what's the "ideal" ph?

[chem geek]

Chem Geek,
What are the units on the CO2 outgasing rate?

By the way, I find the information you and Waterbear share on this board extremely useful, keep up the good work!

Because the actual outgassing of CO2 is partly a chemical process (CO3(2-), HCO3-, H2CO3 and CO2(aq) all in equilibrium) and partly a physical process (dissolved CO2 physically moving from water out into the air [EDIT]that are not in equilibrium[END-EDIT]) I can't show an actual rate. However, for a constant physical process (that is, constant aeration conditions) the rate of outgassing is proportional to the concentration of dissolved CO2 (or combined H2CO3 and dissolved CO2 since that is the usual convention). So the numbers on the Y-axis of the chart are the unitless quantity "(actual - equil.)/equil." where "actual" is the actual concentration of H2CO3 & CO2(aq) and "equil." is the equilibrium amount of this same species based on the normal concentration of CO2(g) in the atmosphere.

So a Y-axis value of 0 would mean that the pool and the air are in equilibrium with no outgassing at all. A value of 1 would mean that there is twice as much dissolved CO2 in the water as there is during equilibrium. So a value of 15 means that there is 16 times as much CO2 in the water as there is when there is equilibrium. Since we don't know the actual rate of outgassing, the numbers are only useful as a relative scale. A value of 30 will outgas twice as fast as a value of 15. The reason for the "actual - equil." in the numerator is that there is both a forward outgassing and a reverse ingassing going on so "actual - equil." is proportional to the NET outgassing. The division by "equil." is arbitrary since any scaling factor could be used, but I chose this so that the numbers had some reasonable meaning with respect to how far things are out of equilibrium.

I'm sure that's more than you wanted to know!

Richard

[waterbear]

I'm sure that's more than you wanted to know!

Richard

I appreciate the technical explanations! They help me better understand what is occuring.

[chem geek]

I appreciate the technical explanations! They help me better understand what is occuring.

No problem. As you can tell, I enjoy this stuff.

The thread A Cheap Pool Cooler shows that two people with seemingly similar conditions can still get different levels of upward pH drift (and alkalinity lowering when acid is added to compensate for this pH drift). There is no question that all else equal, lower pH, higher alkalinity, and more aeration will cause more outgassing of CO2 which is why Ben's approach to lowering alkalinity tries to maximize two of these effects (obviously, making alkalinity higher so that you can lower it faster is, well, not the brightest thing to do). However, trying to predict the actual amount is really hard. There are formulas to approximate it, but they have so many parameters to put in that are really hard to measure that it's really not worth it -- it's much better to just get to "know your pool" and then you can use my graph as a relative guide using your own pool's behavior for a fixed set of conditions.

[gonefishin]

Thanks all

My ph has drifted up a little. Consequently my pool has seemed to lose that super soft feeling that it has previously had. I didn't know if this could have been related to my ph drifting up or perhaps I was just getting used to the water.

I thought I'd get some opinions before bringing the ph down. I'll see what happens.


thanks,
dan