rising ph levels

[chem geek]

I wasn't suggesting that outgassing is caused by pH, I'm suggesting that the alkalinity of the water will -set- the pH due to the natural interaction between the water and air as CO2 in both reaches a balance point.
Tom

Tom,

The problem is that in a pool the extra carbonate (alkalinity) that is put into the pool as a buffer puts the pool way out of balance with respect to CO2 in the air. At standard NSPI middle ranges of pH 7.5, TA 100, CYA 40, CH 300, there is almost 10 times as much dissolved CO2 in the pool water as there should be for equilibrium with air. Yes, that means that if you were to fully aerate your pool, you would lose some of the carbonate in the pool and the pH would rise. My calculations show that full aeration and achieving equilibrium would happen after about 14% of the total carbonate (no TA change since TA is also a function of pH) in the pool were outgassed and the pH rises to 8.4

So there is always this natural tendency for pH to rise with all pools that aren't running at 8+ pH (Ben wrote about having pools operate at high pH, but that's another discussion...). So with this pool situation, we simply can't set a balance point that is at equilibrium because we won't be running the pool at 8+ pH. So we instead have to figure out the sweet spot between the two competing factors -- low TA large pH swing vs. high TA lower pH swing but increasing pH "push". However, my gut tells me that there's some other significant source of base being added to this pool and that's what we need to figure out.

[EDIT]The graph CO2.png (which I think you've seen before in some of my other posts) shows the relative outgas rates of CO2 at various levels of alkalinity and pH. The relative scale is 0 at equilibrium, 1 when there is twice as much dissolved CO2 in the water as in the air, 2 when there is three times as much dissolved CO2 in water vs. air, etc. So a relative number of 20 outgasses twice as fast as one of 10. The "Limit" line is somewhat arbritrary but seems to be close to where many people complain of an upward pH drift that seems excessive.[END-EDIT]

Richard