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