Relationship beteen pH and alkalinity

[stev32k]

I’ve proven to myself that an alkalinity of 80 ppm or greater will require acid additions once or twice a week. The pH will rise from 7.4 to 8.2+ in 7 to 10 days. So if I maintain the pH between 7.4 and 7.8 I need to add acid at least once per week. If the alkalinity is 40 ppm acid additions are required about every 6 weeks to 2 months to maintain the pH between 7.4 and 7.8.

I’ve repeated the testing twice and got the same results both times. I’m now lowering the alkalinity to 40 and plan to keep it there unless there is some overriding reason to maintain a higher level.

The thing that bothers me is the recommendations and discussion on this site about maintaining the alkalinity at 70 to 100 ppm. I’m convinced that the pH in my pool is much more stable at a lower alkalinity than is recommended but I don’t know why.

Is there some other factor involved in maintaining a stable pH or a reason besides pH stability for keeping the alkalinity higher?

 

[ewkearns]

I, too, have had to question this. I was having to add ridiculous amounts of acid to maintain a pH of 7.4 - 7.6, given the suggested TA of 70-90+ ppm. I have allowed my TA is float downwards toward 50 ppm and I almost have the pH under control. The fact that I have 50 ppm of borates may be having an effect, too.

Bear in mind that I have a vinyl liner and don't get very concerned about CSI as it goes negative with the above numbers, so I don't worry about calcium scaling.

Plaster pool owners may need to march to a different drummer....

Thoughts?

 

[sbcpool]

My TA was 100 when I inherited my pool and I had to add acid every couple of days or it would spring right back to 8.0+. Now my TA has fallen some to 70 and the pH pops back up to 8.0, but it takes a few weeks to do so. I'm planning on just adding acid until the TA and pH reach equilibrium with my conditions.

 

[domct203]

This is very interesting... I have my TA at 100ppm and my pH has been solid at 7.4 all season.

My pool is considerably much smaller than any of yours, but does that even matter? A ratio is a ratio, right?

Could this have something to do with total saturation? My CH is around 20ppm (vinyl liner, no heater).

Dom

 

[sbcpool]

My CH is closer to 200 ppm. Makes sense.

 

[stev32k]

My CH stays at about 20 ppm. Don't know if that makes a difference or not.

 

[ewkearns]

Mine is about 200, too.....

 

[chem geek]

The thing that bothers me is the recommendations and discussion on this site about maintaining the alkalinity at 70 to 100 ppm.

Where are you getting this? The Recommended Levels in the Pool School for vinyl pools using bleach says 70-90+. It could legitimately say something lower than 70 and be fine as you have found out. It all depends on the amount of aeration of your water -- do you have any waterfalls, spillovers, fountains, etc.? There is no mystery here. Pools are intentionally over-carbonated and the equilibrium pH is high for most TA levels. For spas using the Dichlor-then-bleach method, we recommend a low TA of around 50 ppm and the use of 50 ppm Borates. With all the aeration in a spa and the hot water temperature there is more outgassing in spite of the spa being covered most of the time. The smaller water depth also means more surface area to volume ratio which means greater pH movement from outgassing.

There isn't a problem keeping the TA lower in vinyl pools. It's in plaster pools where one would need to compensate with a higher pH and CH target to have water that is saturated in calcium carbonate to protect plaster surfaces. The only risk to a low TA level is if you were to add any net acidic chemical to the pool and run out of the carbonate pH buffer at which point the pH would crash.

The CH level is not relevant to the outgassing and you can see that doesn't matter from posts in this thread at both low and high CH experiencing the same effect of rising pH at higher TA levels. Again, no mystery here.

 

[stev32k]

Where are you getting this? The Recommended Levels in the Pool School for vinyl pools using bleach says 70-90+.

OK, I was close. I normally don't have any aeration except when I'm trying to lower the TA. Then I have plenty LOL

 

[ewkearns]

The Recommended Levels in the Pool School for vinyl pools using bleach says 70-90+.

Which could be read from 70-200 TA. The 90+ is indefinite and I've never been able to hold 70, without the pool having an out-of-control thirst for acid. For a noob (and I was no exception), 90+ surely won't work and neither will 70. I think the numbers in the table may need further explanation or maybe a row for "if you have a lot aeration..."

It could legitimately say something lower than 70 and be fine as you have found out. It all depends on the amount of aeration of your water -- do you have any waterfalls, spillovers, fountains, etc.?

Speaking for me, no aeration, but at even 70 TA, the pH would take off at a sprint towards pH of 8+

There is no mystery here. Pools are intentionally over-carbonated and the equilibrium pH is high for most TA levels.

Why? :???:

There isn't a problem keeping the TA lower in vinyl pools. It's in plaster pools where one would need to compensate with a higher pH and CH target to have water that is saturated in calcium carbonate to protect plaster surfaces. The only risk to a low TA level is if you were to add any net acidic chemical to the pool and run out of the carbonate pH buffer at which point the pH would crash.

Good! Would borates help mitigate this happening or do we have a tipping point?

The CH level is not relevant to the outgassing and you can see that doesn't matter from posts in this thread at both low and high CH experiencing the same effect of rising pH at higher TA levels. Again, no mystery here.

Yet, a vinyl lined noob, seeing more into the CSI (as I did) is confused by the apparent requirement for more calcium.

 

[JoyfulNoise]

Quote Originally Posted by chem geek
There is no mystery here. Pools are intentionally over-carbonated and the equilibrium pH is high for most TA levels.

Why?

You can use Henry's Law to calculate the concentration of dissolved carbon dioxide in an aqueous solution. The current, average atmospheric concentration of CO2 is 387 ppm, that is 387 x 10-6 atm.

[CO2] = P/K_H = 3.87 x 10-4 atm/29.41 atm M^-1 = 1.32 x 10-5 M

If this concentration of CO2 were dissolved in water (assuming a pH = 7.5), it would equal ~ 10ppm worth of alkalinity (assuming CYA = 0 and no other sources of alkalinity).

As you can see, the equilibrium amount of dissolved atmospheric CO2 gas is smaller than the amount of CO2 in pool water derived from the chemical equilibrium of bicarbonate/CO2.

As well, carbonates are the only reasonably safe buffering system available to pool water. So, for better or for worse, we're stuck with carbon dioxide.

 

[chem geek]

Why? :???:

Good! Would borates help mitigate this happening or do we have a tipping point?

Yet, a vinyl lined noob, seeing more into the CSI (as I did) is confused by the apparent requirement for more calcium.

Pools are intentionally over-carbonated for two reasons. One is to provide a carbonate pH buffer to prevent wild swings in pH. Of course, it's ironic because that same buffer system is a SOURCE of rising pH in its own right due to carbon dioxide outgassing, but for pools using net acidic sources of chlorine the TA helps to prevent the pH from crashing. The second reason is to provide a source of carbonate which when combined with a source of calcium saturates the water with calcium carbonate to protect plaster surfaces. Obviously this latter effect is not required for vinyl pools.

This chart shows how much a pool is over-carbonated as a function of pH and TA. This is why having a lower TA and also having a higher pH target reduce the rate of carbon dioxide outgassing and the rate of pH rise. You should not try and lower the pH to 7.4 since the pool will outgas faster at that lower pH. If the pH rises to 8.0, lower it to 7.6, and you will find you use a lot less acid. Of course, once you get your TA low enough and use borates, you may find that the pH is more stable so that you can maintain a pH of 7.7 or so and only add a touch of acid to keep it there.

Borates would help slow down the rate of pH rise but probably won't reduce the total amount of acid that needs to be added. However, if you combine the use of 50 ppm Borates with also lowering the TA level, then not only the rate of pH rise but the amount of acid that needs to be added is lessened. The borates provide additional pH buffering but is stronger at higher pH so you still need some carbonates to protect against the pH crashing. If you are careful not to use any net acidic chemicals, then the risk of pH crash is low.

As you have since figured out, you don't need to worry about a negative CSI for vinyl pools so can have the TA be lower.

 

[ewkearns]

Chem Geek,

How do the Europeans (primarily Germans, I think) maintain such low pH values? Most of the anecdotal evidence I hear lauds lowered pH as a cure for stinging eyes..... and it seems to be true in my pool. I don't have a fetish for pH, other than the red-eye issue....

 

[chem geek]

Actually, low pH isn't to avoid stinging eyes and studies show that this isn't a predominant factor. See this paper that no irritation related to pH from 7.0 to 9.0 except at higher HOCl levels (with no CYA) where low pH was more irritating. Other studies (quoted in that same paper) referred to more irritation at lower pH.

The primary reason for lower pH in the DIN 19643 is for the use of flocculants/coagulants where specifically the pH range when using alum salts is 6.5 to 7.2 while it is between 6.5 and 7.5 when using iron salts. They also have the lower pH to have the active chlorine (hypochorous acid) be higher though that could also be accomplished by a higher FC level. Really, the main reason for the low pH is for the coagulation since that is such an important part of their system.

As for how they maintain such a low pH, they don't specify in the standard and in practice they'd have to keep a lower TA level or use a LOT of acid and bicarbonate or inject a LOT of CO₂. They do talk about metal corrosion and using both active (impressed-current anodes) and passive (coatings) are discussed in the standard. There is no discussion of calcium carbonate saturation or protection of plaster surfaces. Many commercial/public pools use tile surfaces though obviously there is still grout to consider in that case.

 

[stev32k]

Just a quick update on my pH/alkalinity/acid addition problem. I lowered the alkalinity to 40 ppm on the 26th of July while aerating the pool. The pH rose to 7.4 - 7.5 on the 29th and I stopped the aeration. That was 7 days ago and the pH has remained stable and is still 7.4 - 7.5 today. With 80 ppm alkalinity I'm pretty sure the pH would be 7.8 - 8.0 by now.

 

[Megsyperk]

Glad you updated because I'm having the same issues. I'm planning on testing my TA on Friday as I've been adding MA like it's nobody's business to keep pH at a good level.


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[ewkearns]

I've had the same experience. With the suggested 70-90+ ppm, I was constantly adding acid. Now sitting at 60 ppm, I no longer have to battle a pH racing towards the big 8.....

 

[JoyfulNoise]

For the vinyl and fiberglass folks, I would not be too worried about a 40ppm TA. For people with plaster pools, you'd want to keep a careful eye on the CSI to make sure the water doesn't get too corrosive. If you have 50ppm borates, then going as low as 40ppm TA isn't too bad. But without borates, I'd be really concerned about crashing the alkalinity and causing pH imbalance.


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[gregsfc]

I too found that if I continue to lower my PH continually, just a couple of tenths each time back to 7.5-7.6 until it quits drifting up, I end up with TA around 40 and then very steady PH thereafter; meaning that now I mostly add bleach daily like TFP is supposed to be. Before Chem Geek helped me on a similar thread, I was in a viscous cycle of adding acid every two days, because I'd be back at 8 every couple of days until I found that my TA had dropped to 50 or lower, and then of course, following the TFP recommendations of 70-90, I'd target and raise my TA back up to 80. The cycle would continue. I started a thread asking if it's okay to let the PH run high; never even thought about TA until making the inquiry.

But since learning this technique, which I don't think is part of pool school (and I don't think many amateurs would ever know as an acceptable practice except for a few threads on this topic in the forum); I think those of us with vinyl pools and constantly rising PH, should get the PH to level off at the upper end of the acceptable range and not worry so much about TA as long as we're just adding bleach. I've been doing this for one month and it's much less work and worry when the PH hardly ever drifts out of range.

I guess making this topic part of pool school would add to the complexity, but when non chemist-minded TFP folks try to follow the recommended levels, we don't necessarily all understand the relationship of TA and PH; and that it's okay to let one drop below the normal recommended level to help maintain the other. We just read that we're supposed to keep them all in range, and I wonder how many folks are either stuck adding acid and baking soda constantly, or running their pools at high PH when all they really have to do is let the TA settle in wherever the PH stabilizes in range?

 

[chem geek]

For vinyl pools there isn't a problem getting the TA lower to have more stable pH. Even with plaster pools one could do it if they also increase their CH and possibly their pH target (i.e not 7.5, but 7.7 or 7.8) to keep things in balance to still protect plaster surfaces.

Where this approach doesn't work well is for pools with high TA fill water and a lot of evaporation and refill. In that case, there's going to be a lot of acid addition regardless, but at least one can get to a point where that acid addition is only making up for the additional TA added and not from the carbon dioxide outgassing.

What is there about your pools that would have outgassing be higher? Any spillovers, waterfalls, fountains, or other sources of aeration?