Mechanism of pH Variation and H2O2 Generation in Water Exposed to Pulsed Discharge Plasma

Positive and negative pulsed discharge plasma is generated above test liquid containing NaCl in Ar atmosphere, and the spatiotemporal variation of pH value in the liquid is visualized by a colorimetric method using bromothymol blue. Furthermore, polarity effects on H₂O₂ generation are examined by electrochemical calculation and a colorimetric method using titanium sulfate. When the positive pulsed discharge plasma is generated, the pH value of the liquid is decreased by H⁺ generation through the dissociation of H₂O⁺, formed by the charge exchange reaction between Ar⁺ and H₂O, while the pH value of the vicinity of an earthed electrode is increased by OH^- generation through the electrolysis of water. When the negative pulsed discharge plasma is generated, the pH value of the liquid is increased by OH^- generation and H⁺ reduction, respectively, through the reaction of hydrated electrons with H₂O and H⁺, while the pH value in the vicinity of the earthed electrode is decreased by H⁺ generation through the electrolysis of water. The generation of H₂O₂ is found to be promoted when the positive pulsed discharge plasma is generated, and this increase rate of H₂O₂ is in approximate agreement with the calculated generation rate of H₂O₂, which is produced from OH generated by electrolysis.