2646 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 11, NOVEMBER 2011 Volume Effects of Atmospheric-Pressure Plasma in Liquids Katrin Oehmigen, Tomáš Hoder, Christian Wilke, Ronny Brandenburg, Marcel Hähnel, Klaus-Dieter Weltmann, and Thomas von Woedtke Abstract—Generation of chemical species by atmospheric-pres- sure plasma treatment of aqueous liquids is a result of reactions at the plasma/gas–liquid interface and subsequent diffusion and convection processes into the liquid volume. Using color forming reactions, acidification as well as generation of nitrite is visualized in the water treated by a surface dielectric barrier discharge under atmospheric conditions in ambient air. Index Terms—Dielectric barrier discharge (DBD), liquid treat- ment, nonthermal plasma, plasma-induced species, plasma medi- cine, volume effects in liquids. P LASMA–LIQUID interactions are an important research topic not only in the field of purification of polluted water but also in plasma medicine to investigate mechanisms of plasma–cell and plasma–tissue interactions [1]–[7]. To understand the complex processes of plasma-induced chemistry in aqueous solutions, plasma diagnostics has to be combined with liquid analytics. Several classical wet-chemical analytic procedures are based on color forming reactions using chemical indicator molecules. In the experiment presented here, indicator reactions are used to visualize volume effects of water treatment by surface dielectric barrier discharge (surface- DBD) plasma under ambient air conditions. The surface-DBD arrangement, which was particularly designed for plasma treat- ment of microorganism or cell cultures and liquid samples in petri dishes (Fig. 1), has been described in detail elsewhere [6], [7]. Using the electrode array which is mounted by a special construction into the upper shell of a petri dish (60-mm diame- ter), a 100-mL beaker containing water was covered (Fig. 2). The plasma was generated at the surface of the electrode arrangement (Fig. 1, small picture). Since the distance between the electrode arrangement and the liquid surface was adjusted at 5 mm, there was no direct contact of the plasma to the liquid. All experiments are performed under atmospheric pressure at ambient air conditions using a pulsed sinusoidal voltage of 10 kV peak (20 kHz) with a 0.413/1.223 s plasma-on/plasma-off time. Energy of 2.4 mJ was dissipated into the plasma in each cycle of high voltage. As it was demonstrated before, plasma Manuscript received November 30, 2010; revised May 12, 2011; accepted May 22, 2011. Date of publication June 23, 2011; date of current version November 9, 2011. This work was supported by the German Federal Ministry of Education and Research within the joint research project “Campus Plas- maMed” under Grant 13N9779. The authors are with the Leibniz Institute for Plasma Science and Tech- nology (INP Greifswald), 17489 Greifswald, Germany (e-mail: oehmigen@ inp-greifswald.de; hoder@inp-greifswald.de; wilke@inp-greifswald.de; brandenburg@inp-greifswald.de; haehnel@inp-greifswald.de; weltmann@ inp-greifswald.de; woedtke@inp-greifswald.de). Digital Object Identifier 10.1109/TPS.2011.2158242 Fig. 1. Schematic drawing of the surface-DBD arrangement and photo of plasma on the surface of the electrode arrangement [6], [7]. treatment of water in ambient air may result in acidification and generation of different chemical species like hydrogen peroxide, nitrate, and nitrite [4]–[8]. To detect acidification, the pH indicator methyl orange (Merck) was dissolved in water forming a yellow-colored solution (pH > 4.4). At pH < 3.1, methyl orange reacts with protons (H + ), resulting in a color change from yellow to orange and finally to red. To detect nitrite ions (NO - 2 ), a commercially available test kit (Spectroquant, Merck), based on sulfanilic acid and N -(1-naphthyl)-ethylene diamine hydrochloride, was used. These noncolored substances react with nitrite ions via azo sulfanilic acid to a magenta- colored azo dye. Fig. 2 shows time-dependent color changes of water contain- ing pH-sensitive (upper picture sequence) as well as nitrite- sensitive indicators (lower picture sequence) during 30-min surface-DBD plasma treatment in air. Note that there was no gas flow and no stirring of the liquid! At the beginning of plasma treatment (0–2.5 min), in both cases, formation of a colored layer near the liquid surface was found, indicating chemical reactions localized at the gas–liquid interface. The thin layer extended into the liquid volume and formed a diffusion front. After about 2.5-min plasma treatment, diffusion seems to be superimposed by additional gradients. Such gradients which lead to a significant increase of velocity of particle transport 0093-3813/$26.00 © 2011 IEEE