Regular Article

The pH value control in air plasma–liquid system by sodium bicarbonate

¹ Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, Institute of Electromagnetics and Acoustics, Xiamen University, Xiamen 361005, P.R. China
² Key Laboratory of Special Function Materials & Structure Design of the Ministry of Education, School of Physical Science & Technology, Lanzhou University, Lanzhou 730000, P.R. China
³ State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, P.R. China

^a e-mail: chenq02lzu@gmail.com

Received: 8 May 2018
Received in final form: 19 December 2018
Published online: 19 February 2019

Abstract

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.