Does Alkalinity Cause pH To Increase Faster?

[chem geek]

Jim
If I may add to this discussion. I guess this is over simplifying but the way I look at ALK and pH they are tied together like 2 balls with a rubber strap between them. If you have a high ALK the pH will slowly want to rise to equalize to the ALK. If you have a low ALK the pH will fall to equalize as well. The pool is trying to reach a point where you have equilibrium between ALK and pH. When the pool reaches this point the pH will not move as much and we have achieved the best buffering capacity for our pool.

Steve,

What you say is true, but the equilibrium point is far outside where most people run their pools. At a TA of 80, you have to have your pH at almost 8.4 before there is balance between dissolved CO2 in the water and in the air. Alternatively, at a pH of 7.5, you would have to have a TA of 14 to achieve this same balance.

Fortunately, the rate of carbon dioxide moving out of the pool is rather slow so you can be quite a bit out of equilibrium and not notice the pH rise (i.e. it is small and dwarfed by other factors that change the pH of your pool water). I'd love to know whether the kinetic rates for outgassing are based directly on the concentration of dissolved vs. external CO2 or if there is some power relationship. Henry's Law and other equilibrium laws don't give the kinetics and what we are interested in is how fast, not the fact that it's out of equilibrium.

The good news is that this discussion has got people thinking about lower alkalinity levels since many many people seem to be fighting rising pH. Some have had success by lowering alkalinity (a lot -- to 60-70, for example), but some have not (though I don't think they tried lowering that far). One example of success at 80 with a pH of 7.5 is described in this thread. If only everyone had the same success story.

Richard