Also, I need to clarify the issue of pKa WRT carbon dioxide:
Carbonic acid is a diprotic acid, so:
H2CO3(aq) + H2O <----> H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O <----> H3O+(aq) + CO3-2(aq)
The first reaction has a pKa of 6.352, while the second has a pKa of 10.329. Notice the bicarbonate ion can act as an acid (2nd reaction) or a base (reverse of 1st reaction) - so a solution made from only the bicarbonate ion (baking soda) will be a good buffer - it can absorb both acids and bases. The pH of such a solution would be midway between the two pKa's, or 8.34 - that number should look familiar to reef/saltwater folks.
In the case of freshwater tanks, our pH is lower - meaning we have a mixture of both bicarbonate ion and carbonic acid. This is the first reaction. If we have an equal mole mixture of the two, then the pH = pKa, or 6.352.
You can calculate the ratio of the two ions for any pH within +/- 1 unit of 6.352 using the Henderson-Hasselbach equation:
pH = pKa + log ([HCO3-]/[H2CO3])
Now the only thing left is to decide how the dissolved CO2 concentration determines the carbonic acid concentration - again, these are equilibrium reactions from my other post.
To sum up then, the pH tells you the ratio of bicarbonate to carbonic acid - or the ratio of kH to CO2 - so if you measure kH, you can use the table or the equation to determine CO2.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.