Algae, TA or what?

[waterbear]

couple o' things:
first, what kind of pool do you have? If it is plaster how old is it? Are you only using bleach for chlorination?

Second, drop the TA a bit more, ti will help

Do you have any water features? Aeration will speed pH rise.
When you add acid to lower the pH how low do you go? The lower the pH the faster it rises. Try not to lower it below about 7.6

If you still have algae the answer is simple, you need more chlorine. Bump tiup to about 6-7 ppm. How much chlorine loss do you have daily. Hot climates like Arizona and Florida often do better with higher CYA in water. If you are having a problem maintaining FC (which your algae problem indicates) then try bumping up the CYA to around 50 or even higher. 80 is not too high in a hot climate that gets a lot of sun. Just adjust your FC levels accordingly and you are good to go.

[CarlD]

I would go further and say if you have algae, you should START by adding sufficient chlorine to achieve the recommend shock level--that's FC of 15ppm, and keep it there at least 24 hours after your algae is all gone.

I wouldn't mess with anything else other than lowering your pH to 7.5/7.6 until the algae is gone. With the high FC reading, though you'll get a higher pH reading than you'd like. As long as it's in the 7's you're OK, but keep the FC up there!

I'm of the firm opinion that algae means you need to raise FC to shock level and anything else is just asking for trouble.

Don't worry about CYA, TA, Borates, rising pH or anything else till it's cleared up.

[PoolDoc]

Hey Guys;

I'm missing some things here.

Waterbear, I'm not clear on your thinking about TA. You may have some really good reasons behind your thinking, but right now, I'm kinda puzzled. Do you have reason to think that lowering Alk will reduced acid demand or pH rise? Have you seen evidence that a really low Alk (below say 50 ppm) is OK for a plaster pool? Why did you feel that lowering Alk was more important than dealing with her algae?


CarlD, are you aware that chlorine becomes less effective against algae as pH increases? The most dramatic problems are with unstabilized chlorine, but as best I can make out, the problem is still significant in stabilized pools, too. Since she reported that her pH was tending to go ABOVE 8, it's my opinion that that's an issue that needs to be addressed in the short term, in addition to raising the chlorine.


Chem Geek has done some stuff with 'carbonics' that I haven't had a chance to digest, or much less fully consider yet. However even once I do, I'm not prepared to toss out all S.I. considerations without some experimental verification first. Stoichiometry is nice, and successful process chemistry often resembles the stoichiometric analysis, but rarely is identical with it.


I'm leaning toward stating some 'standard' recommendations for Pool Forum responses, but I'd like to see what y'all have to say, first.

As always, thanks for all your help.

Ben
"PoolDoc"

[CarlD]

Ben,

Yes I'm aware that lower pH makes chlorine more effective at sanitation. But I'm also concerned that at high FC levels, the pH test can appear higher than it is. Adding extra drops of 0007 can compensate, but 0007 itself affects pH, too, correct? I've forgotten whether it's acidic or basic.

So, given that, I took that leap that if someone's FC is 15, a pH in the 7's is probably OK (since the CYA is moderate).

Carl

[chem geek]

This post is the one showing the difference in active chlorine (hypochlorous acid) level as the pH varies with no CYA and with CYA. The main point is that though it varies much more when there is no CYA, the absolute level is much higher with no CYA so it all depends what you are comparing against. If you compare no CYA against no CYA, then the pH is a big factor.

Even so, if you shock with a lot of chlorine, especially 15 ppm or more, then it's a good idea to lower the pH to 7.2 (or even lower if you are shocking to even higher levels) because the addition of chlorine will raise the pH substantially, even beyond 8.0. Yes, when the chlorine gets used up the pH will mostly come back down but shocking is a sustained high level of chlorine so would be a sustained high pH if you don't lower the pH first.

For a shock of 10 ppm, adjusting the pH isn't that important if it's starting out around 7.5 or so.

As for TA, there was a forum user who found that his pH would rise over time a lot but that when the TA went down to 50 or 40 ppm the pH became stable. It was an extreme case, but we see this a lot in spas (at poolspaforum.com) due to the higher aeration (assuming they are using bleach and not Dichlor, except initially). There are many SWG pools that have lower pH rise at lower TA levels as well. However, in most pools 70 or 80 ppm is OK. A lower TA should be perfectly fine IF one adjusts their target TA and Calcium Hardness (CH) to keep the saturation index near zero. No one is saying to ignore the saturation index if one has a plaster (or fiberglass) pool.

[chem geek]

Ben,

Regarding this post, I want to comment on some of the things that you wrote which I'll quote below.

I can observe that, in my experience with multiple large commercial pools managed with sodium hypochlorite (bleach) feed systems, I universally found that bleach fed pools 'wanted' to be at a pH of 7.6 or higher. I have no analytical explanation for this. But I can tell you that I discovered that if I let the pH float upwards, it usually -- but not always -- stopped before 8.0. I also found that if I operated the pools between 7.6 and 8.0 it took FAR less acid, than if I tried to operate the same pool between 7. 2 and 7.6. (Note to Chem Geek: yes, I tried it both ways on the SAME pools )

What you observed is completely consistent with the underlying chemistry. See this chart which shows how far out-of-equilibrium pool water is in terms of the excess carbon dioxide there is in the water. Pools are intentionally over-carbonated in order to provide a pH buffer and to add carbonates to protect plaster surfaces (when calcium is also added). You can see that at lower TA and higher pH the water is not as far out-of-equilibrium. The rate of carbon dioxide outgassing is related to this number, though Wojtowicz has shown that it is proportional to the square of the TA whereas my table doesn't include that effect (the table shows more of a linear effect with TA).

So basically a pool will tend to rise in pH, but will do so more quickly at low pH and will slow down as the pH rises. It doesn't technically stop until it reaches equilibrium, but in practice it becomes so slow relative to other pH changes that it seems to be stable. This is because the carbonate alkalinity portion of TA is a SOURCE or rising pH itself due to carbon dioxide outgassing. It is not true that a higher TA will produce a more stable pH unless one has some acid added to the pool such as using Trichlor (or even Dichlor which is net acidic when accounting for chlorine usage/consumption since that is an acidic process). Instead, a higher TA will result in a faster pH rise and a higher ultimate equilibrium pH.

The reason that it takes far less acid having the pH settle higher is that a higher pH results in less carbon dioxide in the water, all else equal. If you force a lower pH, you are forcing more of the carbonates in the water to be in the form of carbon dioxide and that will then outgas faster causing the pH to rise.

You have chosen to use a higher target pH as a way of reducing the (further) pH rise, but the other approach is to lower the TA level itself. This will also result in a lower rate of pH rise. Assuming a CYA level of 30 (since some of this measures as TA), a pH of 7.8 with a TA of 80 ppm would have about the same amount of carbon dioxide as a pool with a pH of 7.5 and a TA of 45 ppm.

On another related note: buffering has to do with how difficult it is to change the pH of a solution (your pool water in this case). Highly buffered water has highly stable pH . . . . IF the buffer in question is effective at that pH level.

Most buffers have ranges over which they are effective, and other ranges over which they are not so effective. Cyanuric acid is an effective buffer at LOW pool pH levels, below 7.4 as I recall. Borax is an effective buffer at HIGH pH levels, above 8.0, I think. Total alkalinity tests measure how much buffer you have, but not which levels it's effective at.

This is important, because if you have drifting pH levels like you do, it's your pool's buffer system that determines how hard you'll have to work to maintain your pH level. Absent aeration, sodium bicarbonate is an effective buffer in the 7.0 - 8.0 range where your pool lives. A pool with an effective bicarbonate based "Total Alkalinity" will require larger doses of acid, added less frequently, compared to a pool without that bicarbonate based buffer.

More or less, a well buffered pool will require that you adjust your pH less frequently than a poorly buffered pool. It won't really decrease how much acid you need, but it will change how often you have to add it!

It is not true that highly buffered water has a stable pH, even if the pH is in a good buffering range. The carbonates have TWO competing effects. The pH buffering effect where more is better AND the carbon dioxide outgassing effect where a higher TA is less stable pH (unless their are acid sources such as Trichlor). The latter effect dominates when the TA gets too high. As far as how high is "too high", this depends on the amount of water-air interchange so depends on aeration (i.e. waterfalls, spillovers, fountains, heavy splashing) so generally pools that are covered are more stable in pH even at higher TA levels.

As for effective ranges of buffers, you don't have that quite right. In this post, I give more details about the buffering capabilities of the carbonates, cyanuric acid, and the borates. The borates have more pH buffering capacity against a rise in pH, but they do buffer both ways. The borate buffering system gets stronger as the pH rises. The carbonates, on the other hand, are the opposite where they get stronger in buffering as the pH gets lower (by higher and lower I'm referring to the normal 7.0 to 8.0 pool pH range). The cyanuric acid buffer system is a bit uneven in its buffering in this range.

Total Alkalinity measures the buffer capacity in one direction only -- buffering against a drop in pH. This is why 50 ppm Borates only contributes 5 ppm to TA at a pH of 7.5. The borates have less buffer capacity against a drop in pH and have most of that capacity against a rise in pH. The measure for this would be Total Acidity, but that's not something in standard pool test kits and really isn't necessary to measure anyway since you can measure borates levels directly (the only real reason for the TA test is to indirectly measure the carbonates after adjusting for CYA and borates).

Your statement about a pool with an effective bicarbonate TA needing large doses of acid less frequently is not true if the source of rising pH is due to the TA itself. If we ignore the outgassing of carbon dioxide from higher TA levels, then it is true that greater pH buffering results in a slower change in pH from other sources (say, curing plaster or Trichlor) but requires more acid or base to move the pH back to where it started.

As for the amount of acid or base you need, it is true that the amount of buffering doesn't change that IF the buffer system itself isn't the source of pH change. So IF the rise in pH is due to carbon dioxide outgassing, then lowering the TA will result in less acid being needed over time and in a slower rise in pH. This is most especially true if the loss of pH buffering from the lower TA is made up for by using another pH buffer such as 50 ppm Borates.

Richard

[waterbear]

The following comments are just my own opinions based on my own experience and study. Take them for what you think they are worth:

Hey Guys;

I'm missing some things here.

Waterbear, I'm not clear on your thinking about TA. You may have some really good reasons behind your thinking, but right now, I'm kinda puzzled. Do you have reason to think that lowering Alk will reduced acid demand or pH rise? Have you seen evidence that a really low Alk (below say 50 ppm) is OK for a plaster pool? Why did you feel that lowering Alk was more important than dealing with her algae?

First, I was suggesting several things that can help with rising pH since the original question was about pH control and the algae problem was mentioned as a secondary one. I was giving several general recommendations at controlling rising pH that I have personally seen work. I did make the assumption that the OP was chlorinating with bleach when I suggested dropping the TA a bit since the OP said they had been doing BBB for two months. Lowering the TA will lower the rate of outgassing of CO2 if that is the cause of pH rise. The chemistry is here:
http://www.poolforum.com/pf2/showpos...34&postcount=2
although I was relating it to the lowering TA process.

I personally have never recommended lowering the TA below 50 ppm but I have found that lowering it form 80 to 60 ppm can often have a positive effect on pH stability when using an unstabilized chlorine source, which is basically pH neutral. Higher TA is really only beneficial with stabilized chlorine sources, which are acidic in nature and benefit by the tendency of the carbonic acid/bicarbonate buffer to raise the pH toward 8.2.

Also, the addition of borates will tend to move pH down and can (and does)help with rising pH in most pools and reduces acid demand (Chem geek disagrees and says that total acid use will stay the same but I have seen a reduction in acid use in my pool and in customers pools). The algaestatic effects of borates have been demonstrated to me sufficient times for me to recommend them as a useful pool additive and one that is really beneficial in pool with a SWG, where the reduced chlorine demand can translate into better pH stability because of less on time of the salt cell.

As far as SI, as 'useful' as it is, if the CH is around 350 or so and other water parameters are in the 'normal' ranges and the pH is kept at 7.6 or above then the water will still be in 'balance' with a TA around 60. Certainly in the ballpark, given the precision of our testing and the accuracy of most pool volume measurements. Chem geek gave you the hard chemistry in his post so I don't need to go through it a second time.

My recommendation to not lower pH below 7.6 is to slow the outgassing effect and you commented on it in a recent post that you had observed the same effect on pH and acid demand:
http://www.poolforum.com/pf2/showpos...7&postcount=23

I did also address the algae problem and made some generalize suggestions since, at the time, it was not clear that the algae was embedded in an aggregate finish above the water line and my thinking was that the pool might have been receiving a lot of sun and would benefit from slightly higher CYA levels given the AZ location and that there simply was not enough chlorine staying in the water to keep the algae at bay.

CarlD, are you aware that chlorine becomes less effective against algae as pH increases? The most dramatic problems are with unstabilized chlorine, but as best I can make out, the problem is still significant in stabilized pools, too. Since she reported that her pH was tending to go ABOVE 8, it's my opinion that that's an issue that needs to be addressed in the short term, in addition to raising the chlorine.

I don't intend to speak for CarlD but I do understand where he is coming from. Chlorine kills algae and that is all a newbie needs to keep in their head. Unless the other parameters are WAY out of whack sometimes it's better to leave adjustments until the algae is dead. One thing I learned by working in a pool store is KISS. Many novice (and old time) pool owners cannot multitask so you have to decide what is the most important thing to deal with first. Using just one chem at a time is often the best way for them to do what is needed to solve a problem, even if it is not the most "efficient".

Chem Geek has done some stuff with 'carbonics' that I haven't had a chance to digest, or much less fully consider yet. However even once I do, I'm not prepared to toss out all S.I. considerations without some experimental verification first. Stoichiometry is nice, and successful process chemistry often resembles the stoichiometric analysis, but rarely is identical with it.

FWIW, I find his recommendations and chemistry sound and tend to agree with him. Then again I have not had access to test pools so my observations supporting him are just the empirical evidence I saw in customer's pools (and my understanding of the underlying chemistry). A limited sample in one part of the country at best!

I'm leaning toward stating some 'standard' recommendations for Pool Forum responses, but I'd like to see what y'all have to say, first.

As always, thanks for all your help.

Ben
"PoolDoc"

Standard responses are a good idea but I would not let them get so rigid that they become 'Laws' that get repeated without understanding and mistinterprented too often as they did on a different forum. There is always an exception to the rule, is there not?
IMHO, they should be set up as guidelines but each pool really does need to be taken on a case by case basis IRL.

[CarlD]

I can't imagine us getting rigid beyond belief on items of pool chemistry. There are LOTS of ways to get things done--remember Ben's attack on a high calcium pool by using lots of cal-hypo directly through the skimmer (or something like that). I STILL have no idea what that was about but it worked!

(OK, maybe when it comes to adding calcium to a vinyl pool with a bloom we are going to be rigid and say "Save your money. It won't help!")

[chem geek]

Also, the addition of borates will tend to move pH down and can (and does)help with rising pH in most pools and reduces acid demand (Chem geek disagrees and says that total acid use will stay the same but I have seen a reduction in acid use in my pool and in customers pools). The algaestatic effects of borates have been demonstrated to me sufficient times for me to recommend them as a useful pool additive and one that is really beneficial in pool with a SWG, where the reduced chlorine demand can translate into better pH stability because of less on time of the salt cell.

My comments on the borates having no effect on total acid used, just on the rate of pH rise (so acid is added less frequently, but more needs to be added to move the pH) are based on borate buffering that does not outgas carbon dioxide. However, if waterbear sees a real effect of acid reduction, then it is always possible that the borates affect the rate of carbon dioxide outgassing, perhaps related to their lowering of surface tension in the water or some other effect inhibiting the facilitated transport near the water's surface. In my own pool I haven't seen the total acid change, but my pool is covered most of the time so that only some of the pool's pH rise is due to carbon dioxide outgassing. We'll need to track more users to see the effects. I know that there are some who have added borates and not seen benefits in their pools with regard to acid addition and in some cases even in the rate of pH rise (which doesn't make sense).

Richard