Richard, this all gets quite complicated, and I'm sure that I don't understand it as well as you do. Here is my understanding (please correct as appropriate, thanks)
Virtually everyone in this industry believes the same misconceptions that continue to get taught, at least initially. When I first read about these things, I initially believed them as well until I either read something countering it at The Pool Forum or figured something out that contradicted what was written or read about inconsistencies with what actually happens in pools (and spas). The important thing is to ask questions and try and work together to figure out a theory that is consistent with observation (and scientific principles) to the greatest extent possible.
The equation shows that H
2 (molecular hydrogen gas) is produced. The H
2 can be produced in two ways.
1) H
2O <> H
2 + O. The O can then combine with a variety of other atoms or molecules, including H
3O
+ (hydronium ion) as shown here:
O + H
3O
+ <> H
2O + OH
− . This reduces the effective hydrogen ion concentration and increases the hydroxide ion concentration.
2) H
3O
+<> H
2 + OH. This also decreases the effective hydrogen ion concentration and increases the OH (hydroxide or hydroxyl ion) concentration
The above is not correct. There is no oxygen radical formed when hydrogen is produced. The reaction for the creation of hydrogen at the cathode (negative plate) is the following as I showed in the link I gave earlier:
2H2O + 2e- --> H2(g) + 2OH-
You are forgetting that in electrolysis which occurs in the SWG that there is a transfer of electrons. Now in practice you could have one transfer at a time producing unstable intermediates such as a hydrogen radical as follows:
H2O + e- --> H• + OH-
or more likely the following which is equivalent:
H+ + e- --> H•
or the following which represents how hydrogen ion is usually associated with water:
H3O+ + e- --> H• + H2O
and then two of these hydrogen radicals combine to produce hydrogen gas as follows:
2H• --> H2(g)
for the net result that I showed.
The bottom line is that the hydrogen gas is lost and the reaction decreases the effective hydrogen ion concentration and increases the hydroxide ion concentration. That's the basic definition of a pH rise.
The above is true but only accounts for the production of chlorine in the SWG. It does not account for what happens when that chlorine then gets used up by breakdown in sunlight or oxidation of ammonia. It also doesn't account for what is happening at the other plate since the production of chlorine gas that then dissolves in water is acidic, though the overall net effect is a pH rise from production in the SWG (but again, this ignores the consumption of chlorine that is acidic).
The oxygen can also oxidize ammonia.
The oxidation of ammonia is given by this equation
4NH
4+ + 2OH
− + 8O
2 <> 6H
3O
+ + 4NO
3-
4 ammonium + 2 hydroxide + 8 Oxygen2 <> 6 hydronium + 4 nitrate ions.
The above does not occur to any great extent. If it did, dilute ammonia solutions in water would decompose and this does not happen quickly.
Oxygen can also form O
2 (molecular oxygen gas) and O
3 (Ozone).
I'm not sure what you mean by the above when you say "oxygen" can form from oxygen gas or ozone. If you mean that oxygen in the atmosphere can become dissolved oxygen in the water, that is true, but again it doesn't react quickly with ammonia. Also, the amount of ozone is essentially negligible unless you have an ozonator.
Chlorine gas has a very low pH; therefore any chlorine gas that does not dissolve will result in a higher pH than if it all dissolved.
If what you mean by this is that any chlorine gas that escapes and does not dissolve in the water will result in a net pH rise, then yes that is true and is one of the two things I said can explain a rise in pH.
I suppose that if the Carbonate CO
32− or the Bicarbonate HCO
3− was converted into carbonic acid and then into CO2 (carbon dioxide) and it outgassed then it could raise the pH; but I'm not so sure that is actually happening. Is that the theory? Is the aeration causing that much carbon dioxide to outgas? How does aeration cause the carbonates or bicarbonates to become carbonic acid?
Yes, this isn't just theory, but proven in practice and seen in studies. Pools are intentionally over-carbonated. The equilibrium amount of TA in water at a pH of 7.5 exposed to normal amounts of carbon dioxide in the air would be about 9 ppm (with no CYA in the water). The reason the TA is higher in pools is to provide a pH buffer, but this makes the pools out-of-equilibrium with the air so carbon dioxide will outgas from the pool into the air. This does not happen quickly, but it does happen faster at higher TA levels and at lower pH (because both of these will increase the amount of dissolved carbon dioxide or carbonic acid relative to bicarbonate and carbonate) and it happens faster when there is better air/water mixing such as with aeration or churning of the water surface (this increases the kinetics of net carbon dioxide transfer from water to the air). In fact, you can read more about the procedure for lowering TA and how it works chemically here that depends on this very effect.
Also, we have seen in pool after pool how a high TA level will lead to a much faster rise in pH (or slower drop, if using an acidic source of chlorine) than a low TA level.
As for how carbon dioxide outgassing shifts the equilibrium, note the following equations relating these carbonate species:
CO2(aq) <--> CO2(g) ..... going to the right is outgassing of carbon dioxide
H2CO3 <--> CO2(aq) + H2O ..... going to the right is from carbonic acid to dissolved carbon dioxide
HCO3- + H+ <--> H2CO3 ..... going to the right is from bicarbonate ion to carbonic acid
CO32- + H+ <--> HCO3- ..... going to the right is from carbonate ion to bicarbonate ion
As the first reaction goes from left to right for outgassing, the reactions below go from left to right as well since the concentration of the species on the left of one reaction is getting reduced so the reaction below it moves to the right to produce more to keep things in balance. You can see that going from left to right in the above reactions consumes hydrogen ions so that the pH rises. Technically, the TA doesn't change because the hydrogen ion counts negatively towards TA and the bicarbonate or carbonate are paired up with the hydrogen ion (i.e. both drop by the same amount for each equation).
NaOCl (Sodium hypochlorite) when poured into the pool will cause available hydrogen ions to replace the sodium creating HOCl (Hypochlorous acid). This will reduce the available hydrogen ions causing the pH to rise. When the HOCl dissociates it gives back the hydrogen ion thus lowering the pH back to where it was. So the net effect is neutral. However, the liquid chlorine also contains sodium hydroxide which causes some pH rise.
This is correct though at a pH near 7.5 only about half of the hypochlorite ions become hypochlorous acid. Hypochlorite sources of chlorine are essentially pH neutral except for the small amount of "extra lye". The pH rise in my pool at a rate of 0.1 per month is due to a combination of this "extra lye" which theoretically would have the pH rise by 0.1 every 2 months plus some amount of carbon dioxide outgassing. My use of a pool cover significantly lowers the outgassing compared to a pool without a pool cover.
Note that you talk about HOCl dissociating to give back the hydrogen ion and thus lowering the pH, but this doesn't just happen with hypochlorite sources of chlorine, but ANY source of chlorine including that from an SWG.
The reason a lower pH gives more available HOCl than a higher pH is that there are more hydrogen ions available to combine with the hypochlorite ions.
Though this is technically true, when CYA is present it acts as an HOCl buffer so HOCl does not vary as much with pH as the traditional industry graphs show (while hypochlorite ion, OCl- actually varies more). This post shows the traditional industry graph which is false unless there is no CYA present and the true graph when CYA is present.
The sources you are apparently quoting don't account for that and in fact hardly any do.
Note: What I posted was written mainly by me and is not a wholesale cut and paste of anyone else's work. It is based on my general understanding of what is happening.
OK.
