Please read to the end of this post since I think we are in agreement with the conclusions, but I want to make sure we are also in agreement with the reasons. I disagree with your first point, but agree with your second.
dschlic1 said:
The first is the loss of hydrogen gas. As pH is a measure of hydrogen ion activity in a solution, it stands to reason that if you lose hydrogen from a solution, the pH will rise.
The chemistry in SWG pools and in manually dosed pools is described in
this post where you can see that though the addition of either a hypochlorite source of chlorine or the net production of chlorine in an SWG cell causes the pH to rise, the usage or consumption of chlorine is an acidic process that causes the pH to drop and these effects cancel each other out.
You need to account for the fact that the chlorine gas that is produced in the SWG cell dissolves in water and that this is highly acidic, counteracting part of the loss of hydrogen at the other plate that produces hydrogen gas. When the hypochlorous acid then gets used up, this is also an acidic process. So it is incorrect to just say that hydrogen gas leaving the system is causing the pH to rise long-term. Essentially, the production of chlorine gas has such chlorine gas react with water to produce hydrochloric acid and hypochlorous acid, the former being a strong acid. You can think of this as creating acid out of one of the hydrogen from water while the hydroxyl (basic) portion gets bound to a chlorine atom as a weak acid so the net effect is uneven (i.e. acidic). When the hypochlorous acid (a weak acid) gets reduced by oxidizing something (or getting broken down by sunlight) it produces hydrochloric acid which is a strong acid so is net acidic. If you see something wrong in the chemistry in that post, please let me know.
Another way to look at this is to consider the electrolysis of water itself, if there were no chloride in the water. Both hydrogen gas and oxygen gas are produced yet the pH does not change in the water itself. You can't just look at the loss of hydrogen gas and assume that because hydrogen is being removed from the system that the pH must rise. Oxygen gas is also being removed so in effect it is a different form of water that is being removed, not hydrogen ions alone. As noted above, something similar happens in an SWG cell where the chlorine gas reacts with water to create acids. In fact, if you look at the net equations from the post linked to above, you see that accounting for chlorine breakdown in sunlight, the net result from an SWG is identical to the hydrolysis of water producing hydrogen and oxygen gas. For SWG-generated chlorine oxidizing ammonia (and related compounds), the net result is the production of hydrogen and nitrogen gas. For SWG-generated chlorine oxidizing urea, it is most likely a net result of hydrogen, nitrogen and carbon dioxide gasses (and some nitrates).
If these pH net neutral effects were not the case, then the rate of pH rise in both manually dosed and SWG pools would be far higher than is actually seen. The process of lowering the Total Alkalinity (TA) level to reduce the rate of pH rise was first figured out on The PoolForum and it works very well in manually dosed pools using hypochlorite sources of chlorine and works well, though not completely, with most SWG pools. This proves that at least part of the source of pH rise in SWG pools is the outgassing of carbon dioxide since that occurs faster at higher TA levels (and at lower pH).
dschlic1 said:
The second is the loss of chlorine. If the pool has any organic load (and what pool doesn't), then some chlorine is going to combine with that organic matter. If the organic matter is complex (ie solid not a gas), then that chlorine is lost to the system. It is removed either by the skimmer, filter or some other means.
You are right about any loss of chlorine other than by its reduction (i.e. it oxidizing another chemical or its breakdown from sunlight) will result in a net pH rise. So if combined chlorine is formed and the compound is not fully oxidized, then that is a loss of chlorine. If chlorine gas were to escape and not fully dissolve in the water, then that would also be a loss of chlorine. These will lead to a net pH rise. This has not been disputed.
Let's use specific examples with a starting pH of 7.5, TA of 80 ppm, CYA of 80 ppm and 1 ppm FC generation so 7 ppm FC per week. Adding a hypochlorite source of chlorine such as bleach or chlorinating liquid or Cal-Hypo or lithium hypochlorite, and ignoring any "excess lye" OR if the chlorine is generated by an SWG cell (and the chlorine gas fully dissolves in the water), the pH would rise to 7.8 after one week and 8.3 after the second week and 8.65 the third week if the chlorine built up and did not break down or oxidize any material. This clearly is not the case in real pools -- the FC level does not continue to climb over time and the pH does not climb this dramatically (assuming one has set their SWG ontime properly). If the added or generated chlorine were to completely get broken down by sunlight or to oxidize another substance, then the pH would not rise at all.
If some of the chlorine gas generated from the SWG does not dissolve in the water but instead outgasses, then that will lead to a pH rise. If 1 ppm out of the 7 ppm were to outgas while the rest of the chlorine either broke down in sunlight or oxidized another substance, the pH would rise to 7.58 after one week. If 2 ppm were to outgas, the pH would rise to 7.68 after one week. If 3 ppm were to outgas, the pH would rise to 7.78 after one week. If 4 ppm were to outgas, then the pH would rise to 7.91 after one week. So clearly this is one possible mechanism for pH rise. In outdoor pools, the chlorine demand may be 2 ppm FC per day or more so the pH rise would be twice as fast as indicated.
If some of the chlorine were to form combined chlorine and not get reduced by oxidizing a substance, then 1 ppm out of 7 would result in a pH rise to 7.54 after one week, 2 ppm to 7.58 after one week, 3 ppm to 7.63 after one week, etc. It is not as powerful an effect as having chlorine gas directly leave the pool.
Conclusion
The pH rise caused by either chlorine outgassing or chlorine not getting reduced also causes the TA to rise. So using Muriatic Acid (or any other strong acid) would be the appropriate countermeasure that would lower not only the pH but also the TA back to the original level. The use of carbon dioxide only works well if it is counteracting a loss (outgassing) of carbon dioxide. If that was your point, then I fully agree with that. Since some of the pH rise is likely due to carbon dioxide outgassing while some of the pH rise is due to other reasons, then a combination of carbon dioxide addition and acid addition would lead to long-term stability of both pH and TA. In effect, this is what people are saying -- that carbon dioxide addition does lower the pH, but that the TA rises over time so that acid addition is also required.
I just don't want people to keep thinking that the chemistry of manual chlorine addition using hypochlorite or SWG cell production inherently causes the pH to rise IF the chlorine is all dissolved in the water and fully used up through sunlight breakdown or oxidizing other substances. By understanding the other true sources of pH rise, then one can help mitigate these issues, such as by lowering the TA to reduce carbon dioxide outgassing. We haven't yet figured out how to have the chlorine not leave the pool through other means except by using borates to reduce the chlorine demand in the first place. If we had more data on pH rise in SWG pools as a function of pipe length and direction returns are pointed, then that might indicate some other factors to adjust in design or operation to help chlorine gas fully dissolve rather than escape (IF that is what is happening).
Richard
