Sonochemical formation of nitrate and nitrite in water

The formation of nitrite and nitrate ions in water under irradiation with 900 kHz ultrasound was studied as a function of time, temperature and gas (oxygen/nitrogen) composition. The rate decreases as temperature increases, and is below the detection limit when there is no O2 gas present. The absolute rate of formation of NO−x ions obtained (about 30×10−9 mol min−1 W−1) agrees well with previous similar studies. The differences in the NO−2/NO−3 ratio found between various studies can be satisfactorily explained though a mechanism where HNO2 and HNO3 are formed in the gas phase of the imploding cavity, and then dissolve in the water and dissociate to ions. The NO−2 species is initially substantially favoured, as considerably more NO is formed than NO2. Thermodynamic calculations indicate that at the ‘hot spot’ temperature of about 5000 K believed to be present, there would very large amounts of NO and OH radicals present, and at such high temperatures, thermodynamics would be a good approximation of the situation, since the rates of reactions would be very rapid. The reaction needs O2 in order to proceed to a significant degree; no NO−x was detected in the absence of oxygen gas.