely with increasing ultrasonic power, and 2) the quantity related to the number and/or size of active bubbles increased with increasing ultrasonic power up to a certain power. Additionally, the effects of the ratio of air to Ar on the yields of NO2−, NO3−, and H2O2 were investigated. These yields could be affected not only by the bubble temperature but also by the concentration of reactants and intermediates inside the collapsing bubbles. The chemical reactions are quite complex, but these yields could be valuable analytical tools for understanding the quantity and quality of active bubbles.