Ultrapure water (Purified Water, PW) generally has a pH value that is nearly neutral, i.e., around 7.0 at 25°C. However, the pH value can vary slightly depending on various factors such as the environment and the temperature. If ultrapure water is exposed to the atmosphere, the actual value can be lower than the original pH value of 7.00. In this case, the pH value could drop towards 5.20 due to the formation of carbonic acid. This highlights the importance of strict control over water treatment processes and the quality of ultrapure water.