# What is exactly the pH value of distilled water?

This was asked by one of my students in my class. I know it is the pH of 7, but others say it can be 6.55 pH

• And I say it's 6.2. What others say is not an argument. Atmospheric gases are dissolved in distilled water (mainly oxygen, nitrogen, argon, carbon dioxide). Due to dissolved carbon dioxide, distilled water has a weakly acid medium and its pH is typically between 6 and 7. It vastly depends on how it's stored and what the ambient conditions are. Dec 6 '17 at 22:13
• Does it depend on the temperature?
– Lynn
Dec 6 '17 at 22:16
• Of course it does chemistry.stackexchange.com/questions/39608/… Dec 6 '17 at 22:18
• Yep, it does, and quite significantly. The solubility of $\ce{CO2}$ decreases with the increase of the water temperature (A page ripped off from CRC Handbook which you can use to support your explanation to the students). Dec 6 '17 at 22:20
• So for pure distilled water at $25\ \mathrm{^\circ C}$ the pH would be 7.00. But the $K_\mathrm w$ is a function of temperature and hence the pH of pure water will vary with temperature too. The problem is that ordinary distilled water will contain some dissolved $\ce{CO2}$ which will lower the pH. You have to take extra precautions not to get $\ce{CO2}$ dissolved in the distilled water.
– MaxW
Dec 6 '17 at 23:12