Chemical Reactions by Pulsed Ultrasound: Memory Effects in the Formation of NO−3and NO−2in Aerated Water

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Abstract
The formation of nitrate and nitrite in the sonolysis of aerated water was studied using pulses of 300 kHz ultrasound. At very low on/off ratios, the yield decreases with decreasing pulse duration. At a pulse length of 3 × 10−3s, the yield is zero. This time is identified as the ‘activation’ time τ1 of small gas bubbles formed by cavitation. At larger on/off ratios, a pulse is more effective the shorter the time interval between the pulses. This memory effect is described by a ‘deactivation’ time τ2 of the system, which amounts to about 6 × 10−2s. At large on/off ratios (1:3 and 1:1) the yield never becomes zero. It first decreases with decreasing pulse length (increasing modulation frequency) and increases again for very short pulses. The results are also discussed with respect to the use of pulsed ultrasound in diagnostic applications.

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