Understanding PH

[Beez]

Hello all,

So I've got the basics of the BBB method down, and numbers are pretty much in the optimal range:
FC - 5
CC- 0
PH - 7.5
TA - 80
CH - 290
CYA - 55
Borates - 50
Salt - 1930

The pool looks and feels great, but what I still don't understand is the PH. It won't stay stable for any longer than 2-3 days. It's 7.5 now because I added MA this morning, but if it continues like it has it will be 7.8 or higher by Wednesday or Thursday.

I can accept that adding MA is just part of the BBB method, but it just bugs me when I don't understand what's happening. Does the MA get used up in a similar way that chlorine gets used up? I wonder if I let the water have its own way, how high would it get before it reached an equilibrium? It seems like continually adding MA to combat PH rise will lead to an inverse battle with TA.

Thanks,
Dave

 

[Guest]

There is no easy way to answer that without going into some chemistry which is why I wrote this.

ta-what-is-it-really-t4979.html

Hopefully this will explain the interaction between pH and TA and why pH always wants to rise in a pool (that we purposely overcarbonate) unless you add some form of acid like Muriatic or Trichlor.

First question I have is do you have any significant sources of aeration in your pool?

Second question, Is your plaster less than a year old? New plaster is going to eat a lot of acid, period! No way around it no matter what you do because of chemistry going on in the plaster itself. After about a year (or less in many cases) the pH rise from the plaster really slows down or stops.

Looking at your numbers I would suggest dropping your TA even more to about 60 ppm and bringing up your CH to around 350.
This will help slow the pH rise. As far as pH goes when it hits 7.8 lower it to 7.5 and no lower. The lower you drop the pH the faster it will rise because you will outgas CO2 faster.
If you make the changes to TA and CH that I recommend and keep your pH between 7.5 and 7.8 your CSI will stay within the good range with some room to spare

As far as how high the pH can rise, this depends on how new your plaster is. New plaster will give off very alkaline substances that will raise the pH of the water. In fully cured plaster the pH rise is basically from the bicarbonates (TA) in the water and will stop at about 8.3 which is where the bicarbonate buffer is always moving towards and wants to rest unless we intervene. With your borates in the water you will find that the pH will stay at about 7.7 for an extended period of time but the bicarbonates seem to always win and the pH does rise higher.

 

[Beez]

There is no easy way to answer that without going into some chemistry which is why I wrote this.

ta-what-is-it-really-t4979.html

Well, that helps a little, given that I understand only a fraction of it...

Hopefully this will explain the interaction between pH and TA and why pH always wants to rise in a pool (that we purposely overcarbonate) unless you add some form of acid like Muriatic or Trichlor.

First question I have is do you have any significant sources of aeration in your pool?

No, other than the very little swimming that we do in it once or twice a week. I do have 2 of 3 returns set to just barely break the surface, but no bubbles are created.

Second question, Is your plaster less than a year old? New plaster is going to eat a lot of acid, period! No way around it no matter what you do because of chemistry going on in the plaster itself. After about a year (or less in many cases) the pH rise from the plaster really slows down or stops.

Plaster is very old, it needs to be replaced.

Looking at your numbers I would suggest dropping your TA even more to about 60 ppm and bringing up your CH to around 350.
This will help slow the pH rise. As far as pH goes when it hits 7.8 lower it to 7.5 and no lower. The lower you drop the pH the faster it will rise because you will outgas CO2 faster.
If you make the changes to TA and CH that I recommend and keep your pH between 7.5 and 7.8 your CSI will stay within the good range with some room to spare.

AH-HAH! Now I see one problem with my method: I may have been adding too much acid and lowering the PH too much.

Will make the changes to TA & CH that you suggest. The lower TA should be no problem considering I keep adding acid. However, I'm a little surprised at your suggestion for the CH. I thought I had done good by lowering it to 290.

As far as how high the pH can rise, this depends on how new your plaster is. New plaster will give off very alkaline substances that will raise the pH of the water. In fully cured plaster the pH rise is basically from the bicarbonates (TA) in the water and will stop at about 8.3 which is where the bicarbonate buffer is always moving towards and wants to rest unless we intervene. With your borates in the water you will find that the pH will stay at about 7.7 for an extended period of time but the bicarbonates seem to always win and the pH does rise higher.

Thanks for the excellent info waterbear! But there is one question you didn't answer: What happens to the acid that I keep adding? Is it simply used up?

Dave

 

[chem geek]

What happens to the acid that I keep adding? Is it simply used up?

If it's Muriatic Acid, it just becomes salt, specifically chloride. The hydrogen part of it basically becomes water with the oxygen of the water coming mostly from bicarbonate that gets converted to carbon dioxide that gets outgassed from the pool. So over time with acid addition, the salt level rises. Since you increase TA by adding sodium [EDIT] bicarbonate [END-EDIT], the sodium comes from that so the net result is just plain salt, sodium chloride. This increases TDS, but is not a problem since salt is fairly innocuous (unless it gets extremely high).

Chlorine also gets converted to chloride (salt).

 

[anonapersona]

I see you are in Dallas, do you know what the TA of your fill water is?

I'm near Houston and have had to add water pretty often with evaporation losses recently and I finally tested the tap water to find it has TA of 340. So, rising TA and rising pH will always be an issue for me.

 

[Beez]

What happens to the acid that I keep adding? Is it simply used up?

If it's Muriatic Acid, it just becomes salt, specifically chloride. The hydrogen part of it basically becomes water with the oxygen of the water coming mostly from bicarbonate that gets converted to carbon dioxide that gets outgassed from the pool. So over time with acid addition, the salt level rises. Since you increase TA by adding sodium carbonate, the sodium comes from that so the net result is just plain salt, sodium chloride. This increases TDS, but is not a problem since salt is fairly innocuous (unless it gets extremely high).

Chlorine also gets converted to chloride (salt).

Precisely the info I was looking for.
Thanks Richard!

 

[Beez]

I see you are in Dallas, do you know what the TA of your fill water is?

I'm near Houston and have had to add water pretty often with evaporation losses recently and I finally tested the tap water to find it has TA of 340. So, rising TA and rising pH will always be an issue for me.

I do not know what the TA of my fill water is, but I hope it is pretty high because at the rate I'm adding MA I'm starting to fear losing TA! It seems like every quart of MA I add is reducing the TA by 10. Hopefully, the PH will start to balance out when my TA drops to 60. TA is now 70. :?

 

[Guest]

Well, that helps a little, given that I understand only a fraction of it...

Read through it a few times. You will be surprised at how much more you understand each time you read it. There really is no "easy" way to explain it.

 

[chem geek]

I wonder if I let the water have its own way, how high would it get before it reached an equilibrium? It seems like continually adding MA to combat PH rise will lead to an inverse battle with TA.

Dave,

I agree with waterbear that there really isn't a simpler way to explain it except to say that TA has TWO effects: one is pH buffering while the other is as a SOURCE of pH rise itself. The former would lead one to think that higher TA would make the pH more stable, but the latter counteracts this and becomes more dominant, especially at higher TA levels (technically, the former effect is more linear while the latter effect varies more as the square of the TA).

The only reason to have a higher TA is when using acidic sources of chlorine such as Trichlor. That not only provides more pH buffering, but the latter effect of having pH tend to rise helps to offset the pH dropping from Trichlor usage. If the TA is high enough, then one can get fairly stable pH and just have the TA drop over time (from the acidity of Trichlor -- it's like continually adding acid). When one is using hypochlorite sources of chlorine that are net pH neutral when accounting for chlorine consumption/usage, a lower TA can be used for greater pH stability (ironically) though one doesn't go too low because one still wants at least some pH buffering and for plaster pools one needs some carbonates to prevent pitting/dissolving.

As for how high the pH would go if you just let things be, with your numbers the pH would rise as high as 8.1 before equilibrium with carbon dioxide in the air was achieved. In practice, the pH would slow down in its rise and probably appear to stop below this pH. The equilibrium pH is a function of the carbonate portion of the TA level and the Borates tend to make that equilibrium pH slightly lower (basically, a lot more carbon dioxide has to outgas for the pH to rise so there's less of it in the water at the same pH). Without the Borates, the equlibrium pH would be around 8.3.

There is no "inverse battle" if you don't try and raise the TA too high. It is true that if the pH is rising for other reasons, such as plaster curing or chlorine outgassing (from undissolved chlorine gas from an SWG), then you'll have to add acid to compensate, but the TA should remain fairly stable unless there is carbon dioxide outgassing. That is, these other sources of pH rise are like pure bases where they make both pH and TA rise so adding acid is exactly the opposite where both pH and TA are lowered back to where they were. The only time the TA drops [EDIT] without a drop in pH [END-EDIT] is when there is actual carbon dioxide outgassing [EDIT] in addition to acid addition [END-EDIT] and a lower TA helps to minimize the outgassing (use of a pool cover works well for stopping this as well).

Richard

 

[Beez]

Thanks for the detailed explanation Richard. I'm starting to realize that part of my problem may be an unrealistic expectation for a target PH. I have been struggling to maintain 7.5(where I got this number for an ideal target, I don't remember...), but maybe I should just let it settle where it will without getting up to that equilibrium of 8.1? I think I read somewhere waterbear saying that his pool will ride at like 7.7 for a while...sound reasonable?

Thanks,
Dave

 

[chem geek]

Yes, you should be fighting the pH rise more easily if you target 7.7 for your pool's pH. This won't help for any plaster curing, but it will help for any outgassing from all that aeration in your pool.