Algae, TA or what?

[chem geek]

Ben,

Regarding this post, I want to comment on some of the things that you wrote which I'll quote below.

I can observe that, in my experience with multiple large commercial pools managed with sodium hypochlorite (bleach) feed systems, I universally found that bleach fed pools 'wanted' to be at a pH of 7.6 or higher. I have no analytical explanation for this. But I can tell you that I discovered that if I let the pH float upwards, it usually -- but not always -- stopped before 8.0. I also found that if I operated the pools between 7.6 and 8.0 it took FAR less acid, than if I tried to operate the same pool between 7. 2 and 7.6. (Note to Chem Geek: yes, I tried it both ways on the SAME pools )

What you observed is completely consistent with the underlying chemistry. See this chart which shows how far out-of-equilibrium pool water is in terms of the excess carbon dioxide there is in the water. Pools are intentionally over-carbonated in order to provide a pH buffer and to add carbonates to protect plaster surfaces (when calcium is also added). You can see that at lower TA and higher pH the water is not as far out-of-equilibrium. The rate of carbon dioxide outgassing is related to this number, though Wojtowicz has shown that it is proportional to the square of the TA whereas my table doesn't include that effect (the table shows more of a linear effect with TA).

So basically a pool will tend to rise in pH, but will do so more quickly at low pH and will slow down as the pH rises. It doesn't technically stop until it reaches equilibrium, but in practice it becomes so slow relative to other pH changes that it seems to be stable. This is because the carbonate alkalinity portion of TA is a SOURCE or rising pH itself due to carbon dioxide outgassing. It is not true that a higher TA will produce a more stable pH unless one has some acid added to the pool such as using Trichlor (or even Dichlor which is net acidic when accounting for chlorine usage/consumption since that is an acidic process). Instead, a higher TA will result in a faster pH rise and a higher ultimate equilibrium pH.

The reason that it takes far less acid having the pH settle higher is that a higher pH results in less carbon dioxide in the water, all else equal. If you force a lower pH, you are forcing more of the carbonates in the water to be in the form of carbon dioxide and that will then outgas faster causing the pH to rise.

You have chosen to use a higher target pH as a way of reducing the (further) pH rise, but the other approach is to lower the TA level itself. This will also result in a lower rate of pH rise. Assuming a CYA level of 30 (since some of this measures as TA), a pH of 7.8 with a TA of 80 ppm would have about the same amount of carbon dioxide as a pool with a pH of 7.5 and a TA of 45 ppm.

On another related note: buffering has to do with how difficult it is to change the pH of a solution (your pool water in this case). Highly buffered water has highly stable pH . . . . IF the buffer in question is effective at that pH level.

Most buffers have ranges over which they are effective, and other ranges over which they are not so effective. Cyanuric acid is an effective buffer at LOW pool pH levels, below 7.4 as I recall. Borax is an effective buffer at HIGH pH levels, above 8.0, I think. Total alkalinity tests measure how much buffer you have, but not which levels it's effective at.

This is important, because if you have drifting pH levels like you do, it's your pool's buffer system that determines how hard you'll have to work to maintain your pH level. Absent aeration, sodium bicarbonate is an effective buffer in the 7.0 - 8.0 range where your pool lives. A pool with an effective bicarbonate based "Total Alkalinity" will require larger doses of acid, added less frequently, compared to a pool without that bicarbonate based buffer.

More or less, a well buffered pool will require that you adjust your pH less frequently than a poorly buffered pool. It won't really decrease how much acid you need, but it will change how often you have to add it!

It is not true that highly buffered water has a stable pH, even if the pH is in a good buffering range. The carbonates have TWO competing effects. The pH buffering effect where more is better AND the carbon dioxide outgassing effect where a higher TA is less stable pH (unless their are acid sources such as Trichlor). The latter effect dominates when the TA gets too high. As far as how high is "too high", this depends on the amount of water-air interchange so depends on aeration (i.e. waterfalls, spillovers, fountains, heavy splashing) so generally pools that are covered are more stable in pH even at higher TA levels.

As for effective ranges of buffers, you don't have that quite right. In this post, I give more details about the buffering capabilities of the carbonates, cyanuric acid, and the borates. The borates have more pH buffering capacity against a rise in pH, but they do buffer both ways. The borate buffering system gets stronger as the pH rises. The carbonates, on the other hand, are the opposite where they get stronger in buffering as the pH gets lower (by higher and lower I'm referring to the normal 7.0 to 8.0 pool pH range). The cyanuric acid buffer system is a bit uneven in its buffering in this range.

Total Alkalinity measures the buffer capacity in one direction only -- buffering against a drop in pH. This is why 50 ppm Borates only contributes 5 ppm to TA at a pH of 7.5. The borates have less buffer capacity against a drop in pH and have most of that capacity against a rise in pH. The measure for this would be Total Acidity, but that's not something in standard pool test kits and really isn't necessary to measure anyway since you can measure borates levels directly (the only real reason for the TA test is to indirectly measure the carbonates after adjusting for CYA and borates).

Your statement about a pool with an effective bicarbonate TA needing large doses of acid less frequently is not true if the source of rising pH is due to the TA itself. If we ignore the outgassing of carbon dioxide from higher TA levels, then it is true that greater pH buffering results in a slower change in pH from other sources (say, curing plaster or Trichlor) but requires more acid or base to move the pH back to where it started.

As for the amount of acid or base you need, it is true that the amount of buffering doesn't change that IF the buffer system itself isn't the source of pH change. So IF the rise in pH is due to carbon dioxide outgassing, then lowering the TA will result in less acid being needed over time and in a slower rise in pH. This is most especially true if the loss of pH buffering from the lower TA is made up for by using another pH buffer such as 50 ppm Borates.

Richard