pH rise with SWG

[Dirk]

Sorry to belabor, but this has been really bugging me, the seemingly contradictory data from various TFP experts. Chem Geek clearly states "The hydrogen gas bubbles out of the returns and aerates the water and is the main cause of pH rise in SWG pools." I see those very bubbles. They are definitely from the SWG. I have accurately measured the pH rise rate, both before and after using an SWG. The rate change was instant, to the day I turned on my SWG. The instant increase was not attributable to my TA level nor a seasonal change because those two things we're virtually identical the day before and the day after I turned on my SWG. I'm not saying that those are not contributing to my ph rise, I'm sure they are, I am saying I have very good proof that the SWG also contributes to pH rise, all on its own, and significantly. Sorry Matt, I gotta go with Richard on this one.

More to the point, what I'd really like to see is TFP put this to bed once and for all. I know you all work together to establish the TFP guidelines, methodology and instruction sets. It's what makes this site so great, and the info here so reliable. Clear, agreed upon, consistent data. Can you not collectively come to terms with this SWG/ph-rise issue and either confirm or deny it, as one? And if not, I guess that's fine. It won't affect how much MA my pool requires, one way or the other.

And if I'm speaking out of turn, you can scratch this post, no problem. I'm not trying to stir the pot, just looking for the correct answer.

 

[Steve_in_C]

Re: Stenner liquid chlorine pump - How to choose and install

Sorry to belabor, but this has been really bugging me, the seemingly contradictory data from various TFP experts. Chem Geek clearly states "The hydrogen gas bubbles out of the returns and aerates the water and is the main cause of pH rise in SWG pools." I see those very bubbles. They are definitely from the SWG. I have accurately measured the pH rise rate, both before and after using an SWG. The rate change was instant, to the day I turned on my SWG. The instant increase was not attributable to my TA level nor a seasonal change because those two things we're virtually identical the day before and the day after I turned on my SWG. I'm not saying that those are not contributing to my ph rise, I'm sure they are, I am saying I have very good proof that the SWG also contributes to pH rise, all on its own, and significantly. Sorry Matt, I gotta go with Richard on this one.

More to the point, what I'd really like to see is TFP put this to bed once and for all. I know you all work together to establish the TFP guidelines, methodology and instruction sets. It's what makes this site so great, and the info here so reliable. Clear, agreed upon, consistent data. Can you not collectively come to terms with this SWG/ph-rise issue and either confirm or deny it, as one? And if not, I guess that's fine. It won't affect how much MA my pool requires, one way or the other.

And if I'm speaking out of turn, you can scratch this post, no problem. I'm not trying to stir the pot, just looking for the correct answer.

Richard's 3rd paragraph:

"When the hypochlorous acid gets used up by oxidizing an organic or ammonia or killing a pathogen (bacteria, virus, algae), it gets converted back to chloride ion. The oxidation produces carbon dioxide and/or nitrogen gasses (and water) depending on what the chlorine oxidizes. This oxidation process produces hydrochloric acid and is acidic and exactly compensates for the alkalinity of hypochlorite sources of chlorine (bleach, chlorinating liquid, Cal-Hypo, Lithium hypochlorite) so the addition of hypochlorite or the production of chlorine in an SWG plus the later usage of that chlorine is a pH neutral process."

Looks like the H2 off gassing may encourage more CO2 off gassing. And with the longer run times required by SWG, there is more water exposed to the air for off gassing to occur. Perhaps these are contributors.

But it looks like the off gassing of CO2 would finally reach some equilibrium and that pH change would level out. We are missing something. Science starts out with an observation, then hypothesis and test. Many here are observing a pH increase with SWG but not all. Perhaps this is a "deep end" discussion.

 

[Dirk]

Re: Stenner liquid chlorine pump - How to choose and install

Perhaps this is a "deep end" discussion.

Probably, I'll stop when you do!

But how I read Richard's post, and that paragraph in particular, actually supports Matt's take, in part. They're not at complete odds. I think Matt and Richard are both saying that the SWG process itself, the chemical conversion of salt water to chlorine, is pH neutral. I can accept that. But it seems Matt stops there and declares the use of a SWG ph neutral. Richard explains that the process creates bubbles, which aerate the water and cause pH rise. This is certainly logical, and fits with what I've proven in my pool. I've seen aeration (bubbles) raise my pH. Tested and documented. And I can see bubbles coming out of the return. That explains the SWG pH rise well enough for me.

I don't know what you mean by reaching equilibrium. The bubbles are constant while the SWG is producing. So would be the pH rise they cause, I should think. It won't stop over time.

I cannot explain why some don't see this issue. Perhaps that relates back to TA and how that varies from pool to pool, along with the material the pools are constructed from, and climate, etc. Lots of variables, for sure.

And please excuse me for seemingly putting words in Matt's or Richard's mouth. I'm only trying to explain how I'm interpreting them...

 

[PoolguyinCT]

With a salt system, the ph is not being pounded into the basement by 3” trichlor tabs.

 

[Steve_in_C]

When I referred to the equilibrium, I was referring to the dissolved CO2 in the water. The bicarbonate buffer system is the same system in our bodies.

CO2(g) + H20 <--> H3O+ + HCO3-

Le Chatelier's principle tells us that if CO2 is removed (offgasses) then everything shifts to the left. The bicarb is continuously getting shifted, consumed, replaced, etc. but should reach some steady state (equilibrium).

 

[Silver_2000]

as a data point my SWG has required less than 20 oz of acid all year...

so its NOT all SWG
and we have spa overflow running every day at least 8 hours a day

 

[JoyfulNoise]

How much CO2 gas do you believe your system is liberating?

Let me give you some numbers -

To raise the pH of 10,000 gallons of pool water from 7.5 to 7.8 (CYA 30ppm, TA 100ppm, zero borates), you need to liberate about 0.3lbs of CO2 gas. 1lb of CO2 gas occupies 243 liters or 64 gallons of volumetric space at standard atmospheric pressure and temperature; so 0.3 lbs of CO2 would be equivalent to 19.2 gallons.

So, even if all of those bubbles coming out of your return line are 100% CO2 (which they are most certainly not), do you really think you have 19 gallons of gas coming out of your pipes??

Look, I don't disagree with Richard and I have had conversations at length with him on the subject (feel free to e-mail him yourself) but I am fairly confident that he would agree that the aeration caused by the SWG is not a major contributor to pH rise. Yes, the aeration does cause some pH rise but you would be hard pressed to see it without some very specific equipment capable of measuring small changes in pH. The water coming out of the returns will generally have higher pH as is predicted by the electrolysis equations for the conversion of chloride to chlorine but, by and large, all of that pH rise will eventually go back down once the chlorine has oxidized organics or killed pathogens as those reaction are acidic (lower pH). Your pH goes up because your pool is an open chemical system to the atmosphere where the amount of dissolved CO2 in the water is several times larger than atmospheric CO2 levels. With such a huge difference in concentration, the outgassing of CO2 is easily the greatest source of source pH rise in your pool.

I know you won't do the experiment because I wouldn't expect anyone to waste $150 on a giant roll of bubble wrap BUT, if you covered your pool and adjusted your SWG to maintain the same FC/CYA ratio, you would see your acid demand go to almost zero. I did that very experiment 2 years ago and, when my pool was covered, the pH didn't change for weeks on end. The minute I removed there cover, the pH rose exactly as it had previously done (acid demand frequency of 7-10 days).

 

[Dirk]

Thanks, Matt. I have to admit, based on what it took for my three PVC aerators running 24hrs a day to move my pH (back when I was adjusting TA), it does seem unlikely (very) that those little bubbles streaming from my three returns about 3.5 hours a day could drive my pH substantially. But on the other hand, I was dosing my pool about 3 or 4 oz of MA a day before SWG, the day after SWG it was 6 or 7 oz. That is my recollection. I have the data in my Pool Math app, if it hangs onto it from that long ago. I'd just have to extract it, as I do remember that I noted the day I turned on the SWG. I'll try to get inspired to do that, but like I said, my pool needs much more MA now than it did, and that is my reality, regardless of the science. If that's TA and evaporation and whatever else, so be it.

 

[JamesW]

The real key is what pH and TA you try to maintain. The right combination reduces pH rise to basically zero.

The amount of CO2 is determined by the Carbonate Alkalinity and the pH. If the TA is kept low enough and the pH high enough, there will be no pH rise.

Also, keeping the CSI close to 0, but somewhat negative, reduces pH rise from plaster.

 

[Steve_in_C]

I agree. There are several buffer systems at play, hypochlorous acid, bicarbonate/carbonate, borate (in my case), etc. You must have chlorine, so you can't change the hypochlorous buffer much, you can allow your TA to go down ( decreasing the bicarbonate buffer), and borates are completely optional but add another buffer system. Then allow the pH to drift to the limits of what you will allow.

 

[JoyfulNoise]

I agree. There are several buffer systems at play, hypochlorous acid, bicarbonate/carbonate, borate (in my case), etc. You must have chlorine, so you can't change the hypochlorous buffer much, you can allow your TA to go down ( decreasing the bicarbonate buffer), and borates are completely optional but add another buffer system. Then allow the pH to drift to the limits of what you will allow.

The HOCl/OCl- buffering systems is actually quite weak and insignificant as the presence of cyanuric acid drops the overall active chlorine level to around 100ppb. However, the cyanurate buffering (pKa ~6.5) is important and contributes to the measured total alkalinity, ALK(cya) ~ 0.3 * [CYA]. The mix of buffering species is really all wrong for what’s considered appropriate pool pH (7.2-7.8) since all of the relevant pKa values are at least 1 pH unit away from 7.5. So, in terms of “equilibrium”, pool pH is an unstable equilibrium at best. The closest analogy would be that of trying to balance a basket ball on the tip of pool cue stick - constant correction is needed to keep the ball balanced as it is always being driven by gravity off the end of the cue stick.

If one wanted a more stable pH, then the phosphate buffering system would be the choice as one of the pKa values is right around 7.5. Problem there is that the presence of calcium and carbonate in fill water would cause phosphate scaling as most phosphate salts (aside from ammonium, potassium and sodium) are insoluble in water.

So, the moral of the story is that most pools need some form of frequent pH adjustment.

 

[Steve_in_C]

It's interesting that the hypochlorous acid buffer likely has the most suitable pKa but is not a significant buffer due to low concentrations. I'd like to take pool water and titrate over a wide pH range to see where all the buffered ranges are in real pool conditions. If I had a pH meter, I'd do it.

 

[Dirk]

I very much appreciate and respect the info you all have provided here. I did, and do, encourage this discussion. But to be honest, I've yet to hear the logical reason my pool's acid demand increased somewhere between 50 to 100% the day after I turned on my SWG for the first time. I didn't imagine it. No pucks were involved. Weather consistent. Water temp consistent. Bather load consistent. TA consistent. Testing methods consistent, etc. All exactly the same from before and after? Not likely, but very much "science experiment worthy," IMO. Sorry for being so dense about it, but I watched it happen in my pool. From liquid chlorine only, to SWG only, acid demand increased substantially. There is a lot of anecdotal evidence that this phenomenon occurs in a whole lot of other pools. Similar anecdotal evidence supports the notion that this phenomenon is not consistent in every pool. Both seem to be pretty widely accepted. So that says to me there are at least some variables involved that explain the discrepancy. Fine. What are they? And can they be manipulated? As near as I can gather, TA is one of the suspects. But I don't adjust TA, it's found it's own level. And I don't want to. So what else ya got?

Saying "SWGs don't affect pH." and "SWGs do affect pH." seem to both be valid claims, depending on who is making it, based on which pool they're basing the claim on. And maybe that's as close as we're going to get on this issue. Fascinating to watch it play out though, that's for sure...

So keep workin' on it, please.

 

[Catanzaro]

I very much appreciate and respect the info you all have provided here. I did, and do, encourage this discussion. But to be honest, I've yet to hear the logical reason my pool's acid demand increased somewhere between 50 to 100% the day after I turned on my SWG for the first time.

Dirk:

Where is your CYA now, compared to where the CYA was prior to the SWG being turned on? I am assuming that you either dosed your pool manually or used 3 " tablets. Below is the reason why I am asking this.

Pre-SWG in late August of 2017, I dosed my pool manually. CYA was always between 30-50 ppm. TA started at 110-120 and I have always had 4 rotating "circulators" breaking the surface of my water. Like clockwork, my acid demand was every 4 days (16 oz.). Slowly, I lowered by TA and it took about 18 months. Not until my TA reached 70 (even at 80 I had some rise, not as fast), did my PH stabilize once and for all. In order to get it down to this level I pumped water from the winter back into the pool in the spring. Problem solved.

Skip ahead to this year in the spring. Increased CYA back to 70 ppm as it was back down to about 50 ppm in the spring. Last year, I added enough stabilizer to bring up to 70, but then pool was closed and still used liquid chlorine as I had enough for the season. Only really used SWG for about one week at most, and then pool was put to rest for the season. Pumped all rain water off the cover and had some minor pin holes so removed a little more water than necessary. Filled up with city water with higher TA than of course 70, and it put me in the range of 100. Immediately I went out and purchased a gallon of MA expecting the PH rise automatically. This year, I also add 2 more circulators, totaling 6 in the pool.

Well guess what, no PH rise. I am guessing that the CYA with the SWG stabilized my PH. Nothing else has changed either. I would always test PH each morning after pump was running for at least 45 minutes and prior to any 12.5% additions. I would go from 7.5 to 7.8 every 4 days, again until TA hit right around 70. Even this year, I have not had any fluctuations with the SWG and CYA at 70. I recall reading on the forum that CYA also helps stabilize PH and it is not all about TA either.

- - - Updated - - -

This is why I am asking about CYA now vs. before the SWG. It appears that my PH stabilized even at higher TA levels with higher CYA. But then again, pools are funny and not all the same.

What I have posted only has to do with my own personal experience and not claiming that every pool will operate this way, or this is why your acid demand has increased.

 

[samotlietuvis]

Very normal.
I add 0.5 gallons of muriatic acid per week - nothing else.
My pump with SWG runs 11hr/day.
My pool sits at 94F now; adding muriatic acid once per week week is much more convenient than adding chlorine daily.

 

[JoyfulNoise]

Please re-read the advanced pool water chemistry sticky that Richard wrote many years ago - Pool Water Chemistry

Since everyone prefers to believe Richard, here is a direct quote -0

Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reactions:

At the anode (positive plate):
2Cl- --> Cl2(g) + 2e-

At the cathode (negative plate):
2H2O + 2e- --> H2(g) + 2OH-

which nets out to the following where the chlorine gas dissolves in water:

2H2O + 2Cl- --> Cl2(g) + H2(g) + 2OH-
Cl2(g) + H2O --> HOCl + H+ + Cl-
H+ + OH- --> H2O
----------------------------------------------
2H2O + Cl- --> HOCl + OH- + H2(g)

or equivalently

H2O + Cl- --> OCl- + H2(g)

Note that the products of HOCl and OH- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is "used up".

Notice what he said at the end - the process is basic, but not strongly so.... Also note what he says in the next sentence - So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses.. In other words, all the hydroxide (OH-) created in the process of creating chlorine gas will eventually be neutralized by acidic (hydrogen ion producing) reactions of sanitation and oxidation.

So, since the overall chemistry nets out to nothing more than oxygen gas and nitrogen gas (or nitrate) production and it has already been stated previously that your SWG can not possibly create enough aeration to drive your pH up as far as you think it does, it is then incumbent upon anyone making the claim that “SWGs create acid demand” to demonstrate something in the physics or chemistry of SWGs that would cause the excess demand you claim is coming from the SWG .... I’ll wait for your dissertation on the subject.

Also, hydrogen gas does not dissolve in water to form hydrogen ions and lower alkalinity. Look up Henry’s Law which is the chemical principal that shows the relationship between the partial pressure of a gas above the surface of solvent (in this case, water) and it’s concentration in the solvent. What you see is that the Henry’s Law constant for hydrogen gas is over 100 times smaller than that of chlorine gas therefore chlorine is much more soluble in water while hydrogen will tend to stay in a bubble and escape.

 

[Perpetual summer]

Last year pool was cya 80, ph 7.8, ta leveled out at 110 beginning of July, SWG on 8 hrs a day cya and ph never budged.
this year, cya 80, ph 7.4, ta 100, SWG on 8 hrs a day. Tried to get ph up to 7.6 to improve csi and it won’t budge. In this pool ph stays solid where ever it is.

 

[Dirk]

OK, I'm coming back down from orbit!! I have to correct myself. I did in fact add CYA to the pool right around the time I turned on the SWG. I don't know off hand how many days before, but I have it in my notes somewhere. So there was indeed a variable I did not account for, at some point in time before I fired up the SWG. Was that alone responsible for the increase in acid demand? I'll have to come back to this when I can spare the time to better plot out the time table, and share the data...

 

[Catanzaro]

OK, I'm coming back down from orbit!! I have to correct myself. I did in fact add CYA to the pool right around the time I turned on the SWG. I don't know off hand how many days before, but I have it in my notes somewhere. So there was indeed a variable I did not account for, at some point in time before I fired up the SWG. Was that alone responsible for the increase in acid demand? I'll have to come back to this when I can spare the time to better plot out the time table, and share the data...

Dirk:

If I am interpreting your reply, quite the opposite. My PH stabilized with higher CYA (of course I only increased it because of the conversion) and high TA. With lower CYA, no SWG and high TA, my PH was on the run. Higher CYA, with the SWG, high TA (at least for me), my PH became stable.

Was that alone responsible for the increase in acid demand?

I do not know. With all this, what exactly is your TA at the moment with your CYA? Maybe you need to mess around with Borates to try and stabilize the PH. Again, I do not really have too much experience with Borates, but this may help.

 

[Spa Owner]

Please re-read the advanced pool water chemistry sticky that Richard wrote many years ago - Pool Water Chemistry

Since everyone prefers to believe Richard, here is a direct quote -0



Notice what he said at the end - the process is basic, but not strongly so.... Also note what he says in the next sentence - So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses.. In other words, all the hydroxide (OH-) created in the process of creating chlorine gas will eventually be neutralized by acidic (hydrogen ion producing) reactions of sanitation and oxidation.

So, since the overall chemistry nets out to nothing more than oxygen gas and nitrogen gas (or nitrate) production and it has already been stated previously that your SWG can not possibly create enough aeration to drive your pH up as far as you think it does, it is then incumbent upon anyone making the claim that “SWGs create acid demand” to demonstrate something in the physics or chemistry of SWGs that would cause the excess demand you claim is coming from the SWG .... I’ll wait for your dissertation on the subject.

Also, hydrogen gas does not dissolve in water to form hydrogen ions and lower alkalinity. Look up Henry’s Law which is the chemical principal that shows the relationship between the partial pressure of a gas above the surface of solvent (in this case, water) and it’s concentration in the solvent. What you see is that the Henry’s Law constant for hydrogen gas is over 100 times smaller than that of chlorine gas therefore chlorine is much more soluble in water while hydrogen will tend to stay in a bubble and escape.

As a test there is a relatively easy test to do.

Lower the TA in your pool to around 30 to 40 mg/l. Do not do this in a concrete pool, but in vinyl or fiberglass should be fine.

Measure pH it will be around 7.2 give or take.

Turn on salt chlorinator.

Wait a week.

Measure pH it will still be around 7.2.

This happens consistently and regularly.

The chemistry behind this is:

- SWG don't increase pH they are pH neutral. There are in excess of 40 something reactions occurring and they balance out to neutral
- Pools with high TA have high CO2 this is not me saying this, or JoyfulNoise. This is an extract from a paper I was reading: Aeration of well waters whether from mechanical means ormere exposure to air alters the equilibrium by loss or absorption of CO2.
Larson, T. E., A. M. Buswell, Harvey F.Ludwig, and W. F. Langelier. "Calcium Carbonate Saturation Index andAlkalinity Interpretations [with Discussion]." Journal (American WaterWorks Association) 34, no. 11 (1942): 1667-684.http://www.jstor.org/stable/41233315.

This is a long way of saying pH is altered by amount of TA and as the water is aerated or exposed to air the gassing off increases the pH.

-A lower TA will assist in reducing CO2 gas bringing closer to equilibrium with atmosphere


The above works, has worked and continues to work on every pool I apply it too. The only caveat I would add is that the make up water needs to be considered as moving out of my immediate area may alter the make up water creating a situation I cannot predict in a forum setting.