Originally Posted by JDM
Without going and looking it up (lazy), what difference does the CO2 make for this test? I'd never read this anywhere else yet.
The level of CO2 dissolved in water is one factor in the acidity. CO2 forms carbonic acid, and this lowers pH as the water acidifies.
For example, when high-tech planted tank aquarists bubble CO2 into their tanks, it always lowers the pH. Similarly, in low-tech natural planted tanks the fluctuation in CO2 during each 24 hour period, what we call the diurnal pH fluctuation, is due to the CO2 increasing during darkness and then decreasing during daylight when plants are using it.
In the case of tap water, it may contain CO2. Some tap water is high, some very low. This lowers the pH, so if you test tap water straight from the tap, you may get a pH of say 7.2, but if you out-gas the CO2 either by shaking briskly or letting it sit out overnight you may find the pH is 7.6 or 7.8. These numbers are just for example, every situation can be different.
CO2 will dissipate out of the water due to the gas exchange as oxygen enters.