The pH value control in air plasma–liquid system by sodium bicarbonate
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The pH value of an aqueous solution usually decreases when treated by air discharge plasma due to the formation of HNO2 and HNO3 in solution. Herein, we found that the existence of a pH-buffer material, sodium bicarbonate (NaHCO3), can maintain or even increase the solution pH value in an air plasma–liquid system depending on the discharge conditions such as the temperature and the voltage polarity of the liquid. When acidic compounds (for instance, HNO2 and HNO3) are produced by the plasma–liquid interactions, the bicarbonate ions (HCO3) existing in the solution can combine with the produced hydrogen ions (H+) to form carbonic acid molecules (H2CO3−), resulting in the maintaining of the solution pH value. However, the formed H2CO3 might be decomposed to produce water (H2O) and carbon dioxide (CO2) by the discharge plasma-induced heat, leading to a continuous consumption of H+ ions in the solution. As the quantity of H+ ions consumed by the decomposition of H2CO3 is greater than the quantity of H+ ions produced by air discharge plasma, the solution pH value will increase.
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