A Theory About Rising pH in SWG Pools

[chem geek]

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.

Yeah, I had forgotten about the effect of curing concrete in new pools. This process produces calcium hydroxide, Ca(OH)2, and is highly alkaline. In fact, for every 1 ppm of calcium hardness added to your pool during the curing process, the pH would rise by 0.08 and is equivalent (for pH) to adding 10.7 ounces of Borax to your pool.

Richard

[Katy-Texas]

I add almost 1 USG per week to our 26,000 USG SW pool here in Houston every week in summer and 1/2 that in winter. Pool is currently 92 F and have 2 HP pump circ 20 hours per day of which about 1/3 (guesstimate) goes into spa and then to pool via a cascading face to the spa, about a 1 foot drop.

Silly question, muriatic acid = HCl equals H & Cl, when HCl is used, does it liberate the Cl, I side benefit is that it chlorinates pool, if so then I'll add my HCl after party as a sort of ph & shock all in one.

[waterbear]

no,, when the HCL is added to the pool it forms hydrogen ions and cloride ions. The chloride ions combine with sodium ions in the water to form salt.

[PatL34]

I defer to Richard in the chemistry involved in a SWCG operation, but I did a writeup in the thread mentioned in my signature, which was simplified for the newbies with SWCGs.

I do know that the pH tended to rise before and after I installed the SWCG 8 years ago. The rise is not pronounced as it used to be, as I was maintaining a 5 - 6 ppm chlorine range from algae that occured in the old Diamond Brite plaster.

With the new plaster which is still curing, I am adding a pint of muriatic acid every three days and holding 7.4 - 7.6 pH with hardly any bather load.

One has to remember after all the chemistry that Richard and I have mentioned whether by bleach or SWCG, the net result is an increase in caustic soda (NaOH), which has to be converted back to salt with muriatic acid.

This is my interpretation and Richard can chip in and correct if necessary.

Pat

[chem geek]

Pat,

You do a very good job explaining the chemistry in simpler terms for people to understand. Thank you for doing that. There are a couple of corrections to what you wrote in your thread, but they don't detract from the main points too much except that it is not true that the production AND consumption are an alkaline (basic) process because they are not -- the net of the two (production and consumption) are neutral in pH.

The products from electrolysis are only Hydrogen (H2) gas and Chlorine (Cl2) gas. There is no sodium (Na+) produced nor consumed by this process. There are also no hydroxide ions (OH-) directly produced from the process, but because hydrogen ions (H+) are consumed and due to the balance of the two with water, the net effect is as if hydroxide ions are created (from water).

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

Now compare the above with introducing liquid chlorine into the water:

NaOCl + H2O --> Na+ + HOCl + OH-

You can see that both process result in the same "HOCl + OH-" and therefore have the same slightly alkaline (basic) effect on pH. The effect is not strong because HOCl is a weak acid that partly counteracts the OH- (so some product is OCl- + H2O).

If we just stopped here, then one would conclude that both the SWCG process and adding liquid chlorine cause a rise in pH and though that is true, it is not the whole story. The chlorine doesn't just keep growing and growing in concentration in the pool. Instead, it gets used up. There are several processes that use up chlorine and I list some of these in this post, but the net result is that these processes do the following:

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

Note that the processes that consume the chlorine (breakdown from sunlight and oxidation of ammonia and nitrogenous organics) produce hydrogen ion (from HOCl or no pH change from OCl-) that exactly counteracts what was formed during the production of the chlorine. The net result of producing chlorine from a salt cell and then having it consumed is as follows:

2Cl- + 4H2O --> 2H2(g) + 2HOCl + 2OH-
2HOCl --> O2(g) + 2H+ + 2Cl-
----------------------------------------------
2H2O --> 2H2(g) + O2(g)

6Cl- + 12H2O --> 6H2(g) + 6HOCl + 6OH-
4NH3 + 6HOCl -->2N2(g) + 6H+ + 6Cl- + 6H2O
----------------------------------------------
4NH3 --> 6H2(g) + 2N2(g) -or equivalently- 2NH3 --> 3H2(g) + N2(g)

So you can see that the normal process of producing chlorine through an SWCG results in no net change in pH. Instead, Hydrogen and Oxygen gases are produced (when chlorine is broken down by sunlight) or Hydrogen and Nitrogen gases are produced (when chlorine is consumed by ammonia or nitrogenous organics) and I believe other organics can also produce carbon dioxide since the oxidation is similar to burning.

With liquid chlorine, the net result is somewhat similar except that the net result is the production of salt (sodium Na+ and chloride Cl-) along with oxygen gas from the breakdown of chlorine by sunlight or nitrogen gas when chlorine is consumed by ammonia and nitrogenous organics. So using liquid chlorine increases TDS (specifically salt) over time while an SWCG does not (the chloride ion consumed in production and turned into chlorine simply goes back into chloride ion during chlorine consumption).

Richard

[PatL34]

Pat,

You do a very good job explaining the chemistry in simpler terms for people to understand. Thank you for doing that. There are a couple of corrections to what you wrote in your thread, but they don't detract from the main points too much except that it is not true that the production AND consumption are an alkaline (basic) process because they are not -- the net of the two (production and consumption) are neutral in pH.

The statement above corroborates what SWCG/liner pool owners have been saying in that they see very little change in pH over close to a week. I would have to say that what SWCG/plaster pool owners like me are seeing is a continuous leaching of Ca(OH)² from the plaster, and hence continuous addition of acid to neutralize this. This appears to clear up an anomaly about the steady pH in SWCG/liner pools. Thanks.

The products from electrolysis are only Hydrogen (H2) gas and Chlorine (Cl2) gas. There is no sodium (Na+) produced nor consumed by this process. There are also no hydroxide ions (OH-) directly produced from the process, but because hydrogen ions (H+) are consumed and due to the balance of the two with water, the net effect is as if hydroxide ions are created (from water).

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

Now compare the above with introducing liquid chlorine into the water:

NaOCl + H2O --> Na+ + HOCl + OH-

You can see that both process result in the same "HOCl + OH-" and therefore have the same slightly alkaline (basic) effect on pH. The effect is not strong because HOCl is a weak acid that partly counteracts the OH- (so some product is OCl- + H2O).

If we just stopped here, then one would conclude that both the SWCG process and adding liquid chlorine cause a rise in pH and though that is true, it is not the whole story. The chlorine doesn't just keep growing and growing in concentration in the pool. Instead, it gets used up. There are several processes that use up chlorine and I list some of these in this post, but the net result is that these processes do the following:

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

Note that the processes that consume the chlorine (breakdown from sunlight and oxidation of ammonia and nitrogenous organics) produce hydrogen ion (from HOCl or no pH change from OCl-) that exactly counteracts what was formed during the production of the chlorine. The net result of producing chlorine from a salt cell and then having it consumed is as follows:

2Cl- + 4H2O --> 2H2(g) + 2HOCl + 2OH-
2HOCl --> O2(g) + 2H+ + 2Cl-
----------------------------------------------
2H2O --> 2H2(g) + O2(g)

6Cl- + 12H2O --> 6H2(g) + 6HOCl + 6OH-
4NH3 + 6HOCl -->2N2(g) + 6H+ + 6Cl- + 6H2O
----------------------------------------------
4NH3 --> 6H2(g) + 2N2(g) -or equivalently- 2NH3 --> 3H2(g) + N2(g)

So you can see that the normal process of producing chlorine through an SWCG results in no net change in pH. Instead, Hydrogen and Oxygen gases are produced (when chlorine is broken down by sunlight) or Hydrogen and Nitrogen gases are produced (when chlorine is consumed by ammonia or nitrogenous organics) and I believe other organics can also produce carbon dioxide since the oxidation is similar to burning.

With liquid chlorine, the net result is somewhat similar except that the net result is the production of salt (sodium Na+ and chloride Cl-) along with oxygen gas from the breakdown of chlorine by sunlight or nitrogen gas when chlorine is consumed by ammonia and nitrogenous organics. So using liquid chlorine increases TDS (specifically salt) over time while an SWCG does not (the chloride ion consumed in production and turned into chlorine simply goes back into chloride ion during chlorine consumption).

Richard

I will revise my writeup accordingly to reflect what we discussed.

Pat

[KurtV]

Pat,
Are you (and others) seeing constantly rising calcium hardness? I am not, yet I still have the huge acid demand.

[chem geek]

Kurt makes a good point. We have not gotten to the bottom of this significant rise in pH some users experience, though we are making progress. As far as I can figure out so far, the following are possible sources of pH rise:

1) High TA and/or Low pH and/or high aeration. This refers to my CO2 outgassing chart though this chart is very qualitative due to variance in aeration. This process has been proven through several users successfully eliminating their pH rise by lowering their alkalinity to 80 or even lower (or their pH to 7.2 in one case). This process can be distinguished from the others by the continual drop in TA over time (after adding acid to restore pH). NOTE: It's techically carbonate alkalinity that drives the CO2 outgassing, not TA, so adjustments should be made for CYA levels or Borates (Boric Acid; sodium tetraborate) that are used. My chart assumes no borates and 30 ppm CYA.

2) Curing of plaster (technically, the concrete in plaster/gunite). When enough Calcium Hydroxide has been produced from the curing process to raise the Calcium Hardness level by 10 ppm (which is probably the minimum detectable amount in test kits), this causes a rise (with a TA of 100) of 0.91 in pH so that 43 ounces of Muriatic Acid are needed (for a 16,000 gallon pool) to restore back to a 7.5 pH. So, the rise in CH will be relatively slow, though if you see it rise about 10 ppm after cumulatively adding 5-6 cups of acid over time (per 16,000 gallons), then this may be the cause of the pH rise. I also would expect that this curing process wouldn't last (in significant amounts) much more than 1 year at the most and that most of the curing would occur in the first 3-6 months. Don't forget that backwashing DE and sand filters as well as splash-out will dilute the CH (and everything else) over time so may hide the rise in CH (at a CH of 300, a loss of 10 is a 3.3% dilution or 333 gallons per 10,000 gallons in a pool). On the other hand, evaporation and refill will tend to increase CH unless the fill water has no CH.

3) SWCG systems that do not fully dissolve their generated chlorine gas. For every 1 ppm of chlorine that is generated and does not dissolve, this causes a 0.15 rise in pH which requires about 8 ounces (1 cup) of acid to restore pH (in a 16,000 gallon pool). My hunch is that after #2 above for newer plaster/gunite pools, that this is the main reason for rising pH in SWCG pools, but I have no proof of this (yet). One experiment to try is to use a lower power setting and run for a longer period of time (to make up for the lower power setting in terms of total chlorine produced in a day). In theory, the lower power setting should produce smaller bubbles and amounts of chlorine per volume of water flow and should have more chlorine dissolve in the water vs. outgassing to cause a rise in pH.

4) Other factors. These include fill water that is high in pH. Bather sweat is probably not very alkaline and rain is normally acidic so these are not likely to increase the pH.

Kurt's data showed that at least part of the pH rise was due to the outgassing of CO2 since his TA kept dropping. Kurt, if you are willing, continue to let it drop to about 50 (see this post for an example of where that worked for someone else. That still provides some buffering, but would virtually slow to a crawl the CO2 outgassing (at a pH of 7.5 or above). Someone who has an SWCG system and experiences rising pH can also try the experiment I mention in #3 above to lower your power setting and increase your time on your SWCG. Let us know what happens if you do this! Oh yes, and Kurt, how old is your pool if it's plaster/gunite?

Kurt, if you do conclude that your specific source of rising pH is the outgassing of CO2, then you might consider adding 50 ppm Borates (sodium tetraborate) to act as a substitute buffer replacing part of your carbonate buffer system. That would let you run with much, much lower TA readings. Before we get to that point, however, we need to see if you can get to a low TA level where the pH rise slows down significantly.

Richard

[PatL34]

Kurt,

As I am still curing my Diamond Brite plaster, my last CH reading was 270 ppm.

I filled the pool entirely with softened water, to help the curing, and tried as much as I could to keep the pH to between 7.2 - 7.6 using roughly a pint of acid a day.

During the first month I kept the SWCG off line, and used trichlor and bleach to maintain about 5 - 7 ppm Cl. Once I got to about 90 ppm CYA, I started using bleach only. I have not added any baking soda, letting the TA build up of its own accord.

As of now The PoolPilot SWCG is on at a power level of 1 and 50% output. So far no hint of algae, and I can only ascribe this to the plasterer who really knew what he was doing, and to what I am doing.

I will be taking a full set of figures today, and let you know what I have.

Pat