Alkalinity education wanted!

[TheDeuce]

Hey experts,

I need to edify myself on alkalinity and how it affects one's pool.

Here's my understanding:

"Alkalinity is a measure of your water's ability to resist changes in pH. Higher alkalinity tends to draw one's pH upward, low alkalinity tends to draw one's pH downward. The ideal alkalinity is that which maintains your pH at your desired level. In other words, if you pH is stable at your desired level it doesn't matter if your alkalinity is 10 or 100 or 1000".

Is my understanding correct? Is your TA level irrelevant so long as your pH is stable?


Thanks in advance,


m.

 

[mknauss]

PH TA Relationship - Further Reading

www.troublefreepool.com

TA - Further Reading

www.troublefreepool.com

 

[mgtfp]

Alkalinity is a measure for the water's resistance against pH dropping when an acid is added. It is more or less the sum of all substances in the water that can accept hydrogen ions (H⁺) minus the number of hydrogen ions itself. pH is basically a measure for the number of hydrogen ions - low pH means many hydrogen ions, high pH means fewer of them. If you add an acid, then you basically add more hydrogen ions, but if there are substances in the water that can accept ("buffer") these additional hydrogen ions, then the pH won't change much.

High TA itself doesn't pull pH up. The point is which substance is used to build up a buffer system in the water. The traditional buffer system in pools is the carbonate buffer by adding bicarb to the pool. This is the Carbonate Alkalinity (CA) part of Total Alkalinity (TA). That means you force more carbon dioxide to be dissolved in the water than there would be if just letting the water in equilibrium with atmospheric CO2. Like a bottle of soda water. If you open the soda bottle and the little gas volume from the top of the bottle can start exchanging molecules with the atmosphere, all the CO2 in the water will think "Hey, the guys above us are leaving the bottle, let's follow them!", and CO2 will out-gas from the water. And in the process the pH rises until you reach a pH where the same number of CO2 leaves the water as re-enters the water from the atmosphere - the water reaches equilibrium.

CYA can also accept hydrogen ions, and has at pool pH very comparable buffering capability as the carbonate buffer - with the advantage that you don't create carbonated water by adding it. The CYA-Alkalinity part of TA doesn't create CO2 out-gassing and doesn't contribute to the dreaded pH-drift.

That's part of the reason why TFP recommends higher CYA for SWGs - it increases TA without increasing pH-drift and without increasing the CSI (only the CA-part of TA adds to the CSI), and therefore helps to prevent scaling in the SWG-cell.

Low TA doesn't by itself pull pH down, but it means that a small acid addition will create a relatively large pH-drop. That's why it is important to have enough TA when using acidic forms of chlorine like Trichlor or Dichlor to prevent pH from crashing way down. For non-acidic chlorination methods (bleach, cal-hypo or SWG), less TA is sufficient to prevent pH from crashing. With Trichlor it is actually desirable to have enough CO2 outgassing to compensate the pH-drop due to the Trichlor - with a low TA, pH will drop very quickly when adding Trichlor tabs. That's probably where the perception that low TA pulls pH down comes from.

If you really want to dive into the deep end, then I'd recommend this post:

Chem Geek's Definition of TA

 

[TheDeuce]

Thanks for both posts. I'll have to book some time to go through all of this in detail but it looks like, at first glance, the kind of information I was seeking.


m.

 

[Prickly]

Alkalinity also seems to play a role in protecting the gel coat on some fiberglass pools. I tried to go with a low alkalinity so I could stop fighting the pH rise and ended up with a white haze on my fiberglass pool. This immediately dissapeared when I brought the alkalinity up to the minimum level Leisure recommends (80 ppm).

 

[mgtfp]

Alkalinity also seems to play a role in protecting the gel coat on some fiberglass pools.

Interesting, I haven't heard about that. I think @Texas Splash had a long fight with his gel coat, maybe he has some insight on that.

 

[Prickly]

Interesting, I haven't heard about that. I think @Texas Splash had a long fight with his gel coat, maybe he has some insight on that.

I'd be interested to know exactly how it impacts the gel coat. Leisure Pools sent out this handbook a few months after I discovered the issue in my pool

 

[mgtfp]

I frankly can't imagine that low alkalinity on its own attacks the gel coat. It's probably more a combination of low TA together with acidic forms of chlorination that might be a problem, but I don't have any experience with FG pools.

There are of course things to keep mind. It's for example not a good idea to first add MA to reduce the pH to the min recommended level, and then add 80ppm worth of CYA. That's never a good idea. Let's say you first add MA to reduce pH from 7.8 to 7.2, and then add 80ppm worth of CYA. If you do that with TA 50, then you'll end up at pH 6.5, with TA 80 it'll be pH 6.7, and with TA 120 it's pH 6.8. The higher TA provides some extra protection, but the answer is to just not do that.

The manufacturers will of course take advantage out of the current industry recommended levels, and take every opportunity to wiggle out of warranty obligations.

But it would be interesting to understand what you have observed in your pool.

 

[Prickly]

I frankly can't imagine that low alkalinity on its own attacks the gel coat. It's probably more a combination of low TA together with acidic forms of chlorination that might be a problem, but I don't have any experience with FG pools.

There are of course things to keep mind. It's for example not a good idea to first add MA to reduce the pH to the min recommended level, and then add 80ppm worth of CYA. That's never a good idea. Let's say you first add MA to reduce pH from 7.8 to 7.2, and then add 80ppm worth of CYA. If you do that with TA 50, then you'll end up at pH 6.5, with TA 80 it'll be pH 6.7, and with TA 120 it's pH 6.8. The higher TA provides some extra protection, but the answer is to just not do that.

The manufacturers will of course take advantage out of the current industry recommended levels, and take every opportunity to wiggle out of warranty obligations.

But it would be interesting to understand what you have observed in your pool.

You would think it would happen with a low pH, but actually they say it happens when the pH is over 7.4. They recommend a tight range of 7.2 to 7.4 to combat the chalking gel coat issue.

I didn't have a low pH when I observed this - in fact I was fighting a high pH constantly (which I have since discovered is due to the high flow and surface agitation in my pool).

I've decided to go with CO2 injection so that I can maintain the levels they recommend without doing daily MA additions....

They also note that pool stores cant be listened to as they know nothing about fiberglass gel coats and recommended chemistry. I had a chuckle when I read that.

Also, I wasn't making a warranty claim, nor did I report this issue as I was able to resolve it. They just sent out guidelines on the subject proactively.

 

[mgtfp]

Let's see if Texas Splash chimes in, I think he's got a really good understanding of FG pools and gel coats.

I just mentioned the warranty because it was mentioned in the extract from your handbook, and that just looked to me like an attempt to make warranty claims basically impossible.

 

[Prickly]

Let's see if Texas Splash chimes in, I think he's got a really good understanding of FG pools and gel coats.

I just mentioned the warranty because it was mentioned in the extract from your handbook, and that just looked to me like an attempt to make warranty claims basically impossible.

I'm sure you are right about warranty claims being impossible!

 

[Texas Splash]

I don't think anyone has cracked the code on this issue yet. I have my own suspicions, much like already noted above. To make matters more complicated, there seems to be two chalky scenarios. The first is the kind that wipes off easily by hand like a milky cloud, while the other is a chalky discoloration that appears embedded in the gelcoat and only seems to change when buffed or sanded. For the chalky pools that wipe off easily, some have found an elevated TA and/or CH seems to help. The jury is still out on the other scenario (embedded in gelcoat). I have been testing areas of my pool for a few years now since mine is the embedded type.

According to manufactures and experts who know their science, the TA, CH and CSI levels shouldn't be a factor since these are not plaster pools. Many experts simply believe that the chalkiness is a natural change that occurs when a porous substance is submerged 365 days a year. But not all FG pools get chalky, so what gives? The FG manufactures provide a standard recommended template for chemical levels, and I think we can all agree those aren't perfect. Some manufactures try to put the blame on pools that had an elevated pH coupled with excessive chlorine levels that resulted in a form of oxidation. I also follow other FG pools that experienced a chalky gelcoat and had their pool resurfaced only to have it return.

In my case, I was so obsessed with potential scale when I first got our pool that I kept my (pH, TA, CH) levels quite low. My CSI level, which should not apply to FG pools, was extremely low for a considerable amount of time. Instead of helping, I believe I was doing more harm than good and it seemed like the chalkiness was getting worse. From a scientific position, I can't prove why, but once I stopped trying to keep the pH and TA low, and allowed the CH to remain in the 350 +/- 50 range, my walls seemed to hold their color better. My suspicion is that the gelcoat in my pool needed a bit more buffer protection from either aggressive water and/or the chlorine interaction in a pool managed that way. But this is only my personal hypothesis so far and not that of standard TFP philosophy. Again, some FG pools seem to do just fine across various pH, TA, and CH levels, so it may be a random thing based on how the gelcoat was mixed and/or applied.

So I continue to experiment on my walls and monitor changes based on various TA, pH, CH, and related CSI levels to see if I can substantiate any of my experiences. That's about all I can do for now.