Best Guess Table

[chem geek]

Thanks,

I lowered the alkalinity even more to 80ppm, and now the pH will hold steady at 7.2. Correct me if I am wrong, but it seems to me that there may be a CO2 reaction taking place at the water surface/atmosphere interface that sets the pH dependent on the alkalinity. (We also have 400ppm calcium hardness.)

In any case I'm running the pool at 7.2 pH, 1.5-2ppm chlorine, ~30ppm CYA and 80ppm alkalinity. Clear blue all the way.

Well, now I'm a bit confused. It sounds like you added acid to lower the alkalinity and doing so will also lower your pH as well. However, you probably didn't change anything that was your original cause for the increasing pH over time.

Yes, you are correct that there may be CO2 being released at the water surface, but this normally doesn't happen (at least not very quickly) unless 1) your pH is low, 2) your alkalinity is extremely high, and/or 3) you have water features that aerate your water. Your alkalinity is not high so let's look at something else. Just so you know, when the alkalinity (carbonate) in the water is released as CO2 into the air, it raises the pH without changing the alkalinity (as measured by the test kit). I know that sounds weird, but it's the way the chemistry works. When you add acid to restore the pH back to where it was, then that lowers the alkalinity, so the NET of this combination of outgassing of CO2 and adding acid is a lowering of alkalinity with no change in pH. If I've hopelessly confused you, I apologize.

If you have been cycling up in pH, then adding acid (which lowers pH and alkalinity) and getting to lower pH, how has your alkalinity been moving -- measuring it at the same pH at two points in time? If you seem to be bouncing between a low pH, lower alkalinity and high pH, higher alkalinity, with a net lowering of alkalinity over time, then the CO2 outgassing is most likely what's going on in which case you should see if your water is getting aerated (check your pool jets, see if you have any fountains, water slides, a very windy day, etc.). If, on the other hand, you are bouncing in pH while the alkalinity stays the same (when measured at the same pH), then something in your pool is making the pH rise or you are adding something to your pool to make it rise (though liquid chlorine or bleach makes the pH rise when you add it, the pH goes back down again as the chlorine gets used up).

At this point, it sounds like you are stable so "if it ain't broke, don't fix it" is probably the rule to follow. Personally, I have found an alkalinity level of 80 to be a bit on the low side (I'm usually at 100) and the pH swings more than I would like as a result. I also run my pool at a pH of around 7.4-7.5. If you decide to increase your alkalinity, you might want to do so next time you dilute your pool water since your calcium hardness is on the high side.

Richard

[chem geek]

"aquarium", I must apologize. Though an alkalinity of 100 and pH of 7.4-7.5 works fine for my pool, your pool may be very different and need different parameters. What I do know is that you will outgas more CO2 if you have lower pH and/or higher alkalinity so if you want to keep your pH closer to 7.2, then you will need to keep your alkalinity down so that the amount of aeration in your pool (which may not easily be changed) has the pH rise slowly enough for your tastes.

This whole issue has given me the thought of another graph I can construct that will show the relative CO2 outgas rates as a function of alkalinity and pH. I cannot give absolute rates since that's a physical aeration process that will be different for everyone's pool, but at least I can show the relative effects. I'll put this in the China Shop under my Pool Water Chemistry thread.

Richard

[aquarium]

No, I aerated the water and simultaneously added acid to intentionally push down the alkalinity, which has resulted in the water stabilizing at the lower pH as well.

I'm not sure about the chemistry at the water surface, but all other things being equal, it appears that the recommended alkalinity range of 80-120ppm will yield the recommended pH range of 7.2-7.8 in a fixed relationship. 80ppm yields 7.2 pH, 120ppm yields 7.8 pH, etc. (at least in my pool )

EDIT:

Though an alkalinity of 100 and pH of 7.4-7.5 works fine for my pool, your pool may be very different and need different parameters.

Which is consistent with my experience of the relationship between pH and alkalinity through that range. END EDIT

BTW, I want to run the pool close to 7.2 pH in order to get the most benefit from the bleach. You said:

At a pH of 7.5, FC of 3.0 and CYA of 30.0 (and TDS of 550, Temp of 80ºF), the HOCl is 0.045. At a pH of 7.2, with everything else the same, the HOCl is 0.052 which is not a huge change, but is still about a 15% change.

Is there a reason NOT to run the pool at 7.2 pH?

[chem geek]

No, I aerated the water and simultaneously added acid to intentionally push down the alkalinity, which has resulted in the water stabilizing at the lower pH as well.
:
:
BTW, I want to run the pool close to 7.2 pH in order to get the most benefit from the bleach.
:
Is there a reason NOT to run the pool at 7.2 pH?

OK, at least now I understand what happened. As for running the pool at the lower pH to get more chlorine benefit, that is true, but it's a relatively small benefit compared to what you can simply get by adding more chlorine. Usually you set your pH for other factors including not stinging the eyes, not risking rusting of metals, etc.

I speculate that the biggest factor in the lower pH will be a tendency to outgas more CO2, even though you've lowered your alkalinity. A pH of 7.2 combined with an alkalinity of 80 (assuming CYA is 30, though this isn't a huge factor) produces exactly the same rate of outgassing as a pH of 7.4 combined with an alkalinity of 120 which is the same as pH 7.5 with alk. 150. You can wander over to the Pool Water Chemistry thread in The China Shop to see a graph of this (it's the last graph in the first post in that thread). I don't want to post it here for risk of scaring people into banning me from ever posting outside The China Shop again

Richard