Ultimate pH?

[nunk]

What is the likely water chemistry outcome for a vinyl SWG pool to which NO corrective HCl additions are made?
The water is currently pH 8.2+, the FC is 10+ and the CYA is c.200ppm. The brand new SWG's certainly working well!
This represents my neighbour's pool (no regular testing) and I'm just wondering where it's headed. Is there an ultimate end point to the pH rise and chlorine generated? It'll probably take a disaster with the water for any action to occur.

 

[duraleigh]

Trying to fix your neighbors pool will likely drive you crazy. He obviously doesn't believe in testing so he probably won't believe any outcome you tell him. I would walk away.

That said, Calcium scaling is the most likely condition with very high pH....although not as big a problem with a vinyl pool. Irritation to the skin may come into play with pH that high as well. Again, your neighbor doesn't believe in testing which he would need to do to get his pool back within normal parameters.

 

[lefty51]

Probably the best you could do for your neighbor is expose him to this site. Then sit back and hope he looks around and the light goes on.

Oh yeah, and I would stay out of his pool

 

[nunk]

Thanks. Yes, my neighbour works on the 'set and forget' principle. Preventive maintenance? What's that?
However, I was really wondering whether the pH will continue to rise? The SWG is a self-cleaning type so maybe the electrodes won't readily become scaled. But with the depleted hydrogen ion concentration, will the SWG still produce HClO at the same rate, or will that decline?

 

[lefty51]

Well, this is clearly beyond my level of knowledge, but here goes.

It's been my experience that left unchecked PH will continue to rise. how high it can go is beyond me. there are folks on here who can answer this, so hopefully they will notice this and respond.

As far as the SWG goes, self cleaning or not, with that high of CYA, I believe the cell will be working overtime and eventually shorten it's life span dramatically.

In other words, that "new" SWG, won't be for long.

Hope that helps, Si

 

[chem geek]

The level of pH rise from carbon dioxide is limited by the carbonate alkalinity level (TA with CYA and borates adjustments). For a pool at 85ºF with 70 ppm TA and 80 ppm CYA and 3000 ppm salt, the pH rise would stop at around 8.0. In an SWCG pool, however, there can also be undissolved chlorine gas outgassing and this won't stop so the pH can continue to rise indefinitely though since pH is a logarithmic scale it will appear to slow down as the pH gets higher. If chlorine usage were 2 ppm FC per day and if 10% of the chlorine gas were outgassing, the pH would rise (from 8.0) to 9.0 after one month, to 9.4 after two months, and to 10.3 after six months. This is essentially occurring from the "lye" produced by the SWCG that isn't getting compensated by chlorine usage/consumption because some of the chlorine outgassed instead.

This 10% chlorine outgas assumption corresponds to the need for adding 2-3/4 cups of full-strength Muriatic Acid (31.45% Hydrochloric Acid) per 10,000 gallons per month so is not an outrageous assumption, though usually a significant amount of acid demand is from carbon dioxide outgassing which can be largely controlled via a lower TA.

 

[nunk]

Thanks chem geek. Perhaps I should have also added: TA 120 ppm; and NaCl 3500 ppm.
Could you possibly expand on the statement "The level of pH rise from carbon dioxide". I would have thought that carbon dioxide outgassing would occur at acid pH's where its in equilibrium with bicarbonate ion but at pH's >8.00 its almost non-existent.
By the way, I prefer to think of the SWCG generating its Cl from chloride ions provided by HCl, not NaCl. This way, its easy to explain the rise in pH as the H+ ions are depleted by conversion to hydrogen gas and hence the need for regular additions of HCl to restore (lower) the pH. Testing for NaCl generally reveals little change to the concentration demonstrating that this is not a consumable.
I will be keeping a discrete eye on the neighbour's pool to see how it travels over time.

 

[chem geek]

At a TA of 120 ppm and assuming 80 ppm and 3500 ppm salt, the equilibrium pH where carbon dioxide outgassing would stop is around 8.35. As shown in this chart, there is more carbon dioxide outgassing due to over-carbonation of the water when the TA is higher or the pH lower. So yes, the pH rise from carbon dioxide slows down as the pH gets closer to this equilibrium pH. Any additional pH rise is due to other sources including undissolved chlorine gas outgassing, hypochlorous acid outgassing (not much due to most of chlorine being bound to CYA that does not outgas), plaster curing or degradation.

The pH rise from the SWCG is from the combination of having hydrogen ions converted to hydrogen gas AND not having the exact balance of acidity from chlorine usage/consumption. Chlorine gas dissolving in water is acidic and the usage/consumption of hypochlorous acid is acidic.

Cl₂(g) + H₂O ---> HOCl + H⁺ + Cl^-
Chlorine Gas + Water ---> Hypochlorous Acid + Hydrochloric Acid

Chlorine consumption being acidic is described in detail in this post.

As for chloride ions not being consumed, it has nothing to do with whether you consider it HCl or NaCl. Chloride ions are converted to chlorine gas which dissolved in water becomes hypochlorous acid (and at pool pH, hypochlorite ion). When the chlorine is used/consumed it becomes chloride ion again. That's why it is conserved. The amount of chloride in the water is so high that if you lost 10% of chlorine gas being outgassed, even at 2 ppm FC per day 10% would be only a loss of 6 ppm per month and would not generally be detectable in a salt test, plus there are other ways for the water to get diluted that would hide such subtle changes.

 

[nunk]

Thanks for pointing me to the other posts and chart. That has nicely filled the gaps in my knowledge. As to your comment:

"As for chloride ions not being consumed, it has nothing to do with whether you consider it HCl or NaCl. Chloride ions are converted to chlorine gas which dissolved in water becomes hypochlorous acid (and at pool pH, hypochlorite ion). When the chlorine is used/consumed it becomes chloride ion again. That's why it is conserved. The amount of chloride in the water is so high that if you lost 10% of chlorine gas being outgassed, even at 2 ppm FC per day 10% would be only a loss of 6 ppm per month and would not generally be detectable in a salt test, plus there are other ways for the water to get diluted that would hide such subtle changes."

I completely understand this, being a (long-retired) chemist myself. What I was intending is that i find it easier to explain to non-chemists that the chlorine results from the consumption of HCl in the SWCG, which then causes the pH to rise due to loss of acidity. Thus the need for regular pH checks and appropriate additions of HCl. I know it's over-simplistic and chemically incorrect but I have found it's any easier concept for the average bloke (my neighbour excepted) to grasp and adopt. It also disabuses them of their belief that the chlorine comes from the salt which needs to be closely monitored.
Just trying to make it super simple. If I start talking about ions, hydrogen outgassing, equilibria etc, I find their eyes soon glazing over!

 

[nunk]

Update. Well a week has gone by and no testing, adjustments or maintenance have occurred. The SWCG is still merrily generating chlorine. A section of the bottom of the originally blue vinyl liner has turned white and there is a small area of crazing. The baskets in the skimmer box and pre-pump are nearly clogged with leaves. If this situation continues, it could all end in tears (not of joy!). In the meantime, bathers are enjoying it (35 degrees C here today).

 

[chem geek]

That's odd that the section of the liner that has faded is at the bottom. That would happen if there were manual chemical addition in the same place without good mixing where the concentrated chemical fell to the bottom so bleached out the liner. I wouldn't expect that from an SWCG even if putting out high chlorine. I'd expect more overall fading everywhere in the water in that case.

Why is the CYA about 200 ppm? Usually that only gets high if one is using stabilized chlorine (Trichlor or Dichlor). Maybe they have supplemented by adding chlorine to the pool -- maybe Trichlor granular that got stuck on the bottom or perhaps Cal-Hypo (Dichlor usually dissolves quickly enough to not settle to the bottom).

 

[nunk]

This all started with a beautiful green pool which had developed over winter due to complete neglect. Also, unbeknownst to anyone, the Maltese Terrier had been excavating under the concrete slab on which were positioned the sand filter and pump. The slab eventually collapsed (the dog wasn't under at the time) resulting in fracturing of PVC pipes and the filter valve assembly. A new filter was purchased and this plus the pump were installed on a new concrete slab together with some new PVC pipe. Also a new SWCG was installed.
I had earlier tested the green water and found 100ppm of CYA (he had been using a floater with Trichlor tablets the previous season - again, no regular testing).
I advised him to drain out 25% of the water (24,000 liter pool) and shock dose the rest with 10 litres of Liquid Chlorine. He wasn't happy with this idea and went to the local pool shop who sold him:
2kg of EZI-CHLOR (81% AC as Trichlor + 10% Na tetraborate.5H2O) - add directly down the skimmer
1kg of EZI-CHLOR (81% AC as Trichlor + 10% Na tetraborate.5H2O) - add to pool
2kg of TRIPLE ACTION POOL CHLORINE (56% AC as Dichlor) - add to pool
1 litre of SUPAFLOC (inorganic/organic cationic polymers - polyaluminium chloride?) - add to pool
1 litre 0f MAXI-FLOC PLUS (polyaluminium chloride complex) - add to pool
4kg of ALKALINITY INCREASER (sodium bicarbonate) - add to pool
2.5kg of pH INCREASER (sodium carbonate) - add to pool
Once clear water is achieved, add 80kg of SALT (sodium chloride) then turn on SWCG.

I didn't get to witness just how these additions were made but I'd imagine the contents of the containers would have just been dumped into the water - after all, the circulating pump will take care of the mixing. Yeah, right! The above adds another 110ppm of CYA to the 100ppm already present.

I hope the above answers your questions.

 

[jblizzle]

I do not think it was mentioned that when FC is higher than 10ppm, the pH test typically reads artificially high. Although given the circumstances, I am sure the pH is actually above 8.2

 

[chem geek]

Thanks for the details. Makes more sense to me now. I suspect the fading was from the Trichlor granular added directly to the pool and settling on the floor.

 

[nunk]

To track the pH rise in the pool water, I just wish I had some m-cresol purple indicator (pH range 7.4 - 9.0) or thymol blue (8.0 - 9.6). The phenol red in the test kit has a range of only 6.4 - 8.2. The water currently tests at over 8.2.

 

[nunk]

Well I thought that I would provide an update. No testing or deliberate.additions have been made to the pool since my last post 5 weeks ago, but we have had 125mm of rain in that time. The SWCG has been adjusted to a lower setting (20% on the gauge). Today I took pool water samples to two pool shops with the
results:
pH: 6.8/7.1
TA: 10/60
FC: 6/8
CH: 195/147
CYA: 110/110
NaCl: 3900/4020

Doesn't say much for testing consistency even though both shops use 'identical' Palintest equipment (is it ever calibrated?) and materials.

However, I was most surprised by the low pH and TA results. Not at all what I was expecting. Is this the result of the excessive Cl generation leading to high levels of HClO and HCL being produced?

 

[Salsiccia]

G'day nunk,

Thanks for sharing your experiences on here - this is a fun read !

My silly question for you is why are you spending so much of your valuable time and considerable knowledge in trying to help someone who clearly isn't interested?
Is it just morbid curiosity ?
Like watching a train wreck?

I'm sitting back and gonna crack a beer for the next instalment !!!

Sal

 

[MarianParoo]

Yup. You can take your friends and neighbors to perfect water, but you can't make 'em drink!