## 3 Results and Discussion
### Measurements of pH and ORP Level  
Figure 4 shows the pH level of the PAW versus treatment time for various gas feedings. There was a declining trend in the pH values, and thus the acidity level of distilled water raised using plasma treatment with air and nitrogen gas feedings. We may have nitrous acid because of nitric oxide radicals (Reactions 23-25) in the nitrogen and air DBD plasmas. Following the above reactions, some reactions may occur in both the gas and liquid phases [28; 31]:  
\[\mathrm{NO\bullet+OH\bullet\rightarrow HNO_{2}} \tag{28}\]  
\[\mathrm{OH\bullet+HNO_{2}\rightarrowH_{2}O+NO_{2}} \tag{29}\]  
\[\mathrm{OH\bullet+NO_{2}\rightarrow HNO_{3}} \tag{30}\]  
It is known that nitrous acid is a weak and unstable acid in the liquid. During an oxidation process, nitrite reacts with hydrogen peroxide to generate nitrous acid [18; 26; 32].  
\[\mathrm{OH\bullet+OH\bullet\rightarrowH_{2}O_{2}} \tag{31}\]  
\[\mathrm{NO_{2}^{-}+H_{2}O_{2}+OH\bullet\rightarrow HNO_{2}+H_{2}O} \tag{32}\]  
The decomposition of nitrous acid inside the water leads to the generation of proxymitrous acid [33], which is a strong acid and thus provides an acidic medium.  
\[\mathrm{HNO_{2}\rightarrow HNO_{3}+2 NO+H_{2}O} \tag{33}\]Nitrite (Reaction 32), generated inside water, originally comes from the dissolution of nitrogen oxides. For instance, nitric oxide can be oxidized to generate nitrogen dioxide (NO\({}_{2}\)) [34]. Then it is dissolved into water to generate nitrite and nitrate [18]. Therefore, a significant reduction of the pH value in water is due to nitrite's presence and formation. In comparison, the pH level of treated water with argon and oxygen plasma treatments does not change significantly due to the lack of nitrogen species. In this case, the pH values remain between 6 to 8 during the plasma treatment. Due to the lack of nitrogen and limited interaction with ambient air, it leads to extremely small values of nitrite and nitrate inside the PAW, so the pH value does not decrease significantly.  
Figure 5 shows the ORP values, which reflect electron transfer ability through chemical reaction strength in the PAW versus treatment time with different feeding gases. The figure depicts two main trends: the first corresponds to the argon and oxygen plasmas treatment, which tend to decline. The other corresponds to the nitrogen and air plasma treatment which has a rising trend. It means that nitrogen and air plasma oxidizing species generated inside the PAW are stronger than argon and oxygen plasmas, probably due to nitrous and proxy-nitrous acid in nitrogen and air DBD plasmas (Reactions 28, 30, 32, and 33). As a comparison of trends in pH (Figure 4) and ORP (Figure 5), high ORP and low pH values appeared with increasing treatment time in the cases of air and nitrogen DBD systems. A similar result was reported by [PERSON] et al. [35] and [PERSON] et al. [36] in a PAW generated by an air plasma jet. [PERSON] et al. [37] presented a PAW using an air surface discharge. ORP and pH trends are the same as our air and nitrogen DBD systems, although they measured three timepoints merely. In addition, [PERSON] et al. [38] measured ORP and pH values of a PAW in an arc discharge. ORP versus pH values have the same trend compared to our system, although the discharge regime is different from our air and nitrogen DBD systems. In contrast, the pH value of PAW generated by cold argon/oxygen plasma microjet reported by [PERSON] et al. [20] does not change significantly, although the ORP level increases slightly.  
Figure 4: Variations of pH level for plasma-treated water as a function of time with four gas feedings: argon, oxygen, nitrogen, and air.