## 4 Discussion
when non-refrigerated or stored above room temperature, light exposure accelerated the degradation of electrolyzed water, making the long-term storage of high-concentration electrolyzed water difficult. Since the pH of these high-concentration NW preparations is prone to change during storage, it is essential to store them in shaded and refrigerated condition (S\({}_{\text{LW}}\)) to use them safely in dental applications, and it is important to note the changes in pH value during storage. Ovidation reduction potential (ORP) is a key indicator of a solution's ability to oxidize microbial components and contribute to its disinfection capability. In this study, ORP values decreased over the storage period in standard-concentration NW preparations (NW\({}_{25}\), NW\({}_{35}\), and NW\({}_{50}\)), particularly under L\({}_{\text{RT}}\) conditions, indicating a reduction in oxidative power. Conversely, high-concentration NW preparations (NW\({}_{100}\), NW\({}_{200}\), NW\({}_{300}\), NW\({}_{500}\), and NW\({}_{1000}\)) exhibited increasing ORP values during storage, particularly under L\({}_{\text{RT}}\). This increase suggests that higher initial ACC levels result in greater oxidative potential. In high-concentration NW preparations, the increase in ORP facilitated the decomposition of HOCL, resulting in a significant decrease in ACC. Our results were similar to those of [PERSON] et al. (2020) [32], who used strongly acidic (pH 2.4, ORP 1140, 67 mV, ACC 32.87 mg/L) and slightly acidic (pH 5.6, ORP 850-900 mV, ACC 17.8 mg/L) electrolyzed waters and showed a negative correlation between pH and ORP. In a 101-day storage stability test conducted by [PERSON] et al. (2020) using highly concentrated neutral electrolyzed water (pH 6.96, ORP 916 mV, ACC 480 mg/L), it was reported that there was a significant decrease in pH and an increase in ORP when stored at room temperature compared with the refrigerated storage [33]. Although a detailed comparison is not possible due to differences in storage conditions (such as the material of the storage container, surface area in contact with air inside the container, and the frequency of opening and closing the container), a similar trend regarding the influence of storage temperature was observed in the results of this study. While ACC and pH are important indicators of microbicidal efficacy, ORP values, which indicate the oxidizing power of NW, are also useful as supplementary indicators and are recommended to be monitored to ensure the long-term efficacy of NW. ACC, representing the concentration of free available chlorine, is one of the most critical factors determining the disinfectan capacity of NW. Regarding ACC, we considered that NW should be at least 15 mg/L at the time of use to obtain a bacterial removal rate of 99.999% or higher, based on the results of previous studies that used similar test methods using the same bacteria [35]. In this study, standard-concentration NW preparations (NW\({}_{25}\), NW\({}_{35}\), and NW\({}_{50}\)) maintained sufficient ACC to retain high bactericidal efficacy over the 126-day storage period, particularly under S\({}_{\text{LW}}\). However, NW\({}_{25}\) experienced a significant decrease in ACC (to 5 mg/L) under L\({}_{\text{RT}}\), which compromised its bactericidal effectiveness, as evidenced by the detection of surviving bacteria. This result (bacteria removal rate: 99.8 \(\pm\) 0.1%) of NW\({}_{25}\) is comparable to the findings (99.8 \(\pm\) 0.9%) of our previous bactericidal efficacy test for fresh NW (ACC 10 mg/L) prepared by the same two-stage electrolysis method against the same bacteria [25]. In the study by [PERSON] et al. (2000) [36], which used a strongly acidic type (pH 2.3-2.4, ORP 1155-1160 mV, ACC 54-56 ppm), all ACC was deactivated within 5 days under L\({}_{\text{RT}}\) and 10 days under S\({}_{\text{RT}}\). However, NW\({}_{50}\) retained an ACC of approximately 15 mg/L, even after 126 days of storage under L\({}_{\text{RT}}\), and demonstrated high bactericidal efficacy. In a study by [PERSON] et al.