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Sonochemical species such as nitrite (NO2−) and nitrate (NO3−) were detected in ultrapure aqueous medium with 28 kHz low frequency ultrasound (US) in the range of 200–1200 W output power. The concentration of their anionic ions monitored with a high-performance liquid chromatography increased with increasing US power especially under air atmosphere. When the generation of NO2− and NO3− ions under US exposure was investigated for N2, O2 and Ar-bubbled solutions, no trace of NO2− was observed while NO3− was slightly generated. Under air atmosphere, the concentration of dissolved oxygen in the aqueous medium increased especially when 1200 W power was used. In addition, the bulk pH shifted towards the acidic side with an increase in exposure time, which indicated that NO2− was formed. The formation of oxidizing species under 28 kHz low frequency ultrasonic treatment was confirmed with an absorption spectrum which was dominated by two maxima peaks at 288 nm and 352 nm representing triiodide I3− anion.
Kwedi-Nsah et al. (Tue,) studied this question.