I just finished doing some testing to see how long it takes for a tank sample of water, sitting out in the open air, to reach a stable pH, and how much CO2 is in a sample of distilled water with only bicarbonates in it for alkalinity, after it has sat out in the air for a considerable time. Obviously I am trying to convince myself that the two pH measurement method of determining how much CO2 we have in our tanks is reasonably accurate. My data is graphed on the attachment. I didn't succeed in convincing myself that our CO2 measurements are any good.
The distilled water, which had a KH of 4.5, ended up with only about 1 ppm of CO2 and it was stable at that amount. But, we have assumed that it would have 3 to 4 ppm of CO2. I wouldn't believe that at all, but the KH/pH/CO2 equation for the tank water says it had only about 13 ppm even though the drop in pH from room air condition to tank condition was about 1.0. But, if the room air sample actually did have about 1 ppm of CO2 in it, the 1.0 drop would mean about 10 ppm of CO2. Have I confused everyone, or is my plight clear?
The distilled water, which had a KH of 4.5, ended up with only about 1 ppm of CO2 and it was stable at that amount. But, we have assumed that it would have 3 to 4 ppm of CO2. I wouldn't believe that at all, but the KH/pH/CO2 equation for the tank water says it had only about 13 ppm even though the drop in pH from room air condition to tank condition was about 1.0. But, if the room air sample actually did have about 1 ppm of CO2 in it, the 1.0 drop would mean about 10 ppm of CO2. Have I confused everyone, or is my plight clear?