# Water Modification by Cold Plasma Jet with Respect to Physical and Chemical Properties
## 3 Results and Discussion
### Liquid Treatment--Basic Characteristics
Thus, above the liquid surface, the non-saturated air is renewed more efficiently, and the concentration of the liquid vapors goes down when the plasma is activated. This fact encourages faster evaporation. The more intense cooling effect induced by the argon gas is likely due to an \"adiabatic-like\" cooling that takes place as the gas is leaving the compressed gas reservoir in a short time. On the other hand, small amounts of heat induced by the plasma (see reference [46] and Figure 3c) compensate for this \"adiabatic-like\" cooling and the temperature variation appears lower. Apart from the evaporation and the temperature variations, the DDW pH is exponentially decreases versus the treatment time (Figure 10). Within 30 min, a decrease of about two in the pH scale (\(\Delta\)pH = \(-\)1.98) is obtained. Since the pH scale is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (at that level, this is essentially the number of moles of hydrogen ions per liter of solution), it is deduced that plasma induces an increase of the hydrogen ion concentration in the water. It should be underlined that the initial low pH (around 6.0) is not surprising since, if pure water is exposed to air, it becomes mildly acidic. This happens because water absorbs carbon dioxide from the air which is then slowly converted into carbonic acid (the chemical reaction is reversible). In any case, the observed plasma-induced acidification of water is a crucial result which can account for the deactivation of different biospecimens. On the other hand, PBS pH remains almost constant (\(\Delta\)pH = \(-\)0.18), as was expected for an isotonic solution. Furthermore, another quantity related to pH and the disinfection efficiency of a liquid is the redox potential, \(E_{\text{h}}\). Redox potential is an indicator of the ability of a solution to oxidize and is related to the oxidizer concentration. Substances like dissolved oxygen will contribute to higher oxidation and higher redox. This substance has higher electronegativity (oxidizing agent) and will normally pull electrons from other substances like unwanted  
Figure 10: pH evolution versus the treatment time in the case of (**a**) DDW and (**b**) PBS. Mean values and standard deviations are calculated from two series of experiments carried out on different dates. Operating conditions: 11 kV\({}_{\text{PP}}\), 17.5 kHz, and 1.5 slm. contaminants and bacteria. Under the present conditions, the redox potential of the DDW changes from about 360 mV to 430 mV (\(\Delta E_{\text{h}}\) = +60.5 mV), in an exponential manner, according to Figure 11a. Consistently with the pH measurements, the PBS redox is less affected even after 30 min of treatment (\(\Delta E_{\text{h}}\) = +10 mV); Figure 11b. Finally, Figure 12 bears testament to the ionic species increase in both DDW and PBS as a function of the treatment time. In the case of DDW, an almost seven-fold conductivity is achieved within 30 min of plasma action (\(\Delta\sigma\) = +30.33 \(\mu\)S cm\({}^{-1}\)), whereas in the case of PBS, an increase \(\Delta\sigma\) = +1.34 mS cm\({}^{-1}\) is obtained within 20 min. The dashed line in Figure 12b corresponds to a numerical extrapolation of the experimental data, because the remaining PBS in the 10-mL beaker after 20 min of treatment was not sufficient to cover the immersed probe and any measurement taken could not be considered as reliable. This extrapolation suggests an increase \(\Delta\sigma\) = +3.1 mS cm\({}^{-1}\) for the 30-min treatment. Figure 11: Redox potential evolution versus the treatment time in the case of (**a**) DDW and (**b**) PBS. Mean values and standard deviations are calculated from two series of experiments carried out on different dates. Operating conditions: 11 kV\({}_{\text{PP}}\), 17.5 kHz, and 1.5 slm. Figure 12: Conductivity evolution versus the treatment time in the case of (**a**) DDW and (**b**) PBS (see text for discussion on the extrapolation curve between 20 and 30 min).