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

[medvampire]

Richard
Before we get too far into this subject and get mod slapped or at best moved to the china shop. I do tend to oversimplify my answers here in the general forums. I do agree with you on the equilibrium point is out of normal pool conditions by the models. The equilibrium point would be affected by other reactions going on in the pool we can’t predict. This is might be one of those situations where the math just doesn’t meet the real world and we have to adjust and measure the result. I do intend to lower my ALK a bit more to around 70 and monitor my acid usage.
Steve

[jereece]

Just an update. My TA has now dropped to 50 and I can't be happier. My pH upward drift has slowed to a crawl and the amount of acid to bring it back down is at least half what it used to be. I have checked my pH after each nightly addition of bleach (after an hour of recirc time) and see no increase in pH. I have been running a pH of 7.4 for a week now. I am much happier.

My plan for the rest of the season to let TA continue to trend down (if it does) and see what happens. I will particularly watch for pH swing problems and if so note at what TA that happens.

This is very interesting and I will keep you informed.

Thanks,
Jim

[chem geek]

Jim,

For the most extremes in pH, measure the pH just before you add chlorine, presumably at night. Then measure the pH about an hour after you've added the chlorine (with the pump running during that hour). This would be one of the extremes of pH. Another time to measure would be soon after a heavy bather load since that can sometimes lower pH. Another time would be after a rain storm (if it rains in the summer where you live).

Richard

[medvampire]

Jim
Thanks for the up date. It would be great if you do let us follow your restults and plot at the end of the year.
Steve

[jereece]

My Alkalinity is holding fairly steady at 50ppm. I have been taking notes about how adding bleach initially affects the pH. My pool is about 19,000 gallons. Adding one 3/4 gallon jug of 6% bleach adds about 2.4 ppm chlorine. I ran each test adding bleach after sunset and allowing to mix with the pool pump on for about 2 hours. I ran the tests several times with a starting pH range from 7.4 - 7.8.

In a nut shell, adding one jug of bleach to my pool at 50ppm Alk causes an initial pH increase of .2 or slightly less. I ran a few tests adding 2 jugs which would add 4.8 ppm chlorine to my pool. Adding 2 jugs resulted in an initial pH increase of .4 or slightly less. I say slightly less because some times the color did not look like it was fully developed to the mark on my Taylor comparitor. After 24 hours, my pH was back down to where it started before I added the bleach.

My in pool results are comparable to what I got in my jug tests. So I am convinced that, at least for my pool water, that running low alkalinity is the way to go. I am having to adjust my pH half as often as when I was running alkalinity around 100 and it takes about half as much acid to bring the pH down. So I am using less chemicals and from what I can tell I am getting better results.

If anyone can think of a reason to be concerned, please let me know. Otherwise I plan to continue to run low alkalinity.

Thanks,
Jim

[chem geek]

Jim,

I don't think there is any problem with you running at the lower alkalinity of 50. The main reason to have higher TA would be to prevent pH swings, but you are seeing less of that (due to less CO2 outgassing) so no problem there. You are technically in a more corrosive pool water balance state which normally means either increasing CH or pH to adjust for that, but I think you are fine so long as you don't plan on running at a low pH (much less than 7.4). This only applies to a plaster/gunite pool situation. If you are concerned, you can post a full set of numbers and I can be more specific.

Richard