A Theory About Rising pH in SWG Pools

[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.

[dawndenise]

Still looking for data? Pool is not quite 3 months old and pH is quite comfortable at 7.5. I add muriatic acid only when the color gets a slight pinkish tinge to it, rather than waiting for it to go up to the next #. Am only in my 3rd gallon of MA since pool filling. My target is always 7.5, since that's where my pool has "said" it's comfortable. Who am I to argue?

SWG online for about 2-3 weeks now. Add acid about every 10 days thus far; no appreciable change since the SWG became operational. Waterfall on 12 hours per day.

TA=80, CH=220, CYA=45, FC=5-9 range, salt=3400; generator set at 60% for 12 hours; water temps 84-89.

Fill water is very high ph (over 8), low in calcium hardness (50-ish), about 70 in alkalinity.

I just can't bring myself to drop my FC into the 1-3 range, increasing my CYA into the 60-80 range, as per SWG's recommendations.

[chem geek]

Am only in my 3rd gallon of MA since pool filling.

Just wondering. Do you have any water features (waterfalls, etc.) or would you say that your pool doesn't have much aeration? What brand of SWG system are you using? Do you use a pool cover and is it opaque or a clear "solar" cover?

Since you aren't seeing the same sort of pH rise that others see in SWG pools, I'm just trying to figure out what's different about your pool.

Richard

[dawndenise]

Just wondering. Do you have any water features (waterfalls, etc.) or would you say that your pool doesn't have much aeration? What brand of SWG system are you using? Do you use a pool cover and is it opaque or a clear "solar" cover?

Since you aren't seeing the same sort of pH rise that others see in SWG pools, I'm just trying to figure out what's different about your pool.

Richard

I have a waterfall that's on 12 hours a day, so lots of aeration! I have an Aquarite SWG. No pool cover of any kind, lots and lots of UV on the pool - we're on Day 52 of air temps at 100 degrees or higher.

Also, although I WAS just on my 3rd gallon of muriatic, I've now broken the seal on the 4th gallon.

[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