Originally posted by TCLynx on AGC, June 3, 2010
Here is an interesting thing that most people may not know about tap water. When you first run the water from the tap, the pH will probably test out with a false low level. If you air out that water for a while and test the pH again, the pH will probably be much higher.
For instance, if I test the pH of my tap water right out of the faucet, it usually tests out at right around 7. If I bubble that water for a day and test the pH again, it will be up over 8. I have well water from a limestone aquifer, it is reasonable that the pH is high.
The reason for the false low pH reading right out of the faucet is due to carbon dioxide trapped and dissolved in the water. This acts as a weak acid in the water and will affect the pH reading until the carbon dioxide is outgassed from the water.
Knowing this bit of information can help sort out many issues ahead of time. When I first got into hydroponics I did not know this and was very confused about why adding my pH 7 tap water to the system was somehow causing the pH to rise above 7.
So, when you get ready to set up a system, it is a good idea to do a pH test of you tap water and then leave some to bubble overnight and then test the pH again just to have an idea what your source water is going to do in your system.