A Theory About Rising pH in SWG Pools

[KurtV]

Richard,
I hadn't intended this thread to focus too specifically on my situation but here's the last three weeks or so of my testing (CYA 30 ppm and calcium hardness 220 ppm throughout this period, daytime water temperature between 89 and 92 deg F, about 2800 ppm salt but no SWG):

Date pH Alk. Acid addition
7/26 7.8 100 24 oz.
7/27 7.4 100
7/28 7.8 100 16 oz.
7/29 7.7 100 16 oz.
7/30 7.2 90 32 oz. (before testing)
7/31 Vacation
8/1 Vacation 16 oz.
8/2 Vacation
8/3 Vacation 16 oz.
8/4 7.8 90 28 oz.
8/5 7.6 80 16 oz.
8/6 7.6 80 16 oz.
8/7 7.4 80 8 oz.
8/8 7.6 70 16 oz.
8/9 7.6 70 16 oz.
8/10 7.5 70 16 oz.
8/11 7.3 70 8 oz.
8/12 7.4 70 16 oz.
8/13 7.6 70 16 oz.
8/14 Work trip
8/15 7.6 70 16 oz.
8/16 7.6 70 24 oz.

Sorry about the format, couldn't get it any better than this.

[Phillbo]

3 days ago .. TA = 95. I add acid once a week and the pH is normaly around 7.8 ish when tested weekly. I don't test every time. just add based on past experience.

[DavidD]

Kurt,

Just for comparison & discussion, I have a waterfall and two spa jets (lots of bubbles). I pretty much turn the water fall on and the spa jets always put out some bubbles any time the pump is running. Both features can be independently shut off but we rarely do. I have three returns that are always pointed up as the wife likes the “ripples”. PH is consistently in the 7.5 range and Alk around 110. I may have added 16 oz. of muratic acid since April and I would believe that my pool is at least in the mid to upper realm of high aeration. 16K Fiberglass in ground pool, CH is 150, CYA 40 and FC stays between 2 and 5. I do have 3200 ppm of Salt, no SWC, 1 HP Superpump & DE filter.

Dave

[KurtV]

David,
As that doesn't support my theory at all, we'll have to throw it out as a bogus data point.

(Hey, it's just a theory.)

[chem geek]

Well Kurt's data shows that it is definitely the outgassing of carbon dioxide that is going on, at least initially. If the SWCG system were somehow introducing base into the pool, then that would increase pH and TA such that adding acid would get you right back where you started with pH and TA. Instead, Kurt's data shows a steady decline in TA at the same "target" pH.

Interestingly, Kurt should be seeing a slow down in the amount of pH rise and acid demand as his TA is lowered, but I don't see that from the data. He is also now in the "green" region though still out of equilibrium with over 5 times as much carbon dioxide dissolved in the water as is found in air. If he reaches a state where his TA does not seem to be lowering (and it looks like this may now be the case, or close to it), but he still needs to add acid, then something in the SWCG system is indeed adding base (and was earlier too, but was masked by the CO2 outgassing).

There are several ways that an SWCG system can add base. The simplest is having the generated chlorine gas not dissolve completely into the water since the conversion of chlorine gas into HOCl is a highly acidic process that partly compensates for the highly basic process of hydrogen gas production. The difficulty in detecting this is that you will find bubbles of hydrogen gas flowing from the salt cell into your pool and you can't easily tell the difference between hydrogen gas and chlorine gas except possibly a smell of "fresh" chlorine.

Great data collection Kurt! Keep it up.

Richard

[mas985]

I thought the rising PH was due to the chemical reactions of the SWG as indicated here: http://www.goldlinecontrols.com/Elec...orination.aspx

The hydroxide ions on the cathod increase the pool water PH.

As for my experience, I have no water features at all and my PH can go from 7.2 to 7.8 in less than a week.

One other thing not mentioned which could have a significant effect is that fill water, such as mine, can be quite basic. Mine is over 7.8 in PH.

[waterbear]

You all might be interested with the thread I started in the china shop called "the great tetraborate experiment". I have a SWG, many water features, and since I have added 50 ppm borates to my water I have been exerpiencing less problems with pH rising as quickly. The experiment is only in it's third week but so far has indicated that the extra buffering system introduced into the water by the borax does help stabilize the pH and might require less acid over the long haul.

[chem geek]

The experiment is only in it's third week but so far has indicated that the extra buffering system introduced into the water by the borax does help stabilize the pH and might require less acid over the long haul.

Evan (waterbear),

I agree that the borax helps stabilize the pH, but it won't change how much acid you need to add. Only finding the source of the "extra base" and reducing that reaction will reduce the amount of acid you need to add. We just don't know yet how much is due to carbon dioxide outgassing (which causes TA to drop when you add acid to maintain pH) and how much is due to inefficient SWCG production with outgassing of chlorine gas or some other process.

Anyway, we'll eventually sort this all out -- or not!

Richard

[chem geek]

I thought the rising PH was due to the chemical reactions of the SWG as indicated here: http://www.goldlinecontrols.com/Elec...orination.aspx

You are correct that the generation of hypochlorous acid from the SWCG is a basic (increasing pH) reaction and is equivalent to adding sodium hypochlorite to your pool, except without the sodium (chloride ion is removed instead so there is no net charge difference). However, the chlorine gets used up mostly through the breakdown from sunlight (UV) and some from the disinfection and oxidation of organics and ammonia. The net equation for the entire process is first

2Cl- + 2H2O --> Cl2(g) + H2(g) + 2OH- --> HOCl + Cl- + H2(g) + OH-
so the net of this is
Cl- + 2H2O --> HOCl + H2(g) + OH-

which is the basic (increasing pH) process you are referring to, though HOCl is a weak acid so this is weakly basic overall. Then the using up of chlorine produces the following:

2HOCl + (UV) --> O2(g) + 2H+ + 2Cl-
-or- 3HOCl + 2NH3 --> N2(g) + 3H2O + 3H+ + 3Cl-

These are weakly acidic processes since HOCl is a weak acid so using it up results in the following reaction to keep HOCl and OCl- in balance:

OCl- + H+ --> HOCl

The net reaction for the production and destruction of chlorine is:

2H2O --> 2H2(g) + O2(g)
-or-
2NH3 + 3H2O --> N2(g) + 3H2(g)

Details of all of this may be found at this thread.

This is why using chlorine bleach with a pH of 11 does not cause a rise in pH over time (nor using chlorinating liquid with a pH of 13 though there is some extra base added in this case for stability so this will increase pH a little bit). This is also why using Di-Chlor (1% solution is 6.8) in fact lowers the pH more than one would think and why Tri-Chlor (1% solution is pH 2.8) is even worse (more acidic) that it first appears.

Now if you were to build up combined chlorine such as chlorinated organic compounds that didn't fully oxidize, then yes you would lose the "acid" part of the balance and rise in pH, but I assume your CC is staying near 0.

mas985, I'm curious. What is your current TA? If it's not unusually high, then indeed something in the SWCG systems would seem to be generating more base than it should. Is your SWCG in clear plastic so that the plates are visible to you? If so, can you see gas being generated vigorously at both plates or is it mostly just at one plate (hydrogen) therefore indicating an efficient dissolving of chlorine gas in water. If you find gas bubbles on both plates and that they seem to stream off of both plates without dissolving, then the salt cell is not operating very well in dissolving the chlorine gas into the water. Some of the gas on the chlorine side could be oxygen gas (a competing reaction), but mostly it should be chlorine gas if designed correctly. When oxygen gas is produced, there is no net change in pH which is why I think the culprit is undissolved chlorine gas.

There is also an increase in outgassing of chlorine gas in salt pools, but with the high CYA and relatively low 3-5 ppm chlorine levels, the rise in pH over a week shouldn't be as high as what you and others are seeing (it's about a 0.1 rise in pH over a week). Now hot tubs are an entirely different matter where there is lots of aeration and the temperature is a lot hotter, but they also are typically covered most of the time. Lots of variables, lots of questions, and we may never find the answers, but we can try.

Richard

[mas985]

mas985, I'm curious. What is your current TA?

100 to 110 ppm

If it's not unusually high, then indeed something in the SWCG systems would seem to be generating more base than it should. Is your SWCG in clear plastic so that the plates are visible to you?

Unfortunately not

One more thing besides the fill water being high in PH is that the pool is only 1 year old which may still affect the PH.