Sodium Hypochlorite and Pool pH

[Tom ONeill]

In 2021 I converted our two HOA community swimming pools in Tucson, AZ (continuously in use throughout the year) from Trichlor erosion feeders to sodium hypochlorite using peristaltic pumps in an attempt to reduce overall costs and save water (otherwise periodically partially drained to contain the CYA level). Within a very short period of time I noticed that the pools' pH tended to be substantially higher, requiring daily additions of muriatic acid to keep it in check. During this I continually tested total alkalinity (TA) to maintain an appropriate LSI. I also noted that the TA generally stayed fairly stable. Also, following the conversion, I maintained an average 40 - 45 PPM CYA, adding some as it was depleted due to backwash, etc.

After doing as much "research" as I could online scouring pool related websites and forums for technical advice, including on Trouble Free Pool and the vast information from TFP's ChemGeek, all of the information indicated that although sodium hypochlorite may initially increase pool pH, that the effect of HOCL/OCl- either oxidizing other compounds or biologics and being decomposed by sunlight resulted in the evolution of hydrochloric acid and, therefore, that acidic result would counter the caustic nature of sodium hypochlorite, except for the small amount of excess sodium hydroxide that is normally added to sodium hypochlorite to help stabilize it in transit or storage. "OK", I said. But it never happened for our pools.

On further digging and many more visits and searches on TFP, I happened upon a posting by TFP Expert "JoyfulNoise" of June 16, 2017, who is apparently also in Tucson, AZ, in which he stated: “As for the reduction of chlorine by UV, HCl is not created. The largest contributor to UV photolysis of chlorine in water is the conversion of hypochlorite ion (OCl-) to chloride (Cl-) and oxygen gas. No acid generation occurs.” Wow, I thought. Why the totally reverse information of everything I have seen presented in every pool chemistry forum I could find?

When I looked up the effect of decomposition of hypochlorous acid (HOCl), I found that popping the O out HOCl yields hydrochloric acid (HCL). Simple, straightforward. Could these totally reverse views presented above be the result of "context"? Isolated chemical formulas versus the the practical reactions in the "chemical soup" that is a swimming pool? Multiple equilibrium relationships including that of chlorinated cyanuric acid?

Any help on this topic would be very much appreciated.

 

[mknauss]

Welcome to the forum.
You are missing the fact that Trichlor is very acidic. You stopped using it. You then needed to start using muriatic acid. The liquid chlorine you add is essentially pH neutral through its life cycle.

 

[PoolStored]

High TA and low pH drives pH rise...if that is what you are concerned about...

PH TA Relationship - Further Reading

www.troublefreepool.com

 

[HermanTX]

In 2021 I converted our two HOA community swimming pools in Tucson, AZ (continuously in use throughout the year) from Trichlor erosion feeders to sodium hypochlorite using peristaltic pumps in an attempt to reduce overall costs and save water (otherwise periodically partially drained to contain the CYA level). Within a very short period of time I noticed that the pools' pH tended to be substantially higher, requiring daily additions of muriatic acid to keep it in check.

If you look at what the trichlor was adding to the pool, then determine which of those components you need to add to the pool to maintain the balance you require when you switch to liquid chlorine. It seems you have accounted for the loss of CYA being added by Trichlor but not for the loss of acid that was also added by Trichlor.
In simple terms, 1 3" puck of trichlor (about 8 oz) will add the following to 10,000 & 30,000 gallons. You can then multiply that out for how many trichlor pucks you were using for each. So if using 5 pucks per volume per week multiply by 5 for a close approximation.
10,000 gallons ............ 30,000 gallons
Will raise FC by 5.5ppm .............. 1.8 ppm
Will raise CYA by 3.3ppm ............. 1.1 ppm
Will lower pH by 0.3 .............. 0.1
Will raise Salt by 4.5ppm ............. 1.5ppm

So with trichlor, your pH was kept in check by the acid compound within the trichlor puck. When you switched to LC, then you will be required to also add MA to maintain the pH, similar to that, you have to add stabilizer occasionally to maintain CYA at the desired level.
BTW, the use of LC will also continue to increase your salt content, just so you are aware and may want to test for that occassionally.

 

[JoyfulNoise]

I don't disagree with Richard, he is far smarter than I am. The simplest way to explain it is that HOCl (hypochlorous acid) is mostly unaffected by UV radiation except for UV-C wavelengths which are practically extinguished via atmospheric absorption. OCl- (hypochlorite anion), on the other hand, is very susceptible to breakdown by UV-A/B radiation and lots of that gets to pool water. There is no simple, one line formula for the extinction of chlorine by UV light as it involves many different chemical processes. Not only does it generate oxygen gas and chloride in the case of hypochlorite, but there are also numerous other reactions where chlorine + UV forms both chlorine radicals, oxygen radicals, hydroxyl radicals, etc, etc. The proportions and magnitudes of these reactions are all affected by the pH, temperature, other chemical species in the water (carbonate ions are known to extinguish radical formation whereas Fe2+ ions are known to increase radical formation), etc, etc. By and large, hypochlorous acid is a very powerful oxidizer and disinfectant and it will mainly breakdown in those reactions to chloride, water and small amounts of hydrogen ions (H+) which decrease pH. But the effects are minimal and usually swamped out completely by the off gassing of CO2 which raises pH substantially. When hypochlorite breaks down from UV exposure, the result is mainly oxygen gas and chloride. This is also why bleach bottles are vented - not to release chlorine, they don't, but to release the build up of oxygen gas from the natural thermal decomposition of hypochlorite. When hypochlorite breaks down in pool water, there is no acid generated and the equilibrium reaction between HOCl/OCl- shifts to generate more OCl- from the HOCl that it is in equilibrium with (and chlorine is released from it's bound state to cyanurate). That equilibrium shift itself will result in a small amount of hydrogen ions (H+) combining with excess alkaline hydroxide (OH-) to form water. This is the primary pathway by which chlorine breaks down in pools and why covered pools have such low FC loss. If you take away the sunlight, and the pool water is sanitary and clean, then the hypochlorite levels only very slowly degrade from natural self-extinction reactions.

Your pH is rising because your pools live in a desert with over 100" annually of evaporation. This leads to continuous outgassing of CO2 which raises pH. Also, your fill water has very high alkalinity, greater than 100ppm every month of the year. Adding in highly alkaline fill water increases pH as well. You were using trichlor pucks and the acidity from that was holding the pH in line. With the trichlor replaced by a slightly alkaline chlorine source, your pool water pH was free to rise on its own.

Bottom line - install an acid injection system (either muriatic acid or CO2 gas) to stabilize pH.

 

[Toxophilite]

@JoyfulNoise I keep thinking if I read enough of your posts, my Chemistry knowledge will get better. High School chemistry was 44 years ago! But, then, shortly after reading your post above, FB must have been reading too and solved my lack of education needs. This popped up!

 

[JamesW]

When I looked up the effect of decomposition of hypochlorous acid (HOCl), I found that popping the O out HOCl yields hydrochloric acid (HCL).

Sodium hypochlorite is only hypochlorite.

So, any hypochlorous acid produced causes the pH to rise by removing a hydrogen.

2NaOCl + H₂O --> 2Na⁺ + OCl^-+ HOCl + OH^-

OCl^-+ HOCl + OH^- --> O₂ + 2Cl^- + H₂O.

As the hypochlorous acid is used up either by being lost to the sun or converting back to hypochlorite as the hypochlorite is used up by the sun, the total number of hydrogen ions removed are simply returned.

So, the net effect is zero pH change.

If you start with a level of chlorine and you replenish it at the same rate as it is being lost, the removal and return of hydrogen ions is in equilibrium, which results in zero pH increase.

 

[JamesW]

The person in the above picture is obviously an idiot.

They are staring at a chemistry experiment with no safety glasses on and their lab coat is stained many different colors, which suggests that they regularly get splashed with different chemicals.

 

[JamesW]

Trichlor is acidic.

As the trichlor dissolves, it produces acid in the form of hydrogen ions.

2C₃Cl₃N₃O₃ + 6H₂O --> 2C₃H₂N₃O₃ + 3OCl^- + 3HOCl + 5H⁺

People think that the cyanuric acid in trichlor causes the pH drop, but the cyanuric acid is actually a base that raises the pH when the chlorine comes off.

It becomes cyanurate, which picks up hydrogen ions as shown in the above formula.

As the chlorine breaks down, more acid is produced.

3OCl^- + 3HOCl --> 3O₂ + 6Cl^- + 3H⁺
______________________________

Chlorine gas is also acidic.

2Cl₂ + 2H₂O --> OCl^- + HOCl + 2Cl^- + 3H⁺

OCl^- + HOCl --> O₂ + 2Cl^- + H⁺

 

[Toxophilite]

The person in the above picture is obviously an idiot.

They are staring at a chemistry experiment with no safety glasses on and their lab coat is stained many different colors, which suggests that they regularly get splashed with different chemicals.

New times! It's an online University course. At home. You have to improvise and supply your own equipment.

 

[JamesW]

It's an online University course. At home. You have to improvise and supply your own equipment.

The instructor is still an idiot and I would not want to take a course from that person.

 

[Toxophilite]

The instructor is still an idiot and I would not want to take a course from that person.

Well, now I know they are reading-in here!!!! #2 just now on my feed from Hillsdale College, and, he has donned eye and hearing protection:

 

[Newdude]

Ben Affleck is EVERYWHERE these days.

 

[JoyfulNoise]

Ben Affleck is EVERYWHERE these days.

When JLo lets him off his leash …

 

[AUSpool]

Ohh, you’ve gotta love chemistry, it’s like magic, only real.

 

[JamesW]

Ohh, you’ve gotta love chemistry, it’s like magic, only real.

I always forget what's real and what's not.

For example, which of the following are real and which are not?

Magic, voodoo, illusions, tarot, astronomy, astrology, horoscopes, palm reading, fortune cookies, physics, chemistry, alchemy, witchcraft, philosophy, ontology, epistemology, taxonomy and podiatry.

 

[AUSpool]

I always forget what's real and what's not.

For example, which of the following are real and which are not?

Magic, voodoo, illusions, tarot, astronomy, astrology, horoscopes, palm reading, fortune cookies, physics, chemistry, alchemy, witchcraft, philosophy, ontology, epistemology, taxonomy and podiatry.

Not fair, that’s a trick question.

 

[Rattus Suffocatus]

One thing also not really stated clearly is that TA in Tucson water is insane and yes, for the level of it in the water the pH is low. I think Tucson city water is more rock than water honestly. I use tons of MA in my pool and I'm always chasing my tail. Borates are the possible solution but despite knowing that the levels are "not lethal to dogs" I rather not tempt fate. It's eaten my first SWCG cell in 2.5 years instead of the warranted four. My water looks great but TA and CH are always a problem no matter what I do.

I sometimes wonder since I obviously need to do a half drain every three years for CH then maybe it's not really different than a CYA drain at two. Especially when you eat a cell in a little over two years. Oh well. We are starting the long planning process to sell the Tucson house, might take five years because that's actually early retirement planning...but definitely no pool next time.

 

[JoeSelf]

Oh well. We are starting the long planning process to sell the Tucson house, might take five years because that's actually early retirement planning...but definitely no pool next time.

Go with a hot tub. That's what I did. Lovin' it.

 

[Rattus Suffocatus]

Go with a hot tub. That's what I did. Lovin' it.

I plan on taking my hot tub with me. I have had it drained down and not in use for over six months now though. I was the only one who used it, and until I get a medical situation taken care of I can't use it either. been a bummer of a last two years for me but I'll get through it eventually.