Why is Ph climbing?

[chem geek]

The "slug method" does not work. If you add acid you lower both the pH and the alkalinity so if you are trying to just lower the alkalinity by itself with no change in pH, then you can't do so by just adding acid. Those that have claimed that the slug method works or "could work" claim that you outgas CO2 when you add the acid, and technically in the localized region where they are dumping the acid the pH is in fact so low that it does force the carbonate in that area to more rapidly outgas, but this area is so small that even if all of the carbonate was removed, it would have a negligible effect. By the time the acid is dispersed through more of the pool so that a larger area could be affected, the acid effect is weakened so that the increase in outgassing isn't very high and is so slow anyway that the total release of CO2 is small. Somebody somewhere probably carefully added acid in such a way that it spread near the surface and did in fact slightly lower alkalinity more than normal and hence this procedure was "born", but it is extremely ineffective (especially compared to Ben's sure-fired approach).

When you inject CO2 for "pH control" you are implying that there is some sort of pH drift (demand) already that you are trying to counteract. Since injecting CO2 makes the pH drop, the implication is that you have something else in the aquarium that is trying to make the pH rise (i.e. is basic or alkaline). So the combination of adding CO2, which just makes the pH drop, plus this basic source, causes the pH to be stable (or to drop less) and for the alkalinity to increase.

It is true that by outgassing CO2 you are removing carbonate from the system which is the primary component of the buffering system, but the very nature of the pH shift that is caused by this outgassing increases the buffer "start point" by having less hydrogen ion that needs to be "neutralized". Think of hyrdrogen ion (which is measured by pH) as being "anti-alkalinity" or "negative alkalinity". When you outgas CO2 you lose the alkalinity from carbonate but also lose the "negative alkalinity" from hydrogen ion so there is no net change in alkalinity (and the pH has risen). [EDIT] If you don't like the concept of "negative alkalinity", then consider that H+ and OH- are always in equilibrium (with water) so that when one goes up the other goes down. So when the pH rises, you get more OH- and clearly OH- is alkaline and increases alkalinity.

TA = 2*[CO3(2-)] + [HCO3-] + [OH-] - a(H+)

Even ignoring the a(H+) for the moment, the first two carbonate-like species decline with outgassing, but the pH rises so the OH- increases by exactly the same amount (and H+ decreases -- [OH-]-[H+] is a "net" excess of alkalinity from pH; the reason a(H+) activity is used is that the titration pH test measures the activity of H+, not its concentration, though they are pretty close to the same in pool water). It is as if each CO2 that leaves is the same as removing H2CO3 (CO2 + H2O) from the system and this takes away 2H+ and one CO3(2-), the latter counting double for alkalinity so there is no net change.
[END-EDIT]

Believe me, this is one of the most counter-intuitive things I have encountered in chemistry and it still doesn't "feel" right to me, but it seems to be true as measured by the titration alkalinity test.

Richard