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