Originally Posted by
jereece
Thanks Richard for your very interesting and educational response. Just a few comments.
1. You said "Since your bottles were inside and I assume were not exposed to sunlight". That is not correct. As I said in my post "I cut the top off of each bottle so that the water depth was about 5 inches and set them outside on my deck table. The air temperature was about 95 and there was a good 7 hours of direct sunlight, so the water in each bottle got to about 110F."
It could help if I could read! Sorry to miss that the first time. Well, sunlight will certainly burn through the chlorine and that explains why the levels dropped, even with CYA from the pool. Thanks for clearing that up so I don't have yet another mystery to solve!
2. You said "The amount of CO2 outgassing outweighs the pH buffering effect...". Does this mean that I am better off running at lower alkalinity? If so, why do most people recommend running 100-120 alkalinity? What alkalinity would you recommend me run at?
This is an excellent question and I, too, have been questioning why the standard recommendations for alkalinity are as high as they are. I can see that when using Tri-Chlor tablets, you pretty much have to have higher alkalinity to help slow down the rather rapid decrease in pH from those tablets, but when using liquid chlorine or bleach, I don't see the need as much. When people are seeing a rise in pH due to outgassing of carbon dioxide, then I would suggest running at a lower TA and a normal pH (7.4 or so) and avoid low pH. However, there are some people who report rising pH and regular adding of acid with little or no change in alkalinity over time so that implies other sources of "base" entering into the pool in which case lowering your TA won't help and could even hurt a bit (by allowing larger pH swings). So the jury is still out on exactly what to do and I hesitate to give a catch-all recommendation.
I would say that it doesn't hurt to "try" a low 80 TA and normal 7.4-7.5 pH or even in more extreme situations try a 60-70 TA. If one finds that they are no longer fighting pH rise (and adding acid regularly), then they've solved a problem. If this doesn't help, and especially if it makes things worse, then they can always go back up to more "normal" TA levels. By the way, the recommended range from NSPI is 80-120, not 100-120. In fact, the 100-120 is when you use Tri-Chlor. When you use liquid chlorine or bleach, the recommendation is 80-100.
3. You said "I found it rather surprising that your pH level returned so quickly after adding the chlorine even with the pool water sample that still measured >5 ppm." I agree and do not really understand this.
Well, now that I know you had things exposed to sunlight, there is an explanation for this. When the chlorine gets broken down by sunlight, this is an acidic process so the pH will come back down to where it was, at least when you get to the original chlorine levels. That is probably what happened in your case. When you have very high chlorine levels exposed to sunlight, you will burn through a lot of chlorine quickly, even with CYA, so your pH will drop fairly rapidly.
4. Is it possible that my very low CH concentration has any affect on all of this?
Unlikely since the CH only figures into water balance for plaster/grout preservation and doesn't affect (much) the chlorine and CYA equations. There is a minor effect from TDS but it is very, very small, and even smaller than usual when CYA is present.
5. As I have allowed my alkalinity to get lower and lower, I am consistently seeing that I don't have to adjust my pH as often. I am liking running at lower alkalinity. Are there any potential problems with running at low alkalinity and if not why doesn't everyone just run at low alkalinity?
There really aren't any problems until you get to very low alkalinity at which point you will eventually see that adding small amounts of chemicals to your pool (chlorine or anything else) cause the pH to jump more wildly than you might like. Though you might not get much of a long-term drift, the initial jump in pH when you add chlorine and it's subsequent drop might cause a swing in pH that is too large during the day for swimmers. I wouldn't worry about this until you get to really low aklainity below 50 or so. Just experiment to get to a point you are happy with. If enough people find this is worthwhile, I may create some more graphs showing the amount of pH buffering at different alkalinity levels.
People with plaster/grout pools should probably raise their CH somewhat when they lower their alkalinity or they should raise their pH even more to 7.6-7.7 or so. This is also an area of controversy since it seems that you have to be WAY out of water balance before corrosion or scaling become a problem.
6. You said "The effect of alkalinity buffering pH so that the pH will not move as much when acid or base is added is true for a closed system." Obviously my pool is not a closed system, so I thought you were saying this does not apply. However you go on to say "So higher alkalinity (pH buffering) reduces the pH swing, but does not change actual acid/base demand." But I am not sure that I saw that in my samples as pH increased in most in samples with the most alkalinity. How can this be?
I'm sorry to have confused you. The pool is somewhat of a hybrid open and closed system. It can be treated as a closed system for some things where the equilibria are reached rather quickly and where changes to the system are slow. For other things, or over a longer period of time, it must be treated as an open system and some things may be out of equilibrium. The carbon dioxide dissolved in the pool is one such out-of-equilibrium situation which means it is extremely difficult to predict exactly what will happen and how quickly. Some factors can be calculated since the "rates" are a function of concentration, but without knowing the precise rate reactions (are they linear, or quadratic, etc.) and without knowing the physical factors and formulas, it is nearly impossible to determine absolute rates. Besides, the rate changes over time as splashing, wind, etc. change. Nevertheless, I have put together graphs showing the relative rates for factors that ARE known, but they need to be taken with a grain of salt and are only "accurate" if everything else is held constant.
The pH rise that was higher in the samples with higher alkalinity was due to the outgassing of carbon dioxide which is because your bottles were open to the air. The change showed up much more over a period of a day or more. So in this sense, your pool is open to the air as well and is therefore over days an open system and would also show a rise in pH over time (except the pool's depth and volume to surface area ratio, exposure to wind, splashing, etc. may be different than your bottles).
The greater buffering effect of alkalinity does not stop the pH from rising -- it only slows it down. The outgassing effect is more powerful and therefore wins out. For example, between an alkalinity of 75 and 150, the outgassing rate of carbon dioxide (at pH 7.5) is 2.3 times faster (all else equal). However, the pH buffering effect of the alkalinity is 2.0 so the carbon dioxide wins out in this case. Since the 2.3 vs. 2.0 doesn't seem like much of a difference, it may very well be that the carbon dioxide outgas rate may vary by the square of the concentration instead of being linear (as I assumed) so perhaps the outgas rate increases by over 4 times. I don't have clear answers about this, but regardless of the actual amount of pH rise, what is very clear is that at an alkalinity of 75 you will be adding half the acid (or less) than you have to add at an alkalinity of 150. So, independent of exactly how much the pH rises, the lower alkalinity definitely has you add less acid on a regular basis. At some point, you will get your alkalinity down and pH up to a balance where the amount of acid you need to add will be small and dwarfed by the other changes that happen in your pool and cause pH to go up and down (rain is acidic, bather sweat is perhaps not neutral, liquid chlorine has extra base in it, etc.).
Again I really appreciate the education.
Thanks, and I'm learning too.
Jim
Re: Does Alkalinity Cause pH To Increase Faster?
Reply With Quote
Bookmarks